Candice Tsay

An elastically stretchable TFT circuit

An elastically stretchable, skin-like transistor circuit is demonstrated. The circuit is built on an elastomeric substrate patterned with stretchable gold film interconnects. Inverters were made by surface-mounting flexible amorphous-silicon thin-film transistors on the elastomeric substrate. The electrical performance of the inverters was evaluated prior to, during, and after uni-axial stretching by up to 12%. Performance in the relaxed state before and after stretching was identical. Small changes in circuit performance were seen in the stretched state. This first elastic transistor circuit demonstrates the functionality of stretchable interconnects, and the feasibility of fabricating active circuits on electronic skin.

Turbulence measurements using a nanoscale thermal anemometry probe

A nanoscale thermal anemometry probe (NSTAP) has been developed to measure velocity fluctuations at ultra-small scales. The sensing element is a free-standing platinum nanoscale wire, 100 nm × 2 µm × 60 µm, suspended between two currentcarrying contacts and the sensor is an order of magnitude smaller than presently available commercial hot wires. The probe is constructed using standard semiconductor and MEMS manufacturing methods, which enables many probes to be manufactured simultaneously. Measurements were performed in grid-generated turbulence and compared to conventional hot-wire probes with a range of sensor lengths. The results demonstrate that the NSTAP behaves similarly to conventional hot-wire probes but with better spatial resolution and faster temporal response. The results are used to investigate spatial filtering effects, including the impact of spatial filtering on the probability density of velocity and velocity increment statistics.

Solution-processed chalcogenide glass for integrated single-mode mid-infrared waveguides.

Chalcogenide glass materials exhibit a variety of optical properties that make them desirable for near- and mid-infrared communications and sensing applications. However, processing limitations for these photorefractive materials have made the direct integration of waveguides with sources or detectors challenging. Here we demonstrate the viability of two complementary soft lithography methods for patterning and integrating chalcogenide glass waveguides from solution. One method, micro-molding in capillaries (MIMIC), is shown to fabricate multi-mode As(2)S(3) waveguides which are directly integrated with quantum cascade lasers (QCLs). In a second method, we demonstrate the ability of micro-transfer molding (µTM), to produce arrays of single mode rib waveguides (2.5 µm wide and 4.5 µm high) over areas larger than 6 cm(2) while maintaining edge roughness below 5.1 nm. These methods form a suite of processes that can be applied to chalcogenide solutions to create a diverse array of mid-IR optical and photonic structures ranging from <5 to 10s of µm in dimension.

Monitoring hippocampus electrical activity in vitro on an elastically deformable microelectrode array.

Interfacing electronics and recording electrophysiological activity in mechanically active biological tissues is challenging. This challenge extends to recording neural function of brain tissue in the setting of traumatic brain injury (TBI), which is caused by rapid (within hundreds of milliseconds) and large (greater than 5% strain) brain deformation. Interfacing electrodes must be biocompatible on multiple levels and should deform with the tissue to prevent additional mechanical damage. We describe an elastically stretchable microelectrode array (SMEA) that is capable of undergoing large, biaxial, 2-D stretch while remaining functional. The new SMEA consists of elastically stretchable thin metal films on a silicone membrane. It can stimulate and detect electrical activity from cultured brain tissue (hippocampal slices), before, during, and after large biaxial deformation. We have incorporated the SMEA into a well-characterized in vitro TBI research platform, which reproduces the biomechanics of TBI by stretching the SMEA and the adherent brain slice culture. Mechanical injury parameters, such as strain and strain rate, can be precisely controlled to generate specific levels of damage. The SMEA allowed for quantification of neuronal function both before and after injury, without breaking culture sterility or repositioning the electrodes for the injury event, thus enabling serial and long-term measurements. We report tests of the SMEA and an initial application to study the effect of mechanical stimuli on neuron function, which could be employed as a high-content, drug-screening platform for TBI.

Mid-infrared characterization of solution-processed As2S3 chalcogenide glass waveguides.

An etch-free and cost-effective deposition and patterning method to fabricate mid-infrared chalcogenide glass waveguides for chemical sensing applications is introduced. As(2)S(3) raised strip optical waveguides are produced by casting a liquid solution of As(2)S(3) glass in capillary channel molds formed by soft lithography. Mid-IR transmission is characterized by coupling the output of a quantum cascade (QC) laser (lambda = 4.8 microm) into the 40 microm wide by 10 microm thick multi-mode waveguides. Loss as low as 4.5 dB/cm is achieved using suitable substrate materials and post-processing. Optical absorption and surface roughness measurements indicate that the solution-processed films are of sufficient quality for optical devices and are promising for further development of waveguide-based mid-IR elements.

