Application reportsExpertise on the subject of bioprinting
3D printing of a bioink vein succeeds confidently and with no fluid loss
Puredyne® printheads give the user the necessary flexibility in handling bioinks
Anyone researching bioink knows that developing a good printable ink is not easy and, above all, cost-intensive. It is a complex process to combine water and a biopolymer to form a hydrogel. Only after the gel has been optimized for printing, the mechanical stability of a printed object can be guaranteed.
If researchers for biotechnological applications or start-ups for bioprinters want to make their entire know-how available, a number of points must be taken into account when selecting the technology, especially for the essential process step of printing. On the one hand, the print heads must be optimally designed for the specific properties of a bioink so that print tests succeed with precision and without loss. Secondly, they must be efficient in order to ultimately make it from laboratory scale to series production. Puredyne® provides such a printing technology, which was recently used to print a blood vessel model. The technological challenge was to print the fine vein structures precisely with start and stop points. In the meantime, a variety of biological inks have become established.
For example, artificial models are 3D printed from living tissue, forming a scaffold to which living cells can attach and proliferate. Precise application via the Puredyne® printhead avoids material loss of the cost-intensive bioink, which benefits researchers and is often a customer requirement for equipment manufacturers. The design of the eccentric screw prevents deformation of the cells during printing, which ensures quality. The fact that Puredyne® printheads do not drip and, in addition, volumetric delivery prevents needle clogging makes research efficient and ensures resilient, reproducible results. Scaling up from laboratory scale to production is facilitated. Process reliability is also underlined by intuitive handling of the printhead and process and continuously consistent print quality – ideal for volume production. These facts make Puredyne® products a flexible solution for fast, volumetrically accurate and viscosity-independent printing in material development.
Bioprinting: The challenge of highly filled pastes
Puredyne ensures maximum precision in printing conductive pastes and inks
Although the printing of complete functional organs is still a dream of the future, research, and development of clinical bioprinter applications are making great strides. New technologies are needed to further improve the printing process of organic substances and to establish the three-dimensional construction of living cells layer by layer.
It can be assumed that the path from models in the laboratory to clinical studies and functional organs will also be significantly influenced by the precision of the dispensing technologies, as the materials used are very cost-intensive to develop and manufacture. Currently, bioprinter manufacturers have two options for extrusion-based systems. One is pneumatic systems that use air pressure to squeeze the biomaterial from a cartridge; the other is those that use piston extrusion. This range is extended by extrusion using the endless piston principle, a high-precision process. For example, ViscoTec has developed a print head specifically for bioprinting under the “Puredyne” brand, which is available under the trade name Puredyne® kit b.
In addition to classical biomaterials, the application also focuses on the precise and lossless dispensing of conductive pastes and inks, such as those used for the transmission of electrical signals between nerve cells in the laboratory. In addition, conductive inks can be used for the stimulation of muscle contractions. The idea is to expose cells to targeted electrical impulses. This stimulation can give rise to a functional muscle.
Dead space cost trap
A major challenge when using conductive pastes is to precisely introduce the electrically conductive paths into the system. To ensure that this also succeeds from an economic point of view, factors such as the dead space of the dispensing unit, the particle filling level of the substance to be dispensed and the precision of the dispensing technology must all be included in the decision-making process on the right dispensing technology. The smaller the dead space, the less waste remains in the dispensing component. If there is almost no dead space, as is the case with Puredyne, almost all of the raw material can be processed. Users can therefore make effective use of cost-intensive materials. In addition, it positively influences the process if the particles of the conductive pastes have no influence on the dispensing components and their exact functionality. Such particles almost always generate abrasion. On the one hand, the dispensing technology must not be impaired in its function by particle abrasion; on the other hand, the particles must also always guarantee the conductivity of the paste. In addition – and not only from an economic point of view – absolute precision is also required when applying the extruded line. The conductive track can only fulfill its function if this line does not fluctuate in width and has an exact start and stop point.
The Puredyne® printhead was developed to make the above factors controllable: The novel single-use design is virtually free of dead space and guarantees maximum material utilization.
The endless piston technology used also ensures superior dispensing of highly filled pastes. High-precision results can be achieved, and, thanks to the adjustable suck-back, precise start-stop points can be produced. The low-shear and pulsation-free dispensing results in a constant line width with a possible resolution below 200 µm. The following video illustrates the features of Puredyne technology and its potential.
