Associate Professor UC Davis Davis, California, United States
Introduction: One of the key thrusts in three-dimensional (3D) printing and direct writing is to seamlessly vary composition and functional properties in printed constructs. Most inks used for extrusion-based printing, however, are compositionally static and available approaches for dynamic tuning of ink composition remain few. Here, we present an approach to modulate extruded inks at the point of print, using droplet inclusions. Using a glass capillary microfluidic device as the printhead, we dispersed droplets in a polydimethylsiloxane (PDMS) continuous phase and subsequently 3D printed the resulting emulsion into a variety of structures. The mechanical characteristics of the 3D-printed constructs can be tuned in situ by varying the spatial distribution of droplets, including aqueous and liquid metal droplets. In particular, we report the use of poly(ethylene glycol) diacrylate (PEGDA) aqueous droplets for local PDMS chemistry alteration resulting in significant softening (85% reduced elastic modulus) of the 3D-printed constructs. Furthermore, we imparted magnetic functionality in PDMS by dispersing ferrofluid droplets and rationally designed and printed a rudimentary magnetically responsive soft robotic actuator as a functional demonstration of our droplet-based strategy. Our approach represents a continuing trend of adapting microfluidic technology and principles for developing the next generation of additive manufacturing technology.
Materials and
Methods: A generic glass capillary microfluidic device made from borosilicate glass capillary tubes was used as the droplet microfluidic printhead. The injection capillary consisted of a round 1-mm diameter glass capillary that is pulled using a glass pulling device (MicroData Instruments Inc.) to form a tapered tip. The tip was subsequently polished using fine sandpaper (grit > 1500) to a final diameter of around 20 μm under an optical microscope. The collection capillary consisted of a round 1-mm diameter glass capillary whose length was limited to 0.75” to limit pressure build-up in the device. Both capillaries were aligned coaxially within a square glass capillary with 1.05 mm long sides and secured in place using Loctite clear epoxy glue. Blunt dispensing needles and polyethylene tubing were used to couple the glass capillary with a syringe pump (Harvard Technologies). PDMS and the aqueous dispersed phase were supplied into the microfluidic printhead using a syringe pump (Harvard Technologies). Once the fluid phases in the microfluidic printhead reach hydrodynamic equilibrium, the devices were visually inspected for successful emulsion generation. Next, the printhead was mounted on a 3D printer machine, moved along the x, y and z directions to deposit the emulsion in 3D space according to the print path defined by the .gcode file used. Upon successful printing of the emulsion inks, the printed constructs were then subjected to further processing that varied depending on the emulsion ink system.
Results, Conclusions, and Discussions: We present a versatile strategy for the modulation of printed construct properties across space using droplet inclusions. By coflowing immiscible inks in a microfluidic printhead, we could concurrently generate and extrude highly textured inks with well-organized droplet inclusions for use in extrusion-based 3D printing. The mechanical properties of the printed PDMS were found to depend on the composition and relative proportion of the inner phase, with PEGDA inclusions being especially effective at softening PDMS even at low doses via chemical modulation of the cross-linking network. We then used these insights with our printing setup to design, fabricate, and optimize the operation of soft robotic actuators, demonstrating the utility of our droplet-based strategy to impart both form and function into printed objects. Follow-up work could explore other interactions, physical or chemical, between the inner and outer phases at different length scales to generate materials with tailorable functional properties for use in soft robotics and other applications.