Chalcogenide glass waveguides integrated with quantum cascade lasers for on-chip mid-IR photonic circuits

We demonstrate on-chip hybrid integration of chalcogenide glass waveguides and quantum cascade lasers (QCLs). Integration is achieved using an additive solution-casting and molding method to directly form As(2)S(3) strip waveguides on an existing QCL chip. Integrated As(2)S(3) strip waveguides constructed in this manner display strong optical confinement and guiding around 90° bends, with a NA of 0.24 and bend loss of 12.9dB at a 1mm radius (λ=4.8μm).

Low-loss chalcogenide waveguides on lithium niobate for the mid-infrared

We demonstrate low-loss chalcogenide (As(2)S(3)) waveguides on a LiNbO(3) substrate for the mid-IR wavelength (4.8 μm). Designed for single-mode propagation, they are fabricated through photolithography and dry-etching technology and characterized on a mid-IR measurement setup with a quantum cascade laser. For straight waveguides, propagation loss as low as 0.33 dB/cm is measured and low-loss bends on the order of 100 μm are simulated, with measurement results showing <3 dB for a 250 μm bend radius. The coupling efficiency is estimated to be 81%. In addition, the influences of variations in width and bend radius are also investigated.

Stretchable micro-electrode arrays for dynamic neuronal recording of in vitro mechanically injured brain

Traumatic brain injury (TBI) is the result of sudden external mechanical forces applied to the head. Compression, stretching and shear deform the brain tissue inducing cellular damage that may develop within a couple of days following the initial trauma. TBI can be modeled in vitro by rapidly stretching brain slice cultures grown on silicone membranes. Stretchable micro-electrode arrays (SMEAs) prepared on such elastomeric substrates will enable the electrical recording of brain neuronal activity prior to, during and after mechanical deformation, promising new insights in mechano-biology and possible therapeutic interventions. SMEAs of 2times2 thin gold micro-electrode arrays have been patterned on 250mum thick silicone membranes. We report their fabrication process and electro-mechanical characteristics. The first SMEA prototypes are biocompatible with brain organotypic hippocampal cultures and robust to rapid equi-biaxial stretching at a 2,000% per second strain rate to 20% strain

Architecture, Fabrication, and Properties of Stretchable Micro-Electrode Arrays

Essential components of a stretchable microelectrode array (SMEA) to record from biological tissue include: a compliant and elastic substrate, stretchable conductors forming active electrodes and traces, and an electrical insulation layer. The materials and architecture of these SMEA components must be biocompatible, resistant to electrolytic environments, and electrically functional during and after mechanical stretching. While rigid MEA systems exist, many applications, such as retinal implants and sensitive skin, need soft, conformable, and stretchable electronic devices. This work focuses on the fabrication process, electromechanical characterization, and biocompatibility of stretchable micro-electrodes on silicone membranes. We deposit and pattern thin gold films on elastomeric silicone substrates, encapsulate them with an insulating photopatternable silicone layer, and show that the electrodes remain electrically conducting during stretch to >50% strain. The SMEA supports growths of organotypic brain slice cultures

Stretchable microelectrode arrays a tool for discovering mechanisms of functional deficits underlying traumatic brain injury and interfacing neurons with neuroprosthetics

Traumatic brain injury (TBI) can be caused by motor vehicle accidents, falls and firearms. TBI can result in major neurological dysfunction such as chronic seizures and memory disturbances. To discover mechanisms of functional deficits underlying TBI, we developed a stretchable microelectrode array (SMEA),which can be used for continuous recording of neuronal function, pre-, during, and post-stretch injury. TheSMEA was fabricated on a polydimethylsiloxane (PDMS)substrate with stretchable, 100 pm wide, 25 nm thick gold electrodes patterned there on [1]. The electrodes were encapsulated with a 10-20 microm thick, photo-patternable PDMS insulation layer. Previous biocompatibility tests showed no overt necrosis or cell death caused by the SMEAs after 2 weeks in culture [2]. The electrical performance of the SMEAs was tested in electrophysiological saline solution before, during and after biaxial stretching. The results showed that the electrode impedance increased with the strain to reach 800 kL at 8.5% strain and then recovered to 10 kil after relaxation. The working noise level remained below 20 pV pp during the whole process. New methodologiesf or improving the patterning of the encapsulation layer were tested on gold electrode arrays supported on glass. With these prototype arrays, robust population spikes were recorded from organotypic hippocampal slice cultures of brain tissue. Additionally, seizure-like activity induced with 1 mM bicuculline was also recorded. Our results demonstrate that the prototype arrays have good electrical performance compatible with existing multielectrode array systems. They also indicate the ability to record neuronal activity from hippocampal slices. This novel technology will enable new studies to understand injury mechanisms leading to post-traumatic neuronal dysfunction.