3D Printing Multifunctional Optoelectronic and Microfluidic Devices

Abstract

Functional materials encompass different classes of materials possessing intrinsic or synthetic properties that are responsive to external stimuli. A few examples include semiconducting polymers/crystals, electroluminescent polymers, polymers with controlled cross-linking mechanisms and printable metallic inks with tunable sintering mechanisms and conductivity. The technology of additive manufacturing, or 3D printing, has been extensively investigated with structural plastics and metals to realize rapid prototyping of irregular/customized geometries, demonstrating a few successful examples of commercialization. Yet, a further systematic study is demanded to investigate the methodologies to incorporate multiple functional materials in the 3D printed multifunctional devices. This will lay important foundations for the fabrication of a range of devices under ambient conditions that were conventionally accessible exclusively to the cleanroom-based microfabrication. More importantly, the capability of 3D printing to integrate materials in a freeform manner will facilitate novel device form-factors and functionalities that are challenging to realize with microfabrication. In this work, the methodologies of 3D printing optoelectronic and microfluidic devices were investigated with an emphasis on material selection, device configuration, alignment, performance optimization and scalable fabrication. To this end, a custom-built 3D printing system was utilized to accurately pattern functional materials that possess varying rheological properties. Over the past several decades, 3D printing has demonstrated an array of electronic devices such as batteries, capacitors, sensors, wireless transmitters etc. This progress renders an expectation for fully 3D printed integrated circuits that can be rapidly prototyped and adopt more complicated spatial architectures. However, fully 3D printed optoelectronic devices are still a relatively unexplored paradigm. One major challenge of 3D printed optoelectronics is to optimize the device performance by controlling the thickness and uniformity of the solution-processed layers. An optimized layer thickness maintains the balance between charge injection and light extraction for light emitting diodes (LEDs) or light absorption and charge separation for photodetectors. Layer uniformity affects the contact between adjacent layers and therefore the charge carrier transport. In this work, electroluminescent semiconductors, including silicon nanocrystals (SiNCs) and conjugated polymers, were 3D printed as the active layers of LEDs and photodetectors. The effect of printed layer thickness on the device performance was investigated for the extrusion-based printing. A spray printing method was integrated in the 3D printing system and an improved device performance was observed. Significantly, for the 3D printed polymer photodetectors, an external quantum efficiency (EQE) of 25.3%, comparable to that of spin-coated devices, was achieved by controlling the concentration of the active ink. For the device integration, photodetector arrays were printed on flexible and spherical substrates for a freeform and wide field-of-view image sensing. Novel multifunctional optoelectronic devices consisting of integrated LEDs and photodetectors in a side-by-side layout was printed on the same platform, demonstrating potential applications of wearable physiological sensors. Next, for the 3D printed microfluidic devices, this work demonstrates that yield-stress fluids, such as viscoelastic gels, can be extruded to construct self-supporting hollow microstructures that are highly flexible and stretchable. Several additive manufacturing methods, such as stereolithography and multi-jet printing, have demonstrated 3D printed microfluidic devices with improved automation compared to the conventional soft lithography. However, it remains a challenge to directly incorporate electrical and biological sensing elements in the microfluidic devices. In this study, because of the yield strength of the viscoelastic ink, mechanical equilibrium states were found to exist for the inclined standing walls. Self-supporting microfluidic channels and chambers were 3D printed by stacking silicone filaments according to prescribed toolpaths. Since no sacrificial material was demanded to realize the hollow structures, the microfluidic structures can be directly aligned and printed onto microfabricated circuits without contaminating the electrodes. The high modeling precision of this method was demonstrated via fully 3D printed chemical species mixers that were embedded with herringbone ridges. In addition, automation components, including microfluidic valves and peristaltic pumps, were also 3D printed with overlapping silicone channels that were encapsulated by UV-curable resins. Most compellingly, microfluidic networks integrated with valves transcended the conventional planar form-factors and were directly printed on 3D surfaces. The 3D microfluidics suggests a potential application of microfluidics-based physiological sensors that can be directly printed onto freeform surfaces such as human bodies. Lastly, this work demonstrates that the above two distinct systems can be seamlessly integrated together via 3D printing, yielding fully encapsulated and flexible LED matrices. Liquid metals such as eutectic GaIn are promising candidates for soft and stretchable electronics. As the cathode material of 3D printed optoelectronic devices, it has the desired work function and a high mechanical compliance. However, current challenge of patterning liquid metals lies in the design of a robust encapsulation for the cathodes and simultaneously creating an effective interface with interconnects. To this end, self-supporting microfluidic networks that are highly adaptable and aligned to the layout of LED matrices were printed to encapsulate the liquid metal. The 3D printed liquid metal microfluidics enabled the scalable fabrication of flexible and individually addressable LED matrices. In summary, this research expanded the scope of ink composition for 3D printed multifunctional devices. Transferring these materials from microfabrication to 3D printing significantly improves the manufacturability of optoelectronic and microfluidic devices. The intrinsic capabilities of 3D printing to pattern 3D structures in a freeform manner facilitated novel functionalities for both types of devices, including spherical image sensors, 3D microfluidic networks, flexible organic LED matrices etc.