The Application of 3D Printing and ICT for Green Products and Processes
B A S I C L E V E L
3D printing creates a tangible object by adding successive layers representing cross-sections of the object.
Contents
3D printing principles of operation
Definitions
3D printing creates a tangible object by adding successive layers representing cross-sections of the object. The simplest allusion there is making a layered cake (Fig. 1), only the layers are numerous and typically very thin, only about 1/5 of the millimetre.
A.
B.
Figure 1. Layers and layering in 3D printing.
A – Layered cake, representing the basic principle of 3D printing. Source: Caroline’s Cakes – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12799030
B – 3D printed chocolate castle. Source: FM1418, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
3D printing is close, both in process and material, to routine ink-and-paper 2D printing.
Nevertheless, in typical 2D printing only an illusion of depth is created using perspective and colour or grayscale, whereas 3D printing creates tangible 3D objects. These may have a myriad of purposes, from rapid creation of prototypes for research and industry to printed objects for everyday or special use; machine parts; custom-made culinary items such as the chocolate castle presented in Fig. 1B; printed implantable materials for the regenerative medicine, and, potentially, ‘replacement parts’ for the human body; and novel forms of art.
A basic desktop 3D printer with PC is presented on Fig. 2.
Figure 2. LulzBot TAZ (FAME 3D, Fargo, ND, USA), an open-source 3D printing machine based on FDM technology. Source: Ochs, J. – College, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=59898086
Typically, the process of 3D printing of an object is additive (hence 3D printing is also known as “additive manufacturing”). It is achieved by depositing layers upon layers of a printable material (commonly termed as ‘3D ink’) until the object is complete.
3D printed objects need not to be solid within (though they might be). They may contain any type of cavities that has been set up in the data file used for printing, because every layer represents a cross-section at a specific level in the manufactured object.
A short time-lapse gif illustrating the basic process of 3D printing may be viewed here:
https://upload.wikimedia.org/wikipedia/commons/e/ed/Robot_3D_print_timelapse_on_RepRapPro_Fisher.gif
The most commonly used 3D printing nowadays is fused deposition modelling (FDM), although other types are rapidly gaining popularity.
Brief history of the field
It is commonly believed (and cited) that the very idea of 3D printing did not exist until the 1950’s when the sci-fi writer Raymond F. Jones published a short story describing use of ‘molecular spray’ directed by information from a pre-recorded data tape. Nevertheless, the possibility for controlled hands-off creation of a complex tangible object following virtual directions was foreseen about 50 years earlier by the great visionary Joseph Rudyard Kipling in his children’s story ‘How the camel got its humph’. The first attempt of 3D printing, however, did not arrive until the 1970’s, and it did entail both creation and removal of objects. Specifically, this was the patented ‘Liquid metal recorder’ by Johannes F. Gottwald (US3596285A) using electrically conductive metallic ink propelled under pressure against a surface. The melted ink would flow at printing temperature but will solidify as it contacts the surface, then it could be removed by increasing the temperature above the melting point, enabling the reuse of the ink and the surface for further use.
As with most novel concepts, the idea of 3D printing did not meet early success. In the early 1980’s, a patent was filed by Hideo Kodama in Japan for additive manufacturing of ‘stereoscopic figure drawing device’ using UV-hardening polymer where the areas of UV irradiation were controlled by pre-set masking pattern (JP S56144478 A). Despite prompt publication of his results, Kodama could not raise much interest in his project and it was subsequently cancelled.
Four years later in France the same fate met the concept for stereolithography presented by Le Méhauté, de Witte, and André and their subsequent patent for the process, purportedly “for lack of business perspective” [http://www.primante3d.com/inventeur/].
Around that time the American William E. Masters filed the first of his series of patents (namely, the patent for ‘Computer Automated Manufacturing Process and System’ (US 4665492) that eventually laid the foundations of 3D printing as we know it.
Only two years later the American Charles Hull was granted a patent for his own ‘apparatus for production of stereolithographs’ (US4575330A), founded his own company and eventually released the first ever commercial 3D printer, the SLA-1, in 1987. The patent for the most commonly used in present day type of 3D printing – the FDM – was filed in 1989 in the US by S. Scott Crump (US5340433A) and the first FDM equipment was released in 1992. Hull also gave probably the simplest and yet complete definition of 3D printing to date, namely: “A system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed…“, in the description of his patent for ‘Method for production of three-dimensional objects by stereolithography’ (US5174943A).
Advantages and disadvantages of 3D printing as a method of manufacture
There are numerous advantages to using 3D printing rather than traditional methods of manufacture. Some of these are listed below:
- Complex design
Since the object is not cut out of a block, 3D printing may handle different designs with very complex structure.
- Possibilities for last-minute modifications.
The design is easily modifiable and a variety of different prototypes of an object may be rapidly manufactured and tested.
- Easy and rapid tailoring of design to suit individual needs
Unlike traditional methods of manufacturing, the introduction of needed modifications concerns the data files rather than the machine itself and may be carried out with very little notice.
- Yields lightweight and durable objects
That depends on the material used, but most commercial 3D printers use plastic, although use of other materials such as plaster, metal, metallic alloys, etc. is also possible.
- Relatively rapid production on smaller scales
Depending on size and complexity of the design, most objects may be manufactured within hours, although larger ones may take more time.
- Low (compared to traditional methods of manufacture) waste of material
Again, since there is no cutting, drilling or boring, the wastage of material is negligibly low. An exception is powder-based 3D printing where excess powder is usually lost.
- May be more cost-effective than traditional methods of manufacture
That depends largely on the material and the process used for manufacturing.
- The ease of access of researchers, industry and the public to 3D printers is increasing all the time
At present, gaining access to a 3D printer is relatively easy, although there is typically a queue of customers waiting to use the equipment. The waiting time depends on the make and model of the printer, the institution it is been hosted by and the type of use the 3D printer is being put to.
- May be more environmentally friendly than traditional methods of manufacture
Again, this depends of the type of material used, the intensity of use and the maintenance of the equipment. Discarded objects, wastewater and small particles emission, however, may pose a problem (see below).
- Special uses of 3D printing in reparative and regenerative medicine (3D-bioprinting)
Significant research and development effort is currently invested in developing a new generation of implantable materials. Instead of simply providing inert support, friction-friendly surfaces and bridging between intact portions of damaged tissue (which are the main functions of most implantable materials of today), the new materials may also presumably become a part of the natural habitat of the cells of the damaged tissue. Such materials may be purely of artificial origin but resemble biological tissue (biomimicking materials). One potential use of 3D bioprinting is providing optimal 3D environment of implantable materials for colonisation with living cells from the surrounding tissue. Another very promising application of 3D bioprinting is the potential for creating living tissue (and, potentially, functioning replacement organs and various ‘spare parts’ for living bodies) using living cell-based materials as printable ‘bioink’ (see below).
Potential limitations of 3D printing compared to traditional methods of manufacture may be:
- Limitations imposed by errors in design
While there are virtually no limits to potential design types, a single error in the design may lead to manufacture of unusable object/s. While some of the errors of design may be minimised by finishing, it may increase the time to process an order and may decrease the durability of the object.
- Limitation imposed by material
Most 3D printers use as ink some type of plastic polymer, although some may also print with metal and metallic alloys. Objects that are produced and then discarded may pose a problem if the material is not biodegradable and/or recyclable. Printing and post-processing may result in emission of particles that contribute to environmental pollution and may have carcinogenic potential when breathed and ingested.
Issues concerning the material for bioprinting may be aggravated further in 3D bioprinting where viability of living cells and biocompatibility is of utmost importance.
- Limitations imposed by size of the object
Very large objects may be difficult to manufacture in one piece. Different sections of the object may need to be printed separately and then assembled together. This may increase the time and costs for manufacturing and may decrease the durability of the object (see below).
- Integrity of the produced objects may be compromised
This may be an issue especially when parts are produced separately and then assembled together. Under physical stress, the layers may come apart (especially in FDM-produced objects) and complex objects may be brittle and delicate to handle.
- 3D -printed objects may need finishing
Essentially, this makes no difference with traditional manufacturing. Again, this may increase the time and costs for manufacturing.
- Cost generally not reduced with larger batches
In traditional manufacture, when the equipment is working at full capacity, the costs of manufacture per object or per batch generally tend to be lower. This is not the case with 3D printing and large volumes of manufacture may be costly.
- Copyright Issues
Quite self-explanatory. The issues entail not only the manufactured objects but also the software, hardware and processes used for manufacturing them.
- Ethical issues, concerning mainly the 3D-bioprinting of living tissue
- May indirectly cause reduction of jobs in the traditional manufacturing sector.
Basics of 3D printing processes and materials
Types of 3D printing processes
Many types of 3D printing technologies currently exist. What they all have in common is that tangible objects are created by deposition of successive layers rather than cutting out the object out of a solid block and/or drilling, milling of boring. Every consecutive layer represents a cross-section of the object under construction. The process is controlled by a computer equipped with designated software for creation of design files, file processing and object printing (see further for details).
Several of the most common technologies for 3D printing are briefly presented below. This list is by no means exclusive.
- Liquid or semisolid ink-based technologies: SLA, FDM, EXT, PolyJet
All these three are based on deposition of layers of liquid (more or less viscous) ink that are then fused to one another by heat or directed light beam (emitted by an UV lamp or laser). The ink may be extruded from a nozzle or, when it is of higher consistency, fed from a spool.
- Stereolithography (SLA)
Stereolithographic 3D printing is perhaps the most easily comprehended of all 3D printing technologies, probably because photopolymerisation is commonly used in modern dentistry. Basically, stereolithography is light-curing of photosensitive material extruded in a layer-by-layer fashion onto a movable platform. The wavelength of the light source is most often in the UV range but other wavelengths may also be used. SLA materials polymerise via free-radical polymerisation.
