Open Access Publications from the University of California

## Microfluidic Circuitry via Additive Manufacturing

Abstract

Microfluidics, the science and engineering of fluid at small scales, affords numerous benefits for applications in chemistry and biology, including rapid reaction rates, reaction uniformity and precision, and reagent minimization but the technology remains limited by the availability of appropriate control mechanisms and related microfluidic components. Microfluidic devices have traditionally been fabricated using soft-lithography, which is time-consuming, costly, and reliant on extensive facilities. Over the past decade, research has shifted towards developing alternate methods such as additive manufacturing (widely known as three-dimensional (3D) printing) to fabricate microfluidic structures. This dissertation has developed three methods for resolving the fabrication and control problems of microfluidics using a combination of standard microfluidic/MEMS techniques with newer 3D printing techniques: optofluidic lithographic microfluidic circuitry, 3D-printed transfer molding, and fully-3D-printed fluidic circuitry.

The basic theory of microfluidic control systems, including the hydraulic analogy and fluidic circuitry, the low Reynolds number approximation to the Navier-Stokes equations, and neglectable terms in microfluidic models are first investigated. Optofluidic lithography, where photocurable liquids are selectively solidified under UV exposure, is utilized as a means of fabricating moving microfluidic structures within a microchannel. Several variants of microfluidic control mechanisms are demonstrated, including current sources (for regulation of fluid flow-rates to $29.2 \pm 0.8$ \si{\micro\liter/min} for pressures from $5-10$ kPa), and microfluidic gain valves (for controlling a high-pressure channel with a low-pressure one to achieve a dynamic gain of $6.30 \pm 0.23$).

A hybrid manufacturing process, where 3D printed polymer molds are used to make multi-layer PDMS constructions by employing unique molding, bonding, alignment, and rapid assembly processes. Specifically, a novel single-layer, two-sided molding method is developed to realize two channels levels, non-planar membranes/valves, vertical interconnects (vias) between channel levels, and built-in inlet/outlet ports for fast linkages to external fluidic systems. As a demonstration, a single-layer membrane micro-valve is constructed, dramatically reducing the number of fabrication steps. Additionally, multilayer structures are fabricated through an intra-layer bonding procedure which uses custom 3D printed stamps to selectively apply uncured liquid PDMS adhesive only to bonding interfaces without clogging fluidic channels. Using built-in alignment marks to accurately position both stamps and individual layers, this technique is demonstrated by rapidly assembling a six-layer microfluidic device.

By using the multi-jet-based additive manufacturing, fully-3D-printed microfluidic circuitry components are demonstrated, including fluidic resistors, capacitors, diodes (for a diodicity of $80.6 \pm 1.8$), and transistors (for a dynamic pressure gain of $3.01 \pm 0.78$) through the combination of experimental, simulation, and analytical methods, which are integrated complex microfluidic circuits, such as half- and full-wave microfluidic rectifiers. The hydraulic analogy circuitry exhibits similar transfer functions to its electronic counterparts, but ultimately behaves more similarly to vacuum triodes than to actual transistors. When combined with fully-realized microfluidic IF-gate and CMOS-analog components, this research may allow for fabrication of analog microfluidic operational-amplifiers, which could allow for fully-analog control of microfluidic systems using high-gain amplification of small pressure signals, and the CAD-based nature of the design process makes it simple for researchers to readily combine multiple microfluidic components into larger integrated microfluidic circuit networks.