Using a microneedle array (MNA) format, the potential for continuous, real-time wearable intracutaneous monitoring of multiple molecules can be achieved via electrochemical aptamer-based (EAB) sensors. Here we demonstrate multiplexed electrochemical detection of endogenous and exogenous molecules using the smallest wireless EAB/MNA system reported to date. The small form factor required for wearability was accomplished using laser-micromachined polyether ether ketone (PEEK) to form a multielectrode MNA having two sets of working electrodes responsive to different targets. Multiplexed MNAs responded selectively to phenylalanine and vancomycin, two model targets. Benchtop and in vivo demonstration in rat were achieved towards realizing a translatable wearable intracutaneous electrochemical sensing system.

Adaptation of electrochemical aptamer-based (EAB) sensing to a microneedle format would enable clinically actionable, real-time molecular measurements via an easily applied, minimally-invasive, painless, wearable device. As a prerequisite, here we have explored what substrate materials meet the combined requirements of both microneedles and EAB sensor fabrication. Specifically, we evaluated 17 microneedle-compatible materials for adhesion with gold, the surface required for EAB functionalization. Those exhibiting satisfactory adhesion were functionalized with an aptamer sensitive to vancomycin, challenged with a range of target concentrations, and compared. Finally, we realized a microneedle sensing patch and demonstrated its function in solution and ex vivo.

Background: Extraneural cuffs are among the least invasive peripheral nerve interfaces as they remain outside the nerve. However, compared with more invasive interfaces, these electrodes may suffer from lower selectivity and sensitivity since the targeted nerve fibers are more distanced from the electrodes. New Method: A lyse-and-attract cuff electrode (LACE) was enabled by microfabrication and developed to improve selectivity and sensitivity while maintaining a cuff format. Its engineering design was described in previous work. LACE is a hybrid cuff that integrates both microelectrodes and microfluidic channels. The ultimate goal is to increase fascicular selectivity and sensitivity by focal delivery via the microchannels of (1) lysing agent to remove connective tissue separating electrodes from nerve fibers, and (2) neurotrophic factors to promote axonal sprouting of the exposed nerve fibers into microfluidic channels where electrodes are embedded. Here, we focus on demonstrating in vivo function of microfluidics and microelectrodes in an acute preparation in which we evaluate the ability to focally remove connective tissue and record and stimulate with microchannel-embedded microelectrodes neural activity in rat sciatic nerves. Comparison with existing methods: While extraneural interfaces prioritize nerve health and intraneural interfaces prioritize functionality, LACE represents a new extraneural approach which could potentially excel at both aims. Results: Surgical implantation demonstrate preservation of LACE function following careful and minimal handling. In vivo electrical evaluation demonstrates the ability of microelectrodes placed within microfluidic channels to successfully stimulate and record compound action potentials from rat sciatic nerve. Furthermore, collagen-rich epineurium was focally removed following infusion of collagenase via microchannels and confirmed via microscopy. Conclusion: The feasibility of using a cuff having integrated microelectrodes and microfluidics to stimulate, record, and deliver drug to focally lyse away the epineurium layer was demonstrated in acute experiments on rat sciatic nerve.

We present for the first time the design, fabrication, and preliminary bench-top characterization of a high-density, polymer-based penetrating microelectrode array, developed for chronic, large-scale recording in the cortices and hippocampi of behaving rats. We present two architectures for these targeted brain regions, both featuring 512 Pt recording electrodes patterned front-and-back on micromachined eight-shank arrays of thin-film Parylene C. These devices represent an order of magnitude improvement in both number and density of recording electrodes compared with prior work on polymer-based microelectrode arrays. We present enabling advances in polymer micro-machining related to lithographic resolution and a new method for back-side patterning of electrodes. In vitro electrochemical data verifies suitable electrode function and surface properties. Finally, we describe next steps toward the implementation of these arrays in chronic, large-scale recording studies in free-moving animal models. [2020-0109]

Informational density and relative accessibility of the peripheral nervous system make it an attractive site for therapeutic intervention. Electrode-based electrophysiological interfaces with peripheral nerves have been under development since the 1960s and, for several applications, have seen widespread clinical implementation. However, many applications require a combination of neural target resolution and stability which has thus far eluded existing peripheral nerve interfaces (PNIs). With the goal of aiding PNI designers in development of devices that meet the demands of next-generation applications, this review seeks to collect and present practical considerations and best practices which emerge from the literature, including both lessons learned during early PNI development and recent ideas. Fundamental and practical principles guiding PNI design are reviewed, followed by an updated and critical account of existing PNI designs and strategies. Finally, a brief survey of in vitro and in vivo PNI characterization methods is presented.

