Adrienne T. Higa

Unidirectional mechanical cellular stimuli via micropost array gradients

Diverse cellular processes are influenced by the mechanical properties of the substrate. Here we introduce the methodology of constructing micropost array gradients to investigate the effects of unidirectional substrate stiffness cues on living cells. Experimental results revealed preferential cell migration in the direction of increasing micropost stiffness.

Fabrication and Characterization of a Balloon Actuator Array for Haptic Feedback in Robotic Surgery

Robot-assisted surgery is characterized by a total loss of haptic feedback, requiring surgeons to rely solely on visual cues. A pneumatically-driven balloon actuator array, suitable for mounting on robotic surgical master controls, has been developed to provide haptic feedback to surgeons. The actuator arrays consist of a molded polydimethylsiloxane substrate with cylindrical channels and a spin-coated silicone film that forms the array of balloons. Preliminary human perceptual studies have demonstrated that balloon diameters greater than 1.0mm may provide effective haptic feedback to the index finger. Before conducting further human perceptual tests, refinements of the fabrication process and performance data of the actuator are required. Balloons with diameters ranging between 1.5mm and 4.0mm were fabricated with film thicknesses of 200μm and 300μm. Inflation pressure versus balloon deflection tests and cyclic actuation tests were performed to characterize each balloon type. The results demonstrated a high linearity between inflation pressure and balloon deflection (R2>0.93) and negligible hysteresis effects between inflation and deflation over 100,000cycles. The studies indicated that 300μm films are optimal for 3.0mm and 4.0mm diameter balloons, and 200μm films are optimal for 1.5mm, 2.0mm, and 2.5mm diameter balloons. Due to its compact size and high performance, the described pneumatic actuator can provide sensory input that is otherwise unavailable during robotic surgery.

Finger-powered microdroplet generator

‘Human-powered’ microdroplet generators are ideal for droplet-based point-of-care diagnostics applications. Here we present a versatile ‘finger-powered’ microdroplet generator. The prototype system was fabricated via polymer-based micromachining processes. In this work, we have achieved: (1) the use of a human finger as the actuation force for droplet generation, (2) an integrated pumping system for actuating both droplet and solvent fluids simultaneously, and (3) the formation of microdroplets via a T-junction microchannel. During experimental device runs, both the formation of water droplets in oil and oil droplets in water were accomplished using a human finger, resulting in an average droplet size of 120 µm.

Unidirectional cellular durotaxis via microfabricated posts of varying anisotropy

This paper presents unprecedented accomplishments toward unidirectional guidance of cellular migration via durotaxis-based microtopography. In contrast to previous efforts, micropost arrays of varying anisotropy (μPVAs) optimize unidirectional control of cell migration through dual axis durotaxis cues which restrict movement in the lateral direction in addition to promoting migration in the direction of increasing micropost stiffness. Preliminary results show 79% of bovine aortic endothelial cells (BAECs) cultured on μPVA substrates migrated within ±60° of the direction of increasing micropost anisotropy. µPVAs offer a simple, yet powerful technique for enabling unidirectional control of cellular migration for a variety of applications in tissue engineering, biomaterials, and medical device implantation.

Mechanical regulation of collective cell behavior via microtopographic substrates

This paper demonstrates the feasibility in utilizing microtopographic substrates to maintain or inhibit collective cell behavior via micromachined posts and thin-film stencils. This technique enables groups, or collectives, of cells to be localized and directly cultured onto microposts for studying effects of substrate stiffness on collective cell behavior. Preliminary results show that bovine aortic endothelial cells (BAECs) collectively contract on soft substrates and disperse into single cells on stiff substrates after 16-hour studies. The micropost mechanical stiffness required for collective-to-single-cell transitions is characterized as 65 ± 41 nN/µm. As such, this work provides an essential foundation for growing and controlling collective cells under in vitro environments for interrogating a wide range of growth and pathological biological phenomena.

Cell-to-electrode contact structures for power density enhancements in microbial fuel cells

More than 200% power density enhancement in miniaturized microbial fuel cells (MFCs) has been successfully demonstrated by modifying the contact electrode structure using micro and nano processes. Two fundamental issues are addressed in this work: (1) a methodology to enhance power density of MFCs by changing micro and nano structural configurations of contact electrodes, (2) a study on the effectiveness of charge transfer between living cells with organic nanowire-pili and micro/nano interfacial electrodes. As such, this work represents a step forward toward higher energy conversion efficiency as well as practical application of MFCs.

Real-time biocatalyst loading and electron transfer via microfabricated transparent electrode

Real-time and simultaneous measurements of bacterial catalyst loading and electron transfer have been accomplished via a microfabricated transparent electrode within the setup of a micro- microbial fuel cell (μMFC). By characterizing the electron transfer on a per cell basis, we aim to generate a methodology for species-to-species comparison and formulate a baseline for electrode optimizations. This is critical and fundamental knowledge to be explored within the scope of MFCs as power devices with direct implications to molecular cell biology. Preliminary results are presented showing that, using Geobacter sulfurreducens, a signal to noise ratio (SNR) of 11 can be achieved upon inoculation and single cells can be non-invasively and accurately identified using intrinsic fluorescence.