A microfluidic printhead with integrated hybrid mixing by sequential injection for multimaterial 3D printing

Extrusion-based 3D printing is often performed using a single ink or switching between two or more inks. The ability to create compositional variation in-situ by rapid mixing of two different inks inside the printhead with dynamically changing ratios can produce a broad palette of material composition that is useful in making functionally graded materials for applications in soft robotics or integrated sensors. However, rapid mixing of high viscosity polymer inks in low Reynolds number flows has been a challenge. Here, we demonstrate a multimaterial microfluidic printhead mixer (MM-PHM) that is capable of mixing dynamically controllable ratios of different silicone inks to print graded materials. The MM-PHM uses a pulsatile injection of multiple inks, pneumatically inside a mixing junction, combined with a free-floating static mixer to produce various compositions from up to four different base inks. The performance of the MM-PHM when dissimilar inks are used has been assessed using both experimental and computational (fluids dynamics) methods. To illustrate the versatility of this method, a 2D colored gradient map, and 3D Rubix cube have been fabricated using four-color inks to produce nine different material combinations. This scalable method expands the palette of multimaterial 3D printing materials that can be fabricated and constructed in complex shapes and varying properties.