Different parts of the object may need to be supported in the making of the object so as to avoid the collapse of the whole structure. Successive layers are made to adhere to each other by cross-polymerisation of the just polymerised layer to the newly deposited layer where polymerisation is still incomplete. Since polymerisation may take some time to complete, the printed object usually needs some time in order to harden, then supports are removed (if any of these remain after the last layer has been deposited), then a final post-processing may take place.
A SLA printed 3D model is presented in Fig. 3.
Figure 3. Stereolithograph of bony labyrinth of the inner ear in man. Source: Didier Descouens, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
The ink used in SLA-based printing is usually liquid in its unpolymerised form. It is mostly epoxy or acrylic-based, although trimethylene carbonate, polycaprolactone, and polylactide-based inks may also be used. Use of such materials yields printed objects with significant rigidity that are, nevertheless, quite fragile. Thus, SLA is quite suitable for rapid prototyping and research purposes (e.g., tissue engineering scaffolds) but not for objects that need to be durable. SLA has quite high resolution (minimal layer thickness is 50 μm) and therefore is very reliable when fine detail is crucially important.
Objects created by SLA may be light-sensitive and may tend to disintegrate over time, especially when kept in light environments.
Solvents are rarely used in SLA printing, therefore, the risk of escape of fumes to the atmosphere is low. Due to the fragility of SLA-printed objects, nevertheless, discarded objects may present an environmental hazard. The use of UV calls for special precautions in order to avoid accidental exposure of surfaces, objects and operators to the curing beam. Some of the inks used for stereolithography may have significant neurotoxic and carcinogenic potential. Also, UV-mediated formation of ozone may need to be considered.
- Fused deposition modelling (FDM), also known as fused filament fabrication (FFF)
At present, this is the most commonly used technology for 3D printing. FDM technology is based on extrusion of a filament of thermoplastic ink fed from a spool through a moving heated printer nozzle. The ink within the nozzle is heated just above its melting point and is extruded in semi-solid form that solidifies almost instantly after a layer is deposited. The object is built upon a moving platform that is gradually lowered as the structure grows (Fig. 4).
Figure 4. Working principle of FDM printers. Source: By Gringer – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=73614647.
The minimal layer thickness for FDM is 100 μm. The most commonly used ink materials for FDM are acrylonitrile butadiene styrene (ABS), polylactide (PLA) and polyethylene terephthalate glycol (PETG). Others, such as polyphenyl ether (PPE), polyurethane (PU), polyethylene terephthalate glycol (PETG), polyphenylsulphone (PPSP), polycarbonate (PC), polyetherimide (PEI), polyether ketone (PEEK), polyamide (PA, commonly known as nylon) are also in use. Polyvinyl alcohol (PVA) may also be used, but since it is hydrophilic, PVA is most commonly used for manufacturing easily removable supports for a structure printed using other type of thermoplastic ink.
The heatable ink for FDM may be also in the form of rods or granules. A variation of FDM uses cold extrusion of unheated ink, usually supplied as slurry or paste.
A FDM printer with spool of filament is presented in Fig. 5.
Figure 5. PRUSA i3 MK2 FDM printer (Prusa Research, Prague, Czech Republic) with spool and a completed object. Image source: Wikimedia Commons, http://reprap.org/wiki/File:Prusai3-MK2.jpg.
Being relatively low-cost, FDM is especially popular among hobbyists and desktop model makers. In addition, FDM is commonly used for rapid prototyping in engineering, in manufacture of high-performance parts and machinery, in education and in art.
The objects manufactured by FDM are significantly more durable than these produced by SLA and with very good quality of detail reproduction. Different quality FDM inks yield objects with different properties with regard to durability, rigidity and flexibility (see below).
Disadvantages of 3D printing by FDM are mainly related to potential environmental hazards (use of solvents, emission of small particles during post-processing, and the fact that the discarded objects are quite durable).
- Semi-solid Extrusion (EXT) 3D Printing
EXT 3D printing is another technology based on extrusion. The ink is usually polymer-solvent mixture with the consistency of paste. EXT materials are not thermoplastic, allowing for printing at low temperatures. Again, the object is built upon a moveable platform.
EXT is commonly used in research (e.g., tissue scaffolds) and for pharmaceutical purposes (targeted and/or timed delivery of drugs). Biosafety is ensured by using inks based on polyvinylpirrolidone, polyethylene glycol, methycelullose, hydroxypropyl methylcellulose, polycaprolactone and other materials. EXT is one of the potental approaches for priting living tissue from a hydrogel containing living cells. Also, extrusion using viscous foodstuff paste (icing, chocolate, jelly, etc.) is a preferred method for 3D printing for the food industry (Fig. 6).
Figure 6. Extrusion printing for the food industry. Source: Grup de recerca de la Universitat de Columbia – https://www.creativemachineslab.com/digital-food.html, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=84202502
Since the material for EXT 3D printing is neither photosensitive nor thermoplastic, layers do not typically solidify instantly. Thus, newly printed 3D object may need time to set and/or dry after completion as the solvent/s evaporate. It may also be prone to warping, shrinkage and deformation. Minimal layer thickness for EXT is 800 μm. This low resolution is inherent to semi-solid extrusion techniques.
- PolyJet
PolyJet is another liquid-based 3D printing technology. Like SLA, it is based on layer-by layer UV-curing of a photosensitive polymer. The ink, however, is released from the printer head in droplets that accumulate on a building platform as directed by a designated software. PolyJet technology allows for mixing of inks, resulting in greater variety of properties for the manufactured objects.
PolyJet inks encompass great diversity of materials. Among the commonly used are ABS (different modifications), the related compound acrylic styrene acrylonitrile (ASA, more heat-resistant and UV-resistant than ABS), polycarbonate and polyetherketone ketone (PEKK). Various photosensitive materials that are used in other liquid-based 3D printing technologies may be used in Polyjet as well. There are also the specifically designed series of Vero PolyjetTM inks patented by one of the market leaders in 3D printers and materials, Stratasys. Prominent members of biocompatible PolyJet materials are MED610 and MED620, a methacrylate-based resin used in the medicine and dentistry for rapid prototyping.
Advantages of using PolyJet are the relatively high resolution (minimal layer thickness is 160 μm, allowing for creation of stable structures with complex geometry and exquisite detail) and the great variety of properties (strength, flexibility, colours, etc.). The disadvantages are (beside the common drawbacks of 3D printing described above) the inherently high levels of emission of small particles, the use of UV light and the emission of toxic fumes when curing objects manufactured with some types of inks (e.g., ASA). Printed objects may need post-processing and may be photosensitive.
- Powder based (PB) 3D printing
PB 3D printing technology is based on deposition of successive layers of powdered material and their binding by means of a liquid binder solution or fusing using other methods (see below). The powder is deposited in a powder bed or produced by a powder jet and is then sprayed or jetted with binder, irradiated with light or heated in order to achieve integrity of the object.
Minimum layer thickness may vary greatly in PB 3D printing and layers may be as thin as 20-30 µm with some materials. This very high resolution allows for creating very complex structures with exquisite detail. Generally, however, the layer thickness is about as 200 µm.
Powdered materials are usually produced by cryogenic grinding in a ball mill at a temperature that is below the glass transition temperature of the material. The grinding may be using low-temperature gases (liquid CO2 or liquid nitrogen – dry powdering) or a mixture of low-temperature gas and organic solvents (wet grinding). In its simplest form, PB 3D printing is achievable using materials such as powdered starch or gypsum bound together with glue (Fig.7). Dyes and plasticiser compounds may be added to the binding solution in order to achieve the desired properties.
Figure 7. Rapid prototypes manufactured by powder-based 3D printing. Source: S Zillayali, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons.
Disadvantages of PB-based 3D printing are related, on the one hand, on the physical properties of the objects created with this type of technology, and, on the other hand, to the need for post-processing in order to achieve maximum quality. Objects built by PB 3D are inherently highly porous and, therefore, exhibit poor mechanical resistance and may be fragile.
PB-printed objects invariably need time for evaporation of solvents. Post-processing is usually necessary in order to remove excess powder (hence a certain amount of material is always wasted) and to improve the properties of the manufactured objects (strength, colour, polish, etc.). Emission of potentially harmful small particles is a serious concern in all PB-based technologies for 3D printing.
PB has been successfully used in research, dentistry and medicine for manufacture of implants and fabrication of drugs with specific pharmacokinetic properties such as rapidly disintegrating tablets, drugs with delayed, pulsatile or extended release. Biosafety of these dosage forms is ensured by using methacrylate or ethyl cellulose as carriers of the bioactive substance.
- ColorJet
ColorJet is not exactly a technology, but a brand of powder-based 3D printers and supplies manufactured in India. It offers one of the simplest solutions in PB 3D printing. The material used is most commonly white plaster powder although plastic may also be used (e.g., the company’s own patented Visijet C4 Spectrum powder). The material is layered onto a movable platform and bound by adhesive. The binder may contain different dyes, conferring different colours to the model being built.
ColorJet is most commonly used for rapid prototyping. Minimum layer thickness is about 100 μm but may vary with different materials.
Colorjet provides a budget solution where durability, brittleness and porosity are not of great concern. Therefore, it is an acceptable solution for manufacturing of models for education, research and commercial purposes (e.g., sales models) and in art. A similar process using edible powder and binder solution is commonly used in the food industry to create customised sweets and dishes.
Objects printed by ColorJet may need extensive post-processing. Plaster structures need to harden after their completion and the solvents must evaporate. Then the object is polished and impregnated with another type of binder (usually acrylic) to achieve maximum strength and to decrease surface porosity. Fumes of solvent may be an environmental concern, as well as small particles emission.
- Selective Laser Sintering (SLS)
SLS shares the common characteristics listed above for PB 3D printing. In SLS the powdered material is layered and a directed light beam generated by the printer head partially liquefies and fuses the layers to one another as the structure is being built in a process commonly termed as necking (Fig. 8).