3D Bioprinting with Alginate
Alginate is one of the most used materials in bioprinting. In addition to relatively low material costs, the mechanical properties and printability are also an advantage for the material. However, with existing extrusion systems such as pneumatic- or syringe extrusion, irregularities in the bioprinting of models can occur due to process variations. With the Puredyne® kit b, these problems no longer exist.
Alginate as a universal hydrogel
In a large number of studies, alginate is used as a base material for bio inks. Both pure (sodium) alginate in solution and chemically modified variants make up the majority of scientifically researched hydrogels. Oxidized alginate, for example, is thought to help cells gain freedom of movement more quickly through degrading alginate. In contrast, methacrylate alginate enables photopolymerization and thus other printing processes based on photon curing. The great popularity of alginate stems not only from its favorable mechanical and chemical properties. On a biological level, the hydrogel also offers the advantage of very good biocompatibility, making it universally applicable.
What mechanical problems arise in the additive manufacturing of alginate?
Several options are available for printing with alginate. In addition to light-based processes, which are possible with modified alginate, extrusion-based processes make up most of the biological 3D printing. Currently, the extrusion of bio-inks is done mainly in two ways.
- Pneumatic extrusion: Using air pressure, a plug is propelled forward inside a cartridge. The pressure ensures the formation of a material strand at the connected metering needle. The advantage of pneumatic extrusion is the simplicity of the system. However, controlled thread breakage is made difficult by the lack of material retraction. The compressed air can only be selectively switched on and off. Particularly with non-degassed material, dripping occurs which cannot be compensated. Another disadvantage that should not be underestimated is the dependence on external influencing factors. These include, for example, the ambient air pressure, the ambient temperature, and the filling level of the clamped cartridge, which influences the extrusion pressure. Alginate also tends to dry quickly. This can lead to clogging of the metering needle. If the pressure is set at a constant level, there is no longer any possibility of the needle clearing itself. The user must remove and clean the needle. This can lead to the entire print result becoming unusable.
- Syringe extrusion: Another popular extrusion system is spindle-driven extrusion with a syringe. Here, a syringe with material (for example, alginate) is clamped in a device that exerts pressure on the syringe’s extended piston. The motion is provided by a motor-coupled spindle. This system has the advantage that it can be considered volumetric. A defined rotation of the spindle is largely proportional to the amount of material extruded. Retraction can also be achieved by reverse rotation of the spindle. The precision is reduced by the material used in each case. If the biomaterial is compressible, dripping occurs quickly. This dripping depends on the amount of material still in the syringe, as different amounts of gas are compressed and expanded during start-stop movements. Frequent use of this operation can create a type of oscillatory behavior, reducing precision. Another negative aspect is the increased space requirement, since the filled syringe including the squeezing system must be placed upright in a printer.
Puredyne® kit b for printing alginate
In addition to the well-known extrusion systems, the Puredyne® kit b, based on progressive cavity pump technology, has recently become available for printing alginate. The volumetric process enables constant and process-reliable 3D printing while eliminating almost all disadvantages that abovementioned extruders have. This sets the print head apart from all previous systems. To test the performance, a print test was carried out with alginate 5 %.
- Test setup: A Puredyne® cap b5 is filled with CELLINK Alginate 5 % via the integrated Luer-Lock connector. With a twist, the cap is connected to the print head via the bayonet connection and the dispensing needle is mounted. Applied compressed air ensures the material supply to the extruder (compressed air itself has no extrusion function).
- Execution: The amount of material proportional to the speed and the model to be printed are set on the computer. Several strands of alginate are laid down in a line pattern. Special attention is paid to the start-stop points.
- Printing result: With the retraction set, the pattern can be printed perfectly without excess material at the start or end points. The result of the print test with alginate can be seen in the video. It shows perfect contours, fine lines, and clean start and end points.
Printing alginate reliably
The versatility of alginate is its greatest strength. Until now, however, it lacked an extrusion system that could support this versatility reliably and with high precision. The Puredyne® kit b can close this gap. Not only can alginate with a concentration of 5 % be optimally printed, but also other concentrations with a wide variety of materials. Viscosities are almost irrelevant when printing with the Puredyne® kit b! Perfect process control is the next big step in bioprinting!