(M-483) One-Step Foam Bioprinting for Tissue Engineering

Introduction:: 3D bioprinting technology is being widely accepted by medical professionals and researchers worldwide as a promising approach to enhance the quality of life for patients suffering from various diseases. The term "3D bioprinting" describes a process that involves the use of an automated dispensing system to print cells or other biological entities, encapsulated into a hydrogel, directly onto a substrate. The biomaterials used in this process are referred as "bioinks." Despite the potential of bioprinting, the growth and proliferation of encapsulated cells, within the bioprinted structures can be limited by their dense biomaterial networks that form because of the gelation of bioinks.

Our laboratory has developed hydrogel-based foam bioinks, using a method that involves introducing air into the hydrogels and mixing them. Yet in this method, the foam bioink needs to prepare separately and then transferred to the 3D printer for printing.

Herein, we have developed microfluidics nozzles that enable one-step foaming and bioprinting of bioinks. In this method the hydrogel-based bioink will be extruded and simultaneously incorporated and mixed with air, resulting in printing of porous filaments of the bioink material with controllable thickness and pore size. These nozzles are used in handheld foam 3D printers and can be mounted to stationary printers with minimal modification.

This 3D foam bioprinting method is a highly adaptable and effective approach for engineering tissue constructs with porous structures and has a wide range of potential applications in fields such as tissue engineering, regenerative medicine, drug development, and personalized therapeutics.


Materials and Methods:: Design and fabrication of the foam printing microfluidics nozzles: The nozzles are composed of multiple parts designed in CAD software (SolidWorks) and printed with an SLA 3D printer (Form3B) and then assembled. Nozzles are made in multiple configurations with different internal diameters, lengths, mixing layers, and air introduction steps that can be manipulated to control the porosity and pore size of the extruded foam.

Design and fabrication of the handheld foam 3D printer: The nozzles are mounted on a custom-designed handheld printer that can simultaneously extrude base hydrogel and airflow into the nozzles to 3D print foamed structures. The printer is equipped with a UV source for photo-crosslinking of the bioink upon extrusion. The performance of the device is tested with different printing conditions (air-to-hydrogel flow rate ratio).

Physical characterization of the printed foam: The printed foam is characterized in terms of its average pore size, pore size distribution and porosity for different printing conditions, nozzle configuration and hydrogel concentrations.

Mechanical characterization of the printed foam: Unconfined compression test is conducted to evaluate the mechanical performance of the foams produced with this method. This data is compared with those of the foams produced with other methods and with solid hydrogel.

Results, Conclusions, and Discussions:: Our results show that although increasing the airflow, for a given hydrogel rate, will result in smaller pore size, it will interrupt the continuous extrusion of the foam which leads to an incomplete 3D printing process. Thus, there is an optimum airflow for a given hydrogel extrusion rate.

Figure 1 shows the pore sizes of the printed foams with 1 ml/min hydrogel rate and 6 psi air pressure and different micro meshing layers. Interestingly, we found that with a given printing condition, there is an optimum number of micro mesh layers that yields the most homogenous foam. The micro meshing layers less than this optimized number will lead to a mixture of foam and hydrogel. On the other hand, too many numbers of micro meshing layers can have an adverse effect on homogenous foam formation since although it provides smaller baubles in the foam, it also blocks the movement of the generated foam and increases its internal pressure that leads to merging some bubbles into each other and so the formation of larger bubbles alongside the smaller ones.

Compression tests show that the elastic modulus of the printed foams is 5-10 kPa. Increasing the airflow in the system will result in more foamed regions and less solid hydrogel area in the structure and so a reduction in mechanical properties. Yet, these values are in an acceptable range for soft tissue engineering applications.

We have developed specific microfluidics nozzles and a handheld 3D printer that enable one-step foam bioprinting. The physical and mechanical properties of the foams generated with this method are evaluated and it is shown that this method can provide foams as good as previously developed methods with the advantage of generating foam in one-step thus bypassing the need for storing, preserving, and transferring the generated foam into a 3D printer for bioprinting. Further physical and mechanical characterization will be done to analyze the potential of this method.


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