Figure 8. Formation of a ‘neck’ between two fusing layers (arrows). Source: Mendoza.755 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=76830144
Minimum layer thickness in SLS is about 100 μm.
SLS works very well with materials with high melting point and is therefore most commonly used with metal-based inks. Plastic (polyamide, polycarbonate, polystyrene and others), thermoplastic resin, glass and ceramic powders may also be used. SLS is often used for rapid prototyping in research (especially tissue scaffolding) and in dentistry. As of now, the opportunities for development of SLS-based technology in the pharmaceutical industry are quite limited as the radiant energy and the heat generated in the course of layer fusion may seriously compromise the properties of the bioactive substance.
Powdered foodstuffs (cocoa, sugar, etc.) may be used for 3D printing in the food industry.
- Selective laser melting (SLM)
SLM is often viewed as a derivation of SLS, as it is used mainly for printing with metal and metallic alloy powders. Nevertheless, while SLS printer head laser generates just enough energy to fuse the layers to one another, SLM uses a high-power laser that may melt completely a block of metal and then let is set in the desired shape similarly to casting. SLM produces objects that are significantly less porous than SLS and have better mechanical resilience due to homogenous internal structure. A disadvantage of SLM is that objects require polishing after their completion as it generates structures with increased surface graininess.
Electronic beam melting (EBM) is a subtype of SLM that uses similarly and electronic beam to melt metallic powders.
- Selective heat sintering (SHS)
SHS is based essentially on the same principle as SLS but the ink is thermoplastic and, respectively, the layer-by-layer fusion is obtained using heat generated by a thermal printer head. The most common material used in SHS is plastic (specifically, polyamide, but other polymers may also be used).
All objects produced by SLS or SHS are prone to warping and shrinkage. Mechanical properties of the printed objects may be inconsistent from object to object as well as from batch to batch. SLS and SHS-made objects often require post-processing, such as removal of supports and coating with sealer agents in order to decrease surface porosity.
- Multi jet fusion (MJF)
MJF 3D printing is a hybrid of the ColorJet approach (use of chemical binders) and SHS (use of heat) that works mainly with polyamide (Nylon). Basically, finely grained polyamide powder is layered onto a powder bed, sprayed or jetted with binder and is then heated selectively as instructed by a pre-programmed software. Minimum layer thickness is about 80 µm. When the object is complete, post-processing takes place (removal of excess powder, polishing, etc.). MJF produces objects that are less porous and therefore more consistent in their mechanical properties than SLS or SHS and has better capacity for reproduction of fine detail. Surface finish is also better with MJF. Chemical fumes and generation of small particles is a concern, as with all PB 3D technologies.
Post-processing requirements for different 3D printing technologies
Not all objects made by 3D printing require post-processing. Nevertheless, many do need more work after the structure is complete. Whether an object needs post-processing depends on the material, the technology, the desired aesthetical or mechanical properties, and other factors.
Generally, there are several types of post-processing of 3D-printed objects:
- Setting and/or hardening
Almost all 3D printed structures need some time after their completion in order to mature and achieve optimal mechanical properties. Objects made by extrusion-based processes (especially those that use semisolid mixtures of polymer and solvent) and powder-based processes using materials that have been impregnated with binder solutions are especially sensitive to mechanical stress immediately upon completion. In this case post-processing includes the time for solvent evaporation and hardening. In some cases (but not always), this may be facilitated by increase of the ventilation in the premises where completed objects are kept.
- Supporting of unfinished structures
Complex structures containing overhangs may not be stable during manufacture without supports, albeit temporary. These supports need to be removed after the structure is complete, increasing the time to completion and the production costs (especially if this involves manual labour). In SLA-based 3D printing, supports for a structure printed with thermoplastic ink are most commonly made of polyvinyl alcohol. Since it is soluble, such supports may be easily removed during post-processing with a jet of water. The latter, however, requires plumbing, access to water and decontamination system for wastewater (which may increase the costs as well). Temporary support structures in SLS and SHS are usually made of unsintered material that may be vacuumed out or air-blasted away.
- Removal of excess material
This is crucial for object made by most powder-based methods. Excess powder needs to be removed using vacuum, air blast or a jet of water (again, this raises concerns about water supply and decontamination of wastewater).
- Polishing
Polishing is a must for improving of the mechanical characteristics of mechanical parts (especially those subjected to movement and friction) and objects with increased graininess of the surface (such as those produced by selective laser melting). Objects with aesthetical properties may also need polishing.
- Sealing of surface pores
This is an important post-processing step for technologies that yield objects with increased porosity, such as SLS or SHS. Surface pores may be sealed with impervious coatings such as cyanoacrylate coatings or by hot isostatic pressing.
- Painting
This is not commonly needed, as most 3D printing processes use differently coloured materials or differently coloured binders.
Which process for which? Basic considerations when selecting 3D printing technology suited for a particular application
While all 3D printing technologies may have overlapping applications (e.g., rapid prototyping), there are factors that may weigh in when a 3D printing technology is selected for a particular project or a specific industrial solution. Main considerations are:
- Type of material that may be used to print an object. Many 3D printers on the market may use a variety of different materials. Architectural, sale and exhibition models as well as models made for educational purposes may conveniently be created by ColorJet, SLA and FDM technologies that use plaster or different types of plastic. Small desktop machines that serve hobbyists and artists are usually based on FDM technology. SLS/SLM and SHS are usually employed when the objects must be made from metal and metallic alloys.
- Availability of 3D printing machine. Usually, orders for a particular machine are queued and work in a ‘first come, first served’ fashion. In regions where 3D printers are hardly available, this factor may be of prime importance, prevailing over the type of material.
- Availability of material. Plastic is versatile, available and, except for high-performance polymers, relatively cheap, while metal may be more costly.
- Costs for production per object. These are related to the technology used for creation of objects, the material used and the needs for post-processing. It was already mentioned that, unlike casting, scaling up in 3D printing does not always bring the unit price down. The price per object includes not only the costs for manufacturing but also the post-processing costs (if it is needed) as well as the costs incurred by measures for decreasing environmental pollution. Among commonly used PB technologies, SHS is a more budget-oriented solution than SLS as purchase and maintenance of a thermal printer head is generally less expensive than a laser printer head. Objects manufactured by SHS are therefore generally cheaper that those made by SLS and the process may easily be scaled down for small-sized orders.
- Rapidity of manufacture, including post-processing and the time for queueing of orders. Small objects may be printed in a matter of minutes to hours. The time for printing increases with the size of the objects to a whole day and, ultimately, with very large objects, up to a several days, although the latter is rare. Post-processing times may also be a concern. Extrusion technologies does not typically need finishing, but the object may need time to set and harden. ColorJet-made objects need time for drying and setting. Powder-based technologies almost invariably need time for post-processing and it increases the costs as well as the time for manufacturing,
- Properties of the end product (mechanical properties, aesthetical value)
Plastic 3D printing ink may come in many different varieties with different mechanical properties. Depending on the purpose of the use of the object, the preferences may be for plastic (lightweight and, at the same time, durable, easily dyed, easily polished, thermo-and photocurable) or for metal (resilient, resistant to UV and heat, colouring and polishing may need special equipment and processes).
Among the plethora of varieties of plastic, different polymers with different mechanical resilience are available. Generally, SLA printing allows for finely polished structures with exquisite detail that are, nevertheless, quite brittle. It is therefore unsuited for applications where durability is crucially important. FDM yields significantly more durable objects than SLA, yet there are different grades of mechanical resilience depending on the material used. Generally, PVA, ABS and PLA are ”commodity” inks for FDM. These are relatively cheap but the resulting objects are prone to warping. Objects printed with PLA are also quite brittle. ABS-printed objects are photosensitive. Use of polyurethane yields parts with significant degree of flexibility. Next step in FDM quality is PC, PETG and PPE. These materials are used mainly as engineering materials for manufacture of mechanical parts and tools. PETG is also food-safe. High-resistance FDM inks are PEI and PEEK which are used for parts and devices that need to exhibit very high resistance, such as moving and/or strain-bearing parts and when thermal stability is crucial. All 3D printed objects might be prone to delamination under stress, but PEEK printed objects exhibit excellent layer-to layer adhesion and are therefore very durable. It is, however, quite expensive, and as the melting point is higher than of the other FDM inks, the printer needs to have the capability for heating the nozzle to high temperatures.
3D printed drug dosage forms and tissue scaffolding have specific requirements that are generally met by extrusion methods (especially semisolid extrusion such as EXT) and some of the powder-based technologies (but not these that require large amounts of radiant or infrared energy such as SLS and SHS). As minimal layer thickness for EXT is almost 1 mm, the printed structures are coarser and less detailed.
Powder-based methods have very high degree of resolution and therefore confer better reproduction of fine detail. This is especially true for SLS (it, however, works mainly with metallic powders) but MJF (works readily with plastic) also compares favourably in terms of resolution.
The structures produced by most PB technologies are also durable (except for ColorJet-printed plaster objects) but porosity may compromise their mechanical properties. When the latter is a serious issue, SLM may be a solution, provided that proper post-processing is carried out.
- Environmental concerns (related to manufacture or post-processing or the biodegradability of discarded objects).
Extrusion-based and some of the PB technologies generate solvent fumes. Powder-based technologies generate large amounts of small particles. All these have significant potential harmful effects and need to be filtered out and/or decontaminated.
Commonly used software for 3D printing
3D printing uses previously collected data that has been stored on a digital information carrier. In the course of 3D printing of a complex object, the momentary changes in the pattern of deposition of the ongoing layer are dictated by the information of specific cross-sections in the digital file representing how exactly the structure must look like at this particular point. Thus, structures with simple patterns may be represented by fewer cross-sections while digital files for structures with intricate patterns ought to contain more cross-sections, indicating where and how the structure is altered. Every cross-section represents a closed polygon and corresponds to a different position along the z axis. Cross-sections must not overlap or intersect and, at best, must be without voids in between. Small discrepancies may be cleared out by the software without significant shape and structure alterations.