Parylene C is a promising material for constructing flexible, biocompatible and corrosion-resistant microelectromechanical systems (MEMS) devices. Historically, Parylene C has been employed as an encapsulation material for medical implants, such as stents and pacemakers, due to its strong barrier properties and biocompatibility. In the past few decades, the adaptation of planar microfabrication processes to thin film Parylene C has encouraged its use as an insulator, structural and substrate material for MEMS and other microelectronic devices. However, Parylene C presents unique challenges during microfabrication and during use with liquids, especially for flexible, thin film electronic devices. In particular, the flexibility and low thermal budget of Parylene C require modification of the fabrication techniques inherited from silicon MEMS, and poor adhesion at Parylene-Parylene and Parylene-metal interfaces causes device failure under prolonged use in wet environments. Here, we discuss in detail the promises and challenges inherent to Parylene C and present our experience in developing thin-film Parylene MEMS devices.

A novel Parylene C-based peripheral nerve interface that combines both electrodes and microfluidic channels in an adjustable cuff was designed, fabricated, and characterized. This minimally invasive interface incorporates a drug delivery system for targeted delivery of lysing agents and neurotrophic factors to the nerve surface to locally disrupt the epineurium and allow fascicular selectivity. Multiple platinum (Pt) electrodes were embedded in the microfluidic channels for neural stimulation and recording. The lyse-and-attract cuff electrode (LACE) uses a simple locking mechanism that is adjustable for close contact with nerves of varying diameters. Devices were fabricated using standard Parylene microfabrication techniques resulting in low variability and high device yield. A procedure for the implantation of the LACE was developed and successfully demonstrated in vivo around rat sciatic nerves. The adjustable locking mechanism demonstrated adequate holding strength and fit around the nerves. Benchtop electrochemical characterization of the thin-film Pt electrodes showed that the electrodes possessed high charge storage capacity (>1 mC/cm2) and low impedances ( $< 2~\text{k}\Omega $ at 1 kHz) suitable for neural stimulation and recording. As expected, embedded electrodes demonstrated higher impedance values. Acute neural recording from the rat sciatic nerve verified the capability of the LACE to record evoked neural activity (compound action potentials). Controlled and localized microfluidic infusions were achieved at low flow rates ( $< 1~\mu \text{L}$ /min). Finally, infusion experiments in vivo demonstrated targeted drug delivery to the sciatic nerve fascicles. This multifunctional peripheral nerve interface has the potential to enhance implant-tissue integration in vivo and provide reliable chronic performance not available in the existing extraneural or intraneural interfaces. [2018-0192]

We present a novel polymer-based cuff electrode designed for peripheral nerve recording and drug delivery. The micromachined cuff interface consists of platinum electrodes embedded within Parylene microfluidics. An adjustable locking mechanism ensures close-fitting of the cuff around the nerve for localized drug delivery to induce axonal sprouting from the fascicles towards the embedded electrodes. This Lyse-and-Attract Cuff Electrode (LACE) specifically incorporates drug delivery to improve sensitivity, spatial resolution, fascicle selectivity, and chronic reliability over existing electrode-only interfaces. We present the design, fabrication, and initial testing of fully functional cuffs including evaluation of the locking mechanism, localized drug delivery, and electrochemical properties of recording electrodes.

