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This paper reports the successful fabrication of softening polymer (SP)-based devices with inter-layer coatings for neural interface applications. SPs are flexible biocompatible polymers that soften upon implantation into the body, thus capable of reducing mechanical stress on the surrounding tissue and increasing the conformability of the implant. The softening process relies on water uptake into the polymer that can affect its electrical insulation capabilities. Employing an amorphous silicon carbide (a-SiC) thin film inter-layer between the metal traces and the SP is expected to provide a reliable water-barrier while allowing the SP plasticization once the device is implanted. We report the electrochemical properties and leakage current characteristics of various structures with a-SiC and polyimide inter-layer coatings under simulated physiological conditions. The fabricated structures demonstrated appropriate electrical properties and were found to be reliable up to a 2.5 mm radius of curvature when bending in compression.

Titanium (Ti) and its alloys are currently used as biomaterials for dental implants. However, titanium and its alloys suffer electrochemical corrosion in the oral environment. Ultrananocrystalline diamond (UNCD) coating is a promising coating material with exceptional biocompatibility, excellent osseointegration, high wear resistance and extreme resistance to chemical attack for producing a new generation of long-life dental implants. The main objective of this work was to investigate the basic electrochemical behavior of UNCD-coated samples of commercially pure titanium (Cp-Ti/Ti-2) and titanium alloy (Ti–6Al–4V/Ti-5) at three different pH conditions of artificial saliva. All titanium and titanium alloy samples of four groups were used: (a) control Ti-2, (b) UNCD-coated Ti-2, (c) control Ti-5 and (d) UNCD-coated Ti-5. For each group, electrochemical analysis was performed at pH 3, 6·5 and 9·0. In the electrochemical tests, the evolution of potential was higher in the UNCD-coated Ti-2 and UNCD-coated Ti-5 than in the uncoated samples. Potentiodynamic (E corr, I corr) and impedance measurements also showed improvement in the electrochemical behavior of both UNCD-coated titanium samples for all three pH conditions. These preliminary findings indicate that UNCD coatings could be considered for the next generation of the dental implants, to improve their performance.

In this work, we present three compositions of thiol-click polymers engineered to soften at various degrees upon implantation into the living body. Dynamic mechanical analysis has been used to assess the softening process and the storage modulus under simulated physiological conditions. In detail, we present polymers that are engineered to soften under physiological conditions within 10 to 20 minutes from 1.7-2.5 GPa to 1.7 GPa, 800 MPa, or 20 MPa, dependent on their composition. Critical material parameters such as the glass transition temperature and the cross-link density can be tuned by the choice of monomers and their relative proportions within the pre-polymer solution, as well as by the specific curing conditions used in the polymerization process. Because these polymers are used as substrate material for bioelectronic interfaces, we also present their electrical properties under simulated physiological conditions. For this, electrochemical impedance spectroscopy was used to assess dielectric constant and electrical resistivity. Typical relative permittivity of our compositions ranges from 4 to 6, while volume resistivity ranges from 10E10 to 10E13 Ohm*m, both measured at 37 ° C temperature in phosphate buffered saline solution. We discuss the results in the perspective of building functional electrical circuits on the presented polymer substrates.

In this work, we investigate the electrical performance and compatibility of low-temperature hafnium oxide (HfO2) thin-film capacitors on a novel softening polymer substrate. Metal-insulator-metal HfO2 capacitors were fabricated using HfO2 as the dielectric material, deposited at 100 °C by atomic layer deposition (ALD), and gold as the top and bottom contacts. The HfO2 capacitors were fabricated on silicon and on softening polymer substrates with a dielectric thickness of 50, 40, 30, and 20 nm. The electrical performance of the MIM capacitors was measured and compared to determine the quality and compatibility of the low-temperature HfO2 deposition with the silicon and polymer substrate. The dielectric constant varied from 12 to 17 as the HfO2 thickness increased from 20 to 50 nm. Moreover, the capacitance density and dielectric constant of the capacitors on the polymer substrate differed by 3.9% ± 2% and 3.4% ± 2% with respect to the silicon substrate. The polymer substrate devices also have a higher leakage current, which suggests a higher number of defects in the dielectric film relative to Si substrates. Finally, the devices on the polymer substrate were subjected to bending cycles (up to 105 cycles) with a 5 mm bending radius to evaluate the resilience of the HfO2 capacitors against mechanical stress. The results show that the fabrication of the HfO2 thin-film capacitors on the softening polymer substrate is achievable with high stability and mechanical resilience. Overall, this research could assist the production of flexible biomedical devices on softening polymer substrates.

Objective. We sought to develop a cervical spinal cord stimulator for the rat that is durable, stable, and does not perturb the underlying spinal cord. Approach. We created a softening spinal cord stimulation (SCS) array made from shape memory polymer (SMP)-based flexible electronics. We developed a new photolithographic process to pattern high surface area titanium nitride (TiN) electrodes onto gold (Au) interconnects. The thiol-ene acrylate polymers are stiff at room temperature and soften following implantation into the body. Durability was measured by the duration the devices produced effective stimulation and by accelerated aging in vitro. Stability was measured by the threshold to provoke an electromyogram (EMG) muscle response and by measuring impedance using electrochemical impedance spectroscopy (EIS). In addition, spinal cord modulation of motor cortex potentials was measured. The spinal column and implanted arrays were imaged with MRI ex vivo, and histology for astrogliosis and immune reaction was performed. Main results. For durability, the design of the arrays was modified over three generations to create an array that demonstrated activity up to 29 weeks. SCS arrays showed no significant degradation over a simulated 29 week period of accelerated aging. For stability, the threshold for provoking an EMG rose in the first few weeks and then remained stable out to 16 weeks; the impedance showed a similar rise early with stability thereafter. Spinal cord stimulation strongly enhanced motor cortex potentials throughout. Upon explantation, device performance returned to pre-implant levels, indicating that biotic rather than abiotic processes were the cause of changing metrics. MRI and histology showed that softening SCS produced less tissue deformation than Parylene-C arrays. There was no significant astrogliosis or immune reaction to either type of array. Significance. Softening SCS arrays meet the needs for research-grade devices in rats and could be developed into human devices in the future.