Computer-assisted design (CAD) images are most commonly used for 3D printing. Those may be the conventional 2D DWG images generated by AutoCAD although these need to be converted to 3D by designated software, e.g., Sketchup Pro. Stereolithography file format (Standard Tessellation Language, STL) was the first to be specifically designated for additive manufacturing. It was developed and adapted for use in the 1980-ties by 3D Systems, a company founded by Charles Hull, the inventor of SLA. Apart from AutoCAD, commonly used CAD software capable of exporting or converting data to STL format are Alibre, Mechanical Desktop, I-DEAS, SolidDesigner, ProE and others. Online converters are also readily available, such as AnyConv (https://anyconv.com/stl-converter/).
Images generated by a 3D scanner or by photogrammetry of a typical 2D digital image (taken by a digital camera) may also do for additive manufacturing. Depending on the source of the data for the object, the final result may be more or less true to the initial image. CAD images usually result in objects with significantly less errors than other types of digital data. These errors may be corrected later. There are many different brands of software for photogrammetry, mainly Microsoft Windows-based (Zephyr3D, PhotoModeler, PhotoSynth and others) but there are some may use other operating systems such as Linux and MacOS (e.g., Pix4D Mapper, Metashape, OpenDroneMap and others). A detailed list of commonly used photogrammetry software with their basic characteristics may be viewed here:
https://en.wikipedia.org/wiki/Comparison_of_photogrammetry_software
Online tools for photogrammetry are also available, such as the mentioned above AnyConv (https://anyconv.com/stl-converter/).
3D printing design guidelines
In order to print an object, if a STL file is not available, it must be output from a piece of software such as AutoCAD or others (see above). Most professional 3D-modeling software is proprietary, but free software is also available, mainly for education purposes for beginners in the field and for hobbyists (FreeCAD, 3D Slash, TinkerCAD, Vectary, SketchUp, Leopoly, Sculptris, Rhinoceros and others). Then the resulting STL file is used by another piece of ‘slicer’ software, such as Repetier, OctoPrint, Z-Suite, Tinkerine Suite, Cura, PrusaSlicer, Simplify3D and others to generate a file containing series of thin layers. The resulting file is used by the 3D printer host to generate a 3D object. Some pieces of slicer software function also as printer hosts, such as Cura, 3DPrinterOS, OctoPrint, MatterControl and others. While most professional 3D modelling software is usually stand-alone, free software may be available as a plugin and is much more likely to be available for operating systems other than Windows.
STL files contain raw mesh data based on triangulations of the surface of CAD models. Thus, the surface of every facet of the object is represented by a polygon mesh that must conform to a set of rules. The basic rules are four, namely:
The ‘vertex to vertex’ rule – each triangle must share 2 vertices with its neighbouring triangles;
The ‘right-hand rule’ – the vertices must be listed in counter-clockwise fashion;
The ‘pointing outwards’ rule – the direction of normal vector should point outwards;
The ‘all-positive rule’ – all the coordinates of the triangle vertices must be represented by positive numbers.
An example may be viewed in Fig. 9.
Figure 9. A polygon mesh representing a 3D image of a dolphin. Source: Chrschn, Public domain, via Wikimedia Commons.
STL files may store information in binary encoding or in ASCII encoding. Files in binary are, generally, smaller and may be easily shared, while ASCII files are used to manually read and check the STL file, e.g., when testing new CAD interfaces.
Complex objects may be a challenge to STL files as there are too many surfaces to be taken in to account in the calculations. In addition, the early STL files had very limited properties for representing colour, texture, material and internal structure (although this was resolved later). Some types of software, such as the prototypic AutoCAD may only export designs of solid objects to STL. Non-adjoining surfaces (e.g., containing gaps or overlaps) are difficult to process or adapt in STL files and would not print. Thus, in 2006 an improved version of STL formats was released that eventually was transformed in another, more versatile CAD file format, the Additive Manufacturing File format (AMF). Nevertheless, many 3D printers used today, especially those that work in the small scale, will use STL files.
AMF files can readily represent internal structure using use face-vertex polygon layout and curved triangles to define a discrete unit of volume of material. Multiple volumes may be specified for a single object. AMF files also contains information about the material, the colour and texture of the printed object or its parts and the order for printing.
Jonathan Hiller, one of the inventors of the AMF file format hosts a detailed and user-friendly site providing CAD software or 3D modelling and tutorials: https://sites.google.com/site/jonhiller/software.
Free architectural CAD drawings for AutoCAD and 3D modelling software are readily available on the Internet, e.g. https://www.arcat.com/, https://www.tinkercad.com/, https://www.sketchup.com/ and others. A free 3D CAD image is presented in Fig. 10.
Figure 10. A 3D CAD model for use in 3D printing. Source: OKFoundryCompany from Richmond, USA, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons.
Free STL files, 3D printer files and 3D printer designs are available for use from www.thingiverse.com, www.youmagine.com, www.myminifactory.com, https://library.zortrax.com/, https://www.cgtrader.com/, https://cults3d.com/en, https://www.youmagine.com/ https://www.turbosquid.com/Search/3D-Models/free/stl, and other sites.
STL editor software is available for multiple sources such as https://www.meshlab.net/, https://www.blender.org/, https://www.freecadweb.org/, https://www.3dslash.net/index.php, https://www.sketchup.com/, and others.
Test: LO2 Basic level
References
- Kodama H. 1981. Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer. Rev Sci Instrum, 52(11): 1770–73.
- Melchels FP, Feijen J, Grijpma DW. 2009. A poly(D,L-lactide) resin for the preparation of tissue engineering scaffolds by stereolithography. Biomaterials, 30(23-24):3801-9.
- Shelby A. Skoog, Peter L. Goering, Roger J. Narayan. 2014. Stereolithography in tissue engineering. J Mater Sci Mater Med, 25: 845–56.
- Alhnan A, Okwuosa M, Chijioke T, et al. 2016. Emergence of 3D Printed Dosage Forms: Opportunities and Challenges. Pharm Res, 33 (8): 1817-32.
- Goyanes A, Buanz AB, Basit AW, et al. 2014. Fused-filament 3D printing (3DP) for fabrication of tablets. Int J Pharm. 2014;476(1–2):88–92.
- Wang J, Shaw LL, Cameron TB. 2006. Solid Freeform Fabrication of Permanent Dental Restorations via Slurry Micro-Extrusion. J Am Ceram Soc, 89 (1): 346–9.
- Khaled SA, Burley JC, Alexander MR, et al. 2014. Desktop 3D printing of controlled release pharmaceutical bilayer tablets. Int J Pharm, 461(1–2):105–11.
- Azad MA, Olawuni D, Kimbell G, et al. 2020. Polymers for Extrusion-Based 3D Printing of Pharmaceuticals: A Holistic Materials-Process Perspective. Pharmaceutics, 12(2):124.
- Chua CK, Leong KF, An J. 2014. 1 – Introduction to rapid prototyping of biomaterials. Woodhead Publishing.
- Sun Y, Soh S. 2015. Printing Tablets with Fully Customizable Release Profiles for Personalized Medicine. Adv Mater, 27(47):7847-53.
- Edelmann A, Dubis M, Hellmann R. 2020. Selective Laser Melting of Patient Individualized Osteosynthesis Plates-Digital to Physical Process Chain. Materials (Basel), 13(24):5786.
- Tan W, Gao C, Feng P, et al. 2021. Dual-functional scaffolds of poly(L-lactic acid)/nanohydroxyapatite encapsulated with metformin: Simultaneous enhancement of bone repair and bone tumor inhibition. Mater Sci Eng C Mater Biol Appl, 120:111592.
- Hiller J, Lipson H. 2009. STL 2.0: A Proposal for a Universal Multi-Material Additive Manufacturing File Format”. Solid Freeform Fabrication Symposium (SFF’09), Aug 3-5 2009, Austin, TX, USA.
- Kodama H. 1981. Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer. Rev Sci Instrum, 52(11): 1770–73.
- Melchels FP, Feijen J, Grijpma DW. 2009. A poly(D,L-lactide) resin for the preparation of tissue engineering scaffolds by stereolithography. Biomaterials, 30(23-24):3801-9.
- Shelby A. Skoog, Peter L. Goering, Roger J. Narayan. 2014. Stereolithography in tissue engineering. J Mater Sci Mater Med, 25: 845–56.
- Alhnan A, Okwuosa M, Chijioke T, et al. 2016. Emergence of 3D Printed Dosage Forms: Opportunities and Challenges. Pharm Res, 33 (8): 1817-32.
- Goyanes A, Buanz AB, Basit AW, et al. 2014. Fused-filament 3D printing (3DP) for fabrication of tablets. Int J Pharm. 2014;476(1–2):88–92.
- Wang J, Shaw LL, Cameron TB. 2006. Solid Freeform Fabrication of Permanent Dental Restorations via Slurry Micro-Extrusion. J Am Ceram Soc, 89 (1): 346–9.
- Khaled SA, Burley JC, Alexander MR, et al. 2014. Desktop 3D printing of controlled release pharmaceutical bilayer tablets. Int J Pharm, 461(1–2):105–11.
- Azad MA, Olawuni D, Kimbell G, et al. 2020. Polymers for Extrusion-Based 3D Printing of Pharmaceuticals: A Holistic Materials-Process Perspective. Pharmaceutics, 12(2):124.
- Chua CK, Leong KF, An J. 2014. 1 – Introduction to rapid prototyping of biomaterials. Woodhead Publishing.
- Sun Y, Soh S. 2015. Printing Tablets with Fully Customizable Release Profiles for Personalized Medicine. Adv Mater, 27(47):7847-53.