Laser pulses that act on fragile samples often alter them irreversibly, motivating single-pulse data collection. Amorphous solid water (ASW) is a good example. In addition, neither well-defined paths for molecules to travel through ASW nor sufficiently small samples to enable molecular dynamics modeling have been achieved. Combining nanoimprint lithography and photoinitiation overcomes these obstacles. An array of gold nanoparticles absorbs pulsed (10 ns) 532 nm radiation and converts it to heat, and doped ASW films grown at about 100 K are ejected from atop the irradiated nanoparticles into vacuum. The nanoparticles are spaced from one another by sufficient distance that each acts independently. Thus, a temporal profile of ejected material is the sum of about 106 "nanoexperiments," yielding high single-pulse signal-to-noise ratios. The size of a single nanoparticle and its immediate surroundings is sufficiently small to enable modeling and simulation at the atomistic (molecular) level, which has not been feasible previously. An application to a chemical system is presented in which H/D scrambling is used to infer the presence of protons in films composed of D2O and H2O (each containing a small amount of HDO contaminant) upon which a small amount of NO2 has been deposited. The pulsed laser heating of the nanoparticles promotes NO2/N2O4 hydrolysis to nitric acid, whose protons enhance H/D scrambling dramatically.

Molecular transport and morphological change were examined in films of amorphous solid water (ASW). A buried N2O4 layer absorbs pulsed 266 nm radiation, creating heated fluid. Temperature and pressure gradients facilitate the formation of fissures through which fluid travels to (ultrahigh) vacuum. Film thickness up to 2400 monolayers was examined. In all cases, transport to vacuum could be achieved with a single pulse. Material that entered vacuum was detected using a time-of-flight mass spectrometer that recorded spectra every 10 mu s. An ASW layer insulated the N2O4 layer from the high-thermal-conductivity MgO substrate; this was verified experimentally and with heat-transfer calculations. Laser-heated fluid strips water from fissure walls throughout its trip to vacuum. Experiments with alternate H2O and D2O layers reveal efficient isotope scrambling, consistent with water reaching vacuum via this mechanism. It is likely that ejected water undergoes collisions just above the film surface due to the high density of material that reaches the surface via fissures, as evidenced by complex temporal profiles extending past 1 ms. Little material enters vacuum after cessation of the 10 ns pulse because cold ASW near the film surface freezes material that is no longer being heated. A proposed model is in accord with the data.

Molecular transport, morphological change, and heat transfer in thin films of amorphous solid water (ASW) were investigated. The experimental strategy was centered on the introduction of a buried layer of material (N2O4) that efficiently absorbs ultraviolet radiation and converts the absorbed energy to heat. Specifically, a single 10-ns, 266-nm pulse of 1-2 mJ in a diameter of ~ 0.3 mm turns the N2O4 layer into a hot fluid that heats surrounding material. Resulting temperature and pressure gradients create "vertical" fissures through which material passes from the heated layer to vacuum (UHV). Film thickness up to 2400 ML was examined. In all cases, it was straightforward to eject material from the N2O4 reservoir with just a single laser pulse. The fissures were robust, enabling material transport to vacuum to proceed for many pulses. Material that entered the gas phase was detected using a time-of-flight mass spectrometer that recorded complete spectra every 10 μs. An ASW spacer layer between the MgO substrate and the N2O4 layer provided insulation between the heated layer and the high-thermal-conductivity 100 K MgO substrate. This was verified both experimentally and with heat transfer simulations. Fluid composed of N2O4, its photoproducts, and water was heated throughout its trip to vacuum by the 10 ns pulse. This fluid removed water from the fissure walls, as demonstrated using alternate layers of H2O and D2O. Virtually all of the water that entered vacuum did so via this mechanism. Little material enters the gas phase after cessation of the 10 ns pulse because of rapid cooling by the ASW. This ensures that material passing through the fissures to vacuum does so with a speed of at least 100 m / s. Gaseous water undergoes collisions just above the film surface, where high fissure density and the fluid nature of the material passing through the fissures ensures a high density of ejected material. This system has many features in common with the water jets observed on Saturn's moon Enceladus. In both cases, warm fluid is driven through fissures, material is expelled vigorously, and the majority of the released material is not found in jets. Rather, it appears to ooze from the surface. It is believed that liquid water moves through the fissures in Enceladus. In our experiments, the N2O4 reservoir and the fluid that passes through fissures can be heated in a controlled manner. The use of photons to heat material rising through fissures provides a means of exploring, and perhaps controlling, its movement, particularly how it interacts with the icy walls it on the way out. The qualitative model presented herein can provide insight into astrophysical and astrochemical mechanism. † The first three authors contributed equally. The order of names was chosen randomly. ‡ Corresponding authors: reisler@usc.edu; wittig@usc.edu