- Edelmann A, Dubis M, Hellmann R. 2020. Selective Laser Melting of Patient Individualized Osteosynthesis Plates-Digital to Physical Process Chain. Materials (Basel), 13(24):5786.
- Tan W, Gao C, Feng P, et al. 2021. Dual-functional scaffolds of poly(L-lactic acid)/nanohydroxyapatite encapsulated with metformin: Simultaneous enhancement of bone repair and bone tumor inhibition. Mater Sci Eng C Mater Biol Appl, 120:111592.
- Hiller J, Lipson H. 2009. STL 2.0: A Proposal for a Universal Multi-Material Additive Manufacturing File Format”. Solid Freeform Fabrication Symposium (SFF’09), Aug 3-5 2009, Austin, TX, USA.
The Application of 3D Printing and ICT for Green Products and Processes
ADVANCED L E V E L
3D bioprinting is an evolving technology with applications throughout the complete range of biomedical research.
Contents
Application areas of 3D bioprinting
3D bioprinting is an evolving technology with applications throughout the complete range of biomedical research. The method employing a biological substrate or ‘bioink’, made of structural components, growth media, and cells, can produce on 3D printing equipment biological structures that closely imitate the structure of cells in living organisms. Using the same manufacturing model as traditional 3D printing, a number of 2D slices are divided from a 3D model by a computer to print a correct biological composition layer-by-layer with a precise arrangement of tissues and cells. 3D bioprinting approach gives an opportunity to solve issues in drug development, regenerative medicine, organ donation, and cancer treatment. This emerging technology offers great accuracy on spatial placement of cells, proteins, DNA, drug particles, growth factors and biologically active substances to guide better tissue formation. This powerful technology seems to be more promising for improving tissue fabrication toward physiologically relevant tissue constructs, tissue models, tissues and organs and organs-on-a-chip models for medicine and pharmaceutics.
Bioprinting technology has a wide application in various areas such as tissue engineering and regenerative medicine, transplantation and clinics, drug screening and high-throughput assays and cancer research (Fig. 1). Bioprinting for tissue engineering and regenerative medicine has been studied for more than a decade, and anatomically correct cell-laden constructs and scaffolds have been produced for various tissue types: from connective and epithelial tissues to muscle and nervous tissues. With its great advantage in modelling and precisely placing multiple cell types, bioprinting has overcome one of the major shortcomings of traditional scaffold fabrication techniques and has facilitated the construction of native-like tissues with a heterocellular microenvironment. Although most of the effort has been targeted toward the fundamental science behind major bioprinting techniques such as laser-, droplet- and extrusion-based bioprinting, considerable attention has recently been given to bioprinting for functional tissue production. Particularly, considerable work has been devoted to bioprinting for transplantation. As additional progress has been made in biomaterials, cell, and transplantation technologies, bioprinting will translate from bench to bedside when authorized for human use and has a multitude of advantages in operating rooms in the near future. Before transitioning into clinical practice, bioprinting has already made a great jump in pharmaceutical use because it does not require any regulatory approvals and there is currently an emerging bioprinting market for tissue fabrication for drug testing and high-throughput assays. Bioprinted tissue models (e.g. liver), fabricated from multiple cell types and facilitating a complex heterocellular physiological environment, have been used in drug screening. Moreover, bioprinting has recently been applied in cancer research to investigate cancer pathology, growth, and metastasis in a physiologically relevant microenvironment. Here, a comprehensive review is presented of the application areas of bioprinting technology and in-depth discussion is presented on successfully bioprinted tissue types for tissue engineering and regenerative medicine, transplantation and clinics, drug screening, and high-throughput assays, and cancer research. For each application area, the advantages and limitations of existing technologies are discussed.
Figure 1. Application areas of bioprinting technology.
Tissue engineering and regenerative medicine
Bioprinting of functional organs at clinically level remains elusive because there are several challenges such integration of the vascular network from arteries and veins down to capillaries, combination of various cell types to recapitulate complex organ biology and limited structural and mechanical integrity and long-term functionality. Despite these challenges, a large variety of tissues have been successfully bioprinted such as thin or hollow tissues, for example blood vessel, or tissues that do not need vascularization such as cartilage.
Bone tissue
Bone is an essential tissue of the body and a key component of the musculoskeletal system. Generally, it overcomes insult and degeneration through its highly regenerative capabilities. However, in some cases bone fails to properly heal and treatment is required. The existing standard of care therapies has significant weaknesses. Innovations in bioprinting and tissue engineering hold much promise for improving both the quality and efficacy of bone reconstructive therapies. Bioprinting, theoretically, does not yet occur because custom resorbable scaffolds are not currently constructed with growth factors or autologous cells. However, the latest investigations have been rapidly improving the 3D printing techniques as well as 3D printable materials with properties finely tuned to reconstruct patient-specific defects. This research seems to be leading toward a future of resorbable scaffolds that incorporate appropriate growth factors and osteoprogenitor cells. Time will reveal whether these therapies will become widely available and standard-of-care.
In a recent study, some authors used a thermal inkjet bioprinter to build poly(ethylene glycol) dimethacrylate (PEGDMA) scaffolds. Nanoparticles of bioactive glass and hydroxyapatite (HA) were used for co-printing of bone-marrow-derived human mesenchymal stem cells (hMSCs) under simultaneous polymerization. This approach enabled uniform distribution of hMSCs compared with manually pipetted hMSCs, which accumulated at the bottom of the scaffold because of gravity. The bioprinted structures encapsulating hMSCs and HA exhibited the highest cell viability, collagen production and alkaline phosphate activity with increased compressive modulus after 21-day culture in vitro. Bioprinting HA particles were also carried out for in situ bioprinting purposes, where a laser-based bioprinting system was employed to deposit HA nanoparticles into mouse calvaria defects.
In another study, heterocellular tissue constructs made of Matrigel™ and alginate hydrogels were bioprinted. Endothelial progenitor and multipotent stromal cells were bioprinted in a spatially controlled manner. The obtained constructs were subcutaneously implanted into immune-deficient mice. By integrating osteoinductive biphasic calcium phosphate microparticles, multipotent stromal cells were differentiated into an osteogenic lineage and accelerated bone formation in 6 weeks. In addition to osteoinductive materials, addition of growth factors is also crucial in stem cell differentiation in bone tissue engineering. It was demonstrated that addition of bone morphogenetic protein (BMP)-2 has important role for the stem cell fate. Using inkjet bioprinting of patterned BMP-2 on fibrin-coated coverslips primary-muscle-derived stem cells were bioprinted. It was shown however, that they were differentiated toward osteogenic lineage on the pattern even if they were treated with myogenic differentiation conditions.
Cartilage tissue
Over the last two decades significant progress has been made not only in the field of bone tissue engineering but also in the cartilage one. Consequently, there is a true potential that techniques to regenerate rather than replace damaged or diseased bones and joints will one day reach the hospital. However, several key challenges must still be addressed before this becomes a reality. These mainly include vascularization when large bone defect repair is required. Other important issues are the necessity of usage of engineering complex gradients for bone‐soft tissue interface regeneration and recapitulating the stratified zonal architecture present in many adult tissues such as articular cartilage. However, existing cartilage engineering approaches have great potential for precise construction of cells and biomaterials with sophisticated patterns. They can mimic native tissues with zonally differentiated cells and extracellular matrix (ECM) structure.
Laser-based bioprinting of stem-cell-differentiated chondrocytes was also attempted. This strategy is based on a computer-aided biofabrication technique with the assistance of LIFT. Porcine-bone-marrow-derived mesenchymal stem cells (MSCs) with high viability were successfully bioprinted. They maintained their functionality and differentiation ability into osteogenic and chondrogenic lineages. Inkjet-based bioprinting has also been applied in cartilage tissue engineering as well as for cartilage defect regeneration. HP desktop printer was modified, and human chondrocytes loaded in PEGDMA hydrogel were bioprinted. Thus, fabricated cartilage construct had mechanical properties and biochemical composition close to native cartilage. Most recently a hybrid bioprinting method to fabricate mechanically improved cartilage tissue constructs by combining 3D bioprinting and electrospinning techniques was created. In that study, electrospinning of polycaprolactone (PCL) fibres together with inkjet bioprinting of rabbit elastic chondrocytes in fibrin–collagen hydrogel was shown.
Despite the great success in cartilage tissue bioprinting, fabricating of zonally stratified articular cartilage tissues with different structural, biomechanical and biological properties is still a challenge and further progress is required to produce articular cartilage tissue constructs with zonal differentiation including more horizontal and thinner collagen fibres with high cell density in the superficial zone, and relatively vertical and thicker collagen fibres with less cell density in the deeper zones.
Cardiac tissue
Cardiovascular diseases are the primary cause of death in the world. In this respect the cardiac and heart valve tissue engineering is from utmost importance. In the past decade a significant progress has been made in studying the native properties of the heart for cardiac regeneration and repair using a variety of strategies, including concepts of biomaterials, micro- and nanotechnologies, and decellularization. However, main difficulty in cardiac tissue engineering is incorporating the complex, heterogeneous micro- and macro-cardiac tissue structures for different cell types to adhere, proliferate, and ultimately repair and remodel damaged parts. 3D bioprinting has appeared as a strategy to address this challenge by providing the technology to construct high-resolution scaffolds with heterogeneous biochemical and mechanical characteristics. Though, limited attempts have been made in bioprinting of cardiac tissue models. It has been demonstrated extrusion-based bioprinting of tissue spheroids comprising human vascular endothelial cells (HUVECs) and cardiac cells obtained from myocardial tubes of chicken embryos. Other approach has been based on bioprinting a cardiac tissue construct in a half-heart shape with connected ventricles using inkjet-based bioprinting. Laser-induced forward-transfer (LIFT) has also been utilized to pattern HUVECs and hMSCs in a geometrically defined pattern on polyester urethane urea and the produced samples were transplanted to the infarcted zone of rat hearts after LAD ligation. Eight weeks post-transplantation, it has been observed increased vessel formation compared with randomly bioprinted cells as control groups and the resulted myocardium patch provided considerable functional improvement.
Another important direction is the engineering of heart valves because they do not possess regeneration capacity and dysfunctional heart valves, if the damage or disease is detrimental, need to be replaced by mechanical or biological prosthetic equivalents. However, the fabrication of such replacement valves is restricted by thrombogenicity and calcification. Despite its important role in the cardiovascular system, only a limited amount of work has been performed in the bioprinting of heart valves. The first-time bioprinting of a heart valve has been achieved via using a dual-head bioprinter modified from a Fab@Home printer. In another study, dual-nozzle bioprinting of composite alginate–gelatin hydrogel was used for the construction of thin hydrogel discs and spatially bioprinting of aortic root sinus smooth muscle cells (SMCs) and aortic valve interstitial cells (VICs). In a recent study, the same group performed bioprinting of composite hydrogels using methacrylated hyaluronic acid (Me-HA) and methacrylated gelatin (Me-Gel) loaded with human aortic valvular interstitial cells (HAVICs). Obtained samples were designed in tri-leaflet valve-shape and the cells bioprinted with increased Me-Gel concentration exhibited better spreading.
Liver tissue
The liver is the largest gland in the human body. It plays a key role in metabolism, bile production, detoxification, and water and electrolyte regulation. The toxins or drugs that the gastrointestinal system absorbs reach the liver first before entering the bloodstream. Liver disease is other leading causes of death worldwide. Hence, an in vitro liver tissue model that simulates the main functions of the liver can be an important platform for investigating liver diseases and developing new drugs. Currently, the newly developing 3D bioprinting technologies have the ability to assemble in vitro liver tissue models both in static scaffolds and dynamic liver‐on‐chip manners.
The first-time bioprinting of human-induced pluripotent stem cells (hiPSCs) was performed through stimulating hiPSCs and differentiating them into hepatocytes for liver micro-organ engineering. Additionally, to bioprinting liver tissue structures, liver carcinoma HepG2 immortal cells were bioprinted in larger tissue models.
In fact, 3D-bioprinting liver tissue scaffolds and constructing liver-on-chips can significantly help for the development of in vitro liver tissue models. By employing 3D bioprinting technology in liver-on-a-chip fabrication, the construction of in vitro liver tissue models will become more efficient and cost-effective. Thus, a powerful platform will be provided for screening drugs and determining the molecular mechanism underlying liver diseases.
Lung tissue
3D bioprinting increases the possibility of reproducibly creating complex macro- and microscale biostructures using multiple different cell types. This is promising strategy for fabrication of multilayered hollow organs, which is a big challenge for the more traditional tissue engineering techniques. Currently, the main progress for the pulmonary system has been limited to the trachea. The available techniques for tracheal graft construction are valuable foundations to whole lung bioprinting. Generally, most 3D bioprinting approaches for tracheal production combine biological hydrogels seeded with cells surrounding a sturdy polymer structure.
Generally, the lung is classified surgically as a solid organ. However, for the purposes of tissue engineering it is more useful to consider the entire respiratory tree as a branching set of tubes. Thus, the progress and difficulties detected in bioprinting of other tubular organs can help in bioprinting of the lung. The attempts for lung bioprinting are new and there are only a few attempts exploring the synthesis of a lung tissue model. It was recently achieved bioprinting of an in vitro air–blood barrier model through the development of a zonally stratified tissue construct in a few layers. First, a thin layer was bioprinted. It serves as a basement membrane. Then a single layer of endothelial cells was bioprinted facilitating the attachment of cells on the next layer. Later, a new layer was added on top of the previously assembled construct followed by bioprinting of a single layer of epithelial cells.
Working toward a more complex geometry, some authors has developed a distal lung model with vascular and airway spaces. They used poly(ethylene glycol) diacrylate and a stereolithographic printer and manage to create a “breathing model” with tidal air ventilation and blood flow. Applying this model, they investigated the pulmonary transport by measuring blood oxygenation entering and leaving the construct. Though the printed vasculature was limited to 300 µm, it is still a large step towards creating clinically relevant lung tissues.
Neural tissue
The human nervous system is extremely complex physiological network that is intrinsically challenging to analyse because of difficulties to obtain primary samples. One of the powerful alternatives to study nervous system development, diseases, and regenerative processes, are the animal models. However, they are unable to address some species-specific characteristics of the human nervous system. More and more sophisticated in vitro models of the human nervous system have been developed, but still require further advances to better simulate microenvironmental and cellular features. The field of neural tissue engineering is rapidly implementing new tools that enable researchers to precisely monitor in vitro culture conditions and to better model nervous system formation, function, and repair.
However, the work done in the framework of bioprinting for neural tissue construction is quite scarce. Some of them has been based on the bioprinting of C17.2 cells on a collagen layer next to a fibrin disk loaded with VEGF. Using this model, the effect of vascular endothelial growth factor (VEGF) on the proliferation and migration of murine neural stem cells (C17.2) has been analyzed. The study demonstrated that neural cells migrated toward VEGF-releasing fibrin gel and proliferated successfully. Recently, other investigations presented bioprinting of a thermoresponsive polyurethane hydrogel with tunable stiffness and gelation ability at 37°C without the requirement for a crosslinker. The effectiveness of the bioink was achieved by loading it with neural stem cells and injecting it into a zebrafish embryo neural injury model. Obtained results revealed that the injected gel rescued the function of the impaired nervous system in 6 days.
A scaffold-free approach was also used, where a pellet of Schwann cells (SCs) and bone marrow stem cells (BMSCs) were extruded within a 3D-printed agarose. After cells aggregation in the agarose a nerve tissue graft with three lumina in each was formed. The constructed grafts were then implanted into mice, and their histology and functionality were assessed. The number of samples were limited for drawing a definitive conclusion about the performance of the 3D bioprinted grafts with respect to commercially available collagen ones. However, the presented case demonstrated a proof-of-concept for bioprinting nerve grafts.
Pancreas tissue
Studying and treating pancreatic diseases, such as diabetes and pancreatic cancer, is hindered by the restricted access to pancreatic tissue in vivo. However, three-dimensional (3D) tissue models, which accurately imitate the native organ, have great potential in biomedical applications, by both providing effective platforms for investigating tissue development and homeostasis and for simulating diseases in pharmaceutical testing. Unfortunately, primary pancreatic β-cells do not easily survive in vitro and only a very few attempts have taken place to differentiate β-cells from human stem cells. In this regard the regeneration of pancreas tissue is primarily exemplified to the extent that β-cells from mouse lines or insulinoma cells have been used to produce pancreatic islets. A limited amount of work has been done in the context of bioprinting for pancreatic tissue fabrication. Recently, human and mouse islets as well as rat insulinoma INS1E β-cells were encapsulated within alginate or alginate–gelatin hydrogels and bioprinted them in dual-layer scaffolds. In addition, recent work demonstrated the microfabrication of scaffold-free tissue strands (with strong expression of insulin) for extrusion-based bioprinting, where tissue strands were made of rat fibroblasts and mouse insulinoma TC-3 β-cells in the core and shell, respectively. An innovative approach was proposed comprising the use of tissue strands for scale-up the tissue bioprinting process.
This experimental attempts in the advanced technological field of bioprinting will pave the way towards new technological opportunities of growing functional tissues and organs in a laboratory. It was expected that the knowledge and models generated by the research in the field of bioprinting will not only stimulate major technological developments in tissue-engineering, but will also open radically new possibilities in medicine, such as studying pancreatic diseases ex vivo in fabricated tissue, developing new drugs, and, possibly facilitating the replacement of injured or diseased tissue.
Skin tissue
Skin is the outermost protecting cover of human body and is in direct contact with the external environment. That makes it highly vulnerable to injury. Skin defects or wounds may result from trauma, skin diseases, burn or removal of skin during surgery. Such conditions require urgent therapeutic interventions to repair the structure and function of the skin and allow the normal mobility of the patient. Furthermore, existence of wounds can lead to the bacterial contamination and related complications if not treated immediately. Moreover, even minor deformities can create psychological distress on the affected individuals, especially to children. Currently, the best alternative of skin tissue engineering is the use of autografts. However, limitations exist including the amount and size of available grafts, creation of a secondary wound and other risks. Employing other types of skin grafts such as allografts and possibly xenografts are associated with the risks of immune reactions and transmission of diseases besides some ethical and cultural issues. There is also a possibility to use wound dressing materials such as those based on polymers or their combinations with other substances. However, they are not living and hardly can be cellularized and replaced by native tissue. In this regard, tissue engineering holds great promises for improving the treatment of skin defects, but solutions should be found for the challenges such as development of multi-layered native skin architecture and vascular networks. Some solutions has already been proposed where besides biomaterial, living cells, biological or chemical signals are being used with the purpose of forming functional skin. However, such conventional tissue engineering methods suffer from several problems like non-homogeneous distribution of cells, failure to integrate and vascularize the construct upon its implantation, subsequent rejection of the implanted biomaterial along with formed skin. Although, some tissue engineered skin products are in market, many limiting factors are encountered with a major and critical one for clinical success being the vascularization through the skin substitute.
A variety of tissue engineering approaches have been used in skin tissue construction and different tissue substitutes have been created including autologous split-thickness skin graft (gold standard), allografts, acellular dermal substitutes and cellularized graft-like commercial products. Bioprinting of skin tissue has been presented by using an eight-channel valve-based bioprinter, where a 13-layer-tissue construct was bioprinted employing collagen hydrogel. Keratinocytes were bioprinted on top of alternating layers of human foreskin fibroblasts and acellular collagen layers. The resulted constructs exhibited densely packed cells in epidermis layers as opposed to the dermis with low density of cells and less ECM deposition. In addition to droplet-based bioprinting, laser-based bioprinting has also been applied for bioproduction of skin tissue substitutes. In a recent study the effect of bioprinting endothelial cells within skin substitutes on formation of macrovasculature during new tissue remodeling was analysed. The approach included encapsulation of neonatal human dermal fibroblasts and epidermal keratinocytes (NHEKs) in collagen. Next the dermis layer is laid down followed by patterning human dermal microvascular endothelial cells (HMVECs) on the dermis layer of the construct. The process was completed by wrapping the fibrin layer with collagen-laden NHEKs. Then, the produced skin substitutes were implanted on the dorsa of mice and compared with commercially available skin substitutes as a control. Obtained results showed that bioprinted HMVECs formed microvessels and the implanted constructs generated contraction compared with the control groups. Moreover, in other studies it was demonstrated in situ bioprinting of stratified skin substitutes by alternating layers of fibrinogen–collagen and thrombin loaded with amniotic-fluid-derived stem (AFS) cells. With this approach, skin substitutes were 3D bioprinted directly onto full thickness wounds on pigs and repaired the native skin more closely than control groups. Despite the efforts in skin tissue bioprinting, bioconstruction of skin substitutes that virtually simulate native skin is still a challenge because integrating sweat glands and hair follicles has remained elusive.
Vascular tissue
Three-dimensional (3D) printing technology has rapidly become from a futuristic technology to a promising science which is believed to revolutionize clinical practice by making available customized equipment, organ models for surgical practices, organ replacement parts, and modern drugs. Though originally this technology was intended to provide surgical models and prototypes, scientists have rapidly advanced the technology to produce tissue constructs for regenerative medicine. However, large-scale implementation has still been constrained due to the anatomical complexity, compositional (cellular and extracellular) diversity, and vascularization of native tissues.
Vascular tissue synthesis has been performed by different bioprinting approaches including extrusion, droplet and laser-based bioprinting. In extrusion-based bioprinting, a broad spectrum of extrusion techniques has been utilized. Some scientists use coaxial-nozzle extrusion, where hydrogels including sodium alginate and chitosan were bioprinted directly in tubular form with encapsulated cells. During co-axial (core-shell) flow, ejected crosslinker (flowing through the core) make a contact with the precursor hydrogel solution (flowing through the shell), and accelerated the gelation and formation of tubular constructs. That approach facilitates direct bioprinting of vascular structures in a practical manner. There are also other direct vascular tissue bioprinting strategies, such as bioprinting droplets of cell-laden hydrogels layer-by-layer using inkjet-based bioprinting. Using bottom-up construction, inkjet-based bioprinting allowed building of branched tubes in horizontal and vertical directions. A related approach was also carried out using laser-based bioprinting. In all the above-mentioned studies, scaffold-based techniques are utilized. However, some investigations are based on a scaffold-free approach in bioprinting vascular tissues, where tissue spheroids were fabricated one by one and self-assembled into larger tissue units.
Though the sphere of 3D bioprinting has brought tremendous advances in producing vessel structures with sound structural and functional integrity, it can still be observed that constructing clinically relevant vascular and vascularized tissue constructs are still far in the future. Innovative inventions enhancing the resolution and obtaining more anatomically accurate constructs while allowing scaling up of bioprinted tissues are required to provide potential solutions. Progressively, it may be expected that independent research on materials and cellular bioink materials for vascularization may cede place to more integrated and holistic approaches for developing bioprinters for vascularized tissue synthesis.
Other tissue types
Additionally, to the described tissue types, some scientists are also working in bioprinting of retinal and brain tissues, although these studies are still at their early fundamental level. A piezoelectric inkjet bioprinting was applied using retinal ganglion cells (RGCs) and glia and the effect of bioprinting parameters on the viability of cells and their growth-promoting properties were investigated. Recently, manual deposition of primary cortical neuron-laden gellan-gum-RGD for brain-like tissue fabrication was also presented. Although no computer control motion system was employed, the presented work demonstrated for the first time the use of layer-by-layer approach for brain tissue engineering.
Transplantation and clinics
Regenerative medicine is probably one of the most thrilling future applications of 3D bioprinting. Scientists hope that bioprinting can be used to directly print tissues such as skin, and to create organs, tissues, and structures for transplantation. 3D bioprinting is believed to be successfully applied for the transplantation and production of multilayered tissue types like nerve, cardiac, blood vessel, bone, and skin (Fig. 2). Researchers anticipate that in the future tissues and eventually whole organs could be transplanted to patients produced from their own cells. With the help of 3D bioprinting, transplant rejection and long donor lists could become an issue of the past. In addition, numerous tissue constructs were bioprinted and implanted subcutaneously to evaluate in vivo differentiation of cells and functionality of implants. Despite these experiments, none of the bioprinted tissues has been clinically used for humans because no approval has been granted yet. There are no regulations laid down for bioprinters or bioprinted products. However, with the growing interest worldwide and emerging businesses in the bioprinting market, the success with the first technology going through regulations will be exemplary for preceding technologies and products.
Although bioprinting technology is still in its early stages in clinics, 3D-printed plastic, ceramic or metallic implants for bone tissue replacement have been already successfully transplanted into humans. In addition to permanent implants, a recent work published in New England Journal of Medicine presented a unique case of transplantation of a 3D-printed bioresorbable airway splint into an infant. No unexpected problems have been observed with the splint and full resorption of the device is anticipated to take around 3 years. This was an exemplary case for clinical use of 3D-printed scaffolds and optimistically will lead to similar success with bioprinted tissues and organs. Despite the achievements in bioprinting research, bioprinting for transplantation in a hospital requires further improvements and translational efforts. Organs and tissues that do not need significant vascularization (i.e. skin and cartilage) are expected to be clinically translated very soon. However, tissues and organs that are metabolically highly active (i.e. heart, pancreas, and liver) are extremely challenging.
Figure 2. 3D printing strategy
One of the arguments for the clinical translation potential of bioprinted tissues and organs originates from the bioprinting technology itself. Bioprinting involves living cells in bioink and the use of patient-specific cells is relatively new in bioprinting. Stem cells, such as embryonic stem cells and induced-pluripotent stem cells, have been prospective unlimited sources of patient-specific cells for production of tissues and organs. Patient-specific cells can be differentiated and then bioprinted or bioprinted and then differentiated toward multiple lineages to construct tissues and organs that will cause minimum immunogenicity risk.
Pharmaceutics and high-throughput screening
Another key area where 3D bioprinting is expected to succeed is in the drug development. Most of the drug testing is currently carried out by using 2D cell cultures. Like cancer research, these cultures do not simulate natural cell behaviour. Xenografting (human cell transplantation) to animals is now employed to help solve this issue. However, this approach is challenging due to the biochemical, metabolic, and genetic differences between the species. It has been suggested that these issues are the main reason for the high rate of failure in assessment of the new drug candidates. Despite continuing efforts to improve the drug development process, only one out of 10,000 new chemical substances and one out of ten drug candidates enters clinical trials, reaches the final approval stage, and enters the market. On the other hand, the use of 3D bioprinting for pharmaceutical testing rather than in vivo studies does not need regulatory approval. Thus, the throughput of drug development will possibly be extensively multiplied. Among different methods for engineering 3D in vitro systems, bioprinting has serious advantages such as controllability on size and microarchitecture, high-throughput capacity, co-culture opportunities, and low risk of cross-contamination. That is why the bioprinted tissues and organs have been increasingly considered as potential models to be used in drug toxicology and high-throughput screening.
Liver and tumour tissue have been a focus in construction of tissue models for pharmaceutics. In this respect the fabrication of a bioprinted liver micro-organ model for drug metabolism has been investigated (Fig. 3). The bioprinted 3D liver micro-organ has been used for simulation of the physiological liver response to drug administrations and toxic chemical exposure. The drug metabolism was demonstrated by the transformation of the nonfluorescent pro-drug, 7-ethoxy-4-trifluoromethyl coumarin (EFC), to an effluent fluorescent metabolite 7-hydroxy-4-trifluoromethyl coumarin (HFC). Epithelial cells and hepatocytes encapsulated in Matrigel™ were utilized to show the path of drug diffusing from the bloodstream to the tissue. Epithelial cells are the cells covering the lumen through which the drugs pass from bloodstream to the target, where hepatocytes were used as the target cell type. Based on the presented tissue model, a computational macroscale model was developed using a convection-diffusion-cell kinetics numerical framework. Latest is proven to be very helpful for future research in 3D micro-organ pharmacokinetics and toxicity.
Figure 3. Bioprinted tissue models for drug testing and high-throughput screening. (a) Bioprinted liver tissue. (b) bioprinted kidney tissue (c) bioprinted skin tissue (d) 3D bioprinted bone repair material (e) printed cell-containing hydrogel (f) multicellular 3D bioprinted cardiac tissue constructs.
Evolving microengineering technologies allow adaptable production of 3D cell-based microarrays including soft lithography, surface patterning, microfluidic-based manipulation and bioprinting. Among them, bioprinting technology has the advantage of creating co-culture and single-cell array with precise control on cell density. A recent study utilized a novel inkjet-based bioprinting method for fabricating a high-throughput miniature drug-screening platform. The authors bioprinted Escherichia coli-laden alginate to array a chip on coverslips. Using layer-by-layer approach droplets of three antibiotics were printed on the spots of cells. Results showed similar cell viability, functionality and antibacterial effects of antibiotics in inkjet bioprinted and micropipetted samples. Thus, the inkjet-bioprinted high-throughput arrays is proven to be effective method to minimize the duration of typical drug screening tests. Furthermore, cell-based biosensors (CBBs) were presented, where acoustic-based bioprinting was used to create a microarray of smooth muscle cell droplets in collagen. The bioprinted microarray was then subjected to different environmental conditions (e.g. temperature) and the effect of the applied stimuli as well as the bioprinting process parameters on cell viability was assessed. In addition to 3D bioprinting of high-throughput cell microarrays, controlled delivery of drug candidates into cell microarrays is also an important factor for effective drug screening. Different methods have been proposed for controlled drug delivery onto cell microarrays, including drug patterning, drug stamping, aerosol sprays and microfluidic drug loading.
Cancer research
The immense future possibilities of 3D bioprinting technology in biomedical field has already been well documented. One of the areas where scientists are hoping 3D bioprinting to prove beneficial is the cancer research. The tumour microenvironment, as well as the biological structures and substances surrounding the tumour is an essential elements of cancer behaviour and is not yet fully understood. Up to now animal models and 2D cell cultures have been employed in these investigations. However, they do not precisely demonstrate the behaviour of human cancer cells in vivo, which includes a complicated interaction between tumour cells with both the cellular environment and other cells. Latest research suggest that 3D models more precisely reflect tumour cell behaviour and bioprinting is a promising technique that could be effective in such studies. One advantage that 3D bioprinting possesses is that cells can be explicitly designed into a 3D model with its extracellular and structural elements. Alternative approaches differ in that they require self-assembly, which is much more time-consuming, or scaffolding, where the support matrix is developed independently from the cell culture. Furthermore, 3D bioprinting provides both the opportunity to print particular tissue and cell structures and also to be specific to the patient. The cells of a patient can be biopsied, grown, and bioprinted, appropriate structures to be fabricated, and drug response or effective dose to be investigated.
However, the use of bioprinting approach in cancer research is new and only a few studies have been performed in this emerging application area. For the first time, the bioprinting of tumor tissue was demonstrated with human ovarian cancer (OVCAR-5) cells and MRC-5 fibroblasts. In this study, an inkjet-based bioprinting platform with dual-ejectors was used. The proposed approach did not only show a tool for cancer research but also offered a great platform for high-throughput screening. The same researchers performed bioprinting of HeLa cells to form cervical tumor models. Although these two studies presented bioproduction of tumor spheroids using bioprinting technology, the construction of larger tissue models to examine cancer cell migration and metastasis is also crucial. Another approach was concentrated on the use of a laser-based 3D projection printing system to bioprint HeLa cells and noncancerous 10 T1/2 fibroblasts in PEGDA along with a microvascular network with channel widths of 25, 45, and 120 μm to simulate blood vessel diameters. In addition to scaffold-based approaches, scaffold-free bioprinting of a breast cancer model using the NovoGen Bioprinting™ platform was also performed. In this attempt cancer cells were surrounded by a physiologically relevant stromal medium comprising MSC-differentiated adipose cells, mammary fibroblasts and endothelial cells.
Concluding remarks and future outlook
Recent investigations have demonstrated that 3D bioprinting is an emerging approach having broad application in various areas including basic research in tissue engineering, regenerative medicine and cancer pathogenesis, tissue bioprinting for transplantation and clinics, and could be successfully applied in pharmaceutics for drug testing and high-throughput screening. Currently, there are around 15 different tissue types used in different experimental studies. However, there still exist several other human tissues that have not been touched yet. In this regard, future attempts must be made in bioprinting of other tissue and organ models that are challenging but can revolutionize medicine. This also depends on the innovations in the field of tissue engineering as bioprinting research vitally depends on our understanding of unexplored tissue types. In addition, one of the new directions in bioprinting should be focused on biofabrication of new types of organs, such as bionic organs or organs that do not exist in nature.
The majority of 3D bioprinting research has developed around homocellular tissue models using a single cell type. However, native tissues have a heterocellular nature with multiple cell types organised in highly complex anatomy. Although simple bioprinting tissue models are relatively acceptable in fundamental research, fabrication of functional tissue for clinical or pharmaceutical studies requires inclusion of multiple cell types. Thus, some of the functionality of cells can be enabled or further boosted by cell–cell interactions. Therefore, bioprinting multiple cell types need further investigations of the optimum culture conditions including the right medium and reagents to support their growth and behaviour.
Contemporary attempts in translation of bioprinting to transplantation are mainly limited to murine models. However, murine models are small with physiological conditions that significantly differ from those of humans. For example, a small pancreatic tissue with 3D bioprinted islets can be implanted into a diabetic murine model and can regulate insulin secretion. On the other hand, the average volume of islets in a mouse pancreas is approximately 100,000 times smaller than those in human pancreas. Therefore, bioprinting of scale-up tissues and organs is extremely important to construct them at clinically relevant dimensions. Larger animals such as a porcine model can be a transitional step toward trials on humans because these animals correspond to human physiology more closely than small animals. To scale-up bioprinting organs for clinical use, the fabrication of hierarchical vascular networks is crucial in addition to their mechanical strength, elasticity, and long-term structural stability. Additionally, integration of nerve tissues and forming innervation is a critical step toward functional tissue and organ production because tissues such as cardiac, muscle, and skin require innervation, which is currently a challenge in bioprinting. The majority of bioprinting research involves the use of hydrogel-based bioinks as they are a favorable environment for the 3D growth of cells. However, hydrogels have weak mechanical properties when they are used at low concentrations to support cell proliferation. In addition, biodegradable materials such as thermoplastics can be utilized as a background frame for the scale-up of tissues and organs. Available biomaterials, such as synthetic polymers, are strong, but their degradation is difficult, time-consuming, and does not match with the tissue regeneration process closely. Therefore, new biomaterial development is vital for the progression in bioprinting of tissues and organs for clinical use. Apart from the biomaterials, selection of the right cell source is also a critical factor or successful clinical and pharmaceutical use. For personalized medicine it is challenging to obtain different primary cell types. Stem cells represent a promising source of cells and further investigations are needed to establish standard protocols for differentiation of stem cells into various stable and functional cell lineages.
Another prominent way of tissue fabrication for high-throughput screening are the miniature 3D-array platforms such as tissue-on-a-chip or organ-on-a-chip models. Blend of bioprinting technology with other technologies such as microfluidics is essential for producing physiologically simulated, complex and long-term viable target micro-organ arrays. Application of technologies generating picoliter droplets with low level of cell damage and having single cell sorting capabilities should be encouraged in bioprinting of cell microarrays. Controlled drug delivery onto cell microarrays should be also considered and implemented.
Although several attempts are made in engineering cancer microenvironment using bioprinting technology, further advancements are needed for construction of a physiologically relevant complete microenvironment, comprising tumour site, healthy site, and microvascular network in between, to perform cancer metastasis research. Tumour tissue models should be precisely placed within bioprinted vascularized parenchymal tissues to study their growth, tumour cell intravasation and extravasation.
Test: LO2 Advanced Level
References
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- Harris M. 2018. Print me an organ. https://physicsworld.com/a/print-me-an-organ/
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- Ozbolat IT, Peng W, Ozbolat V. 2016. Application areas of 3D bioprinting. Drug Discovery Today. doi:10.1016/j.drudis.2016.04.006
- Park JY, Jang J, Kang HW. 2018. 3D Bioprinting and its application to organ-on-a-chip. Microelectron. Eng.. doi:10.1016/j.mee.2018.08.004
- Wang C, et al. 2014. Three-dimensional in vitro cancer models: a short review. Biofabrication, doi: 10.1088/1758-5082/6/2/022001.
- Yao R, et al. 2016. Three-dimensional printing: review of application in medicine and hepatic surgery. Cancer Biol Med
- Grigoryan B, Paulsen S, Corbett D, et al. 2019. Functional intravascular topologies and multivascular networks within biocompatible hydrogels. Science, 464: 458-464.
- Augustine R. 2018. Skin bioprinting: a novel approach for creating artificial skin from synthetic and natural building blocks. Prog Biomater., 7: 77–92.
- Datta P, Ayanb B, Ozbolat IT. 2017. Bioprinting for vascular and vascularized tissue biofabrication. Acta Biomaterialia, 51: 1-20.
- AZoMaterials. 3D bioprinting and its applications (2018). https://www.azom.com/article.aspx?ArticleID=17370
- Bhise NS et al. 2014. Organ-on-a-chip platforms for studying drug delivery systems. Journal of Controlled Release. doi:10.1016/j.jconrel.2014.05.004
- Harris M. 2018. Print me an organ. https://physicsworld.com/a/print-me-an-organ/
- Liang M, Yutong W, Li Y, et al. 2020. Current Advances on 3D-Bioprinted Liver Tissue Models. Adv. Healthcare Mat., doi: 10.1002/adhm.202001517
- Murphy SV, Atala A. 2014. 3D bioprinting of tissues and organs. Nat. Biotechnol., 32: 773–785.Varotto A. 2015. Global Market for 3D Printing., BCC Research, ISBN: 1-62296 -006-8
- Ozbolat IT, Peng W, Ozbolat V. 2016. Application areas of 3D bioprinting. Drug Discovery Today. doi:10.1016/j.drudis.2016.04.006
- Park JY, Jang J, Kang HW. 2018. 3D Bioprinting and its application to organ-on-a-chip. Microelectron. Eng.. doi:10.1016/j.mee.2018.08.004
- Wang C, et al. 2014. Three-dimensional in vitro cancer models: a short review. Biofabrication, doi: 10.1088/1758-5082/6/2/022001.
- Yao R, et al. 2016. Three-dimensional printing: review of application in medicine and hepatic surgery. Cancer Biol Med
- Grigoryan B, Paulsen S, Corbett D, et al. 2019. Functional intravascular topologies and multivascular networks within biocompatible hydrogels. Science, 464: 458-464.
- Augustine R. 2018. Skin bioprinting: a novel approach for creating artificial skin from synthetic and natural building blocks. Prog Biomater., 7: 77–92.
- Datta P, Ayanb B, Ozbolat IT. 2017. Bioprinting for vascular and vascularized tissue biofabrication. Acta Biomaterialia, 51: 1-20.