Complex prosthetic limbs, the bionic eye, or brain-computer interfaces implement microelectrode arrays to connect the nervous system to an electronic device. The aim is to partly restore lost body functions or to gather bioelectrical activity from the nervous system. Microelectrode arrays are fabricated to have specific electrical and mechanical properties to match the biological requirements of the intended application. Polyimide and parylene-C are favorable polymers for substrate and insulation layers in thin-film applications. This article reviews the materials and the mechanical and electrical properties of electrode arrays. It emphasizes the often ignored but crucial influence of adhesion of the thin-film layers on the device’s longevity and reliability. Adhesion promotion techniques using layers of silicon carbide (SiC) are also discussed. Even though the main focus is on thin-film devices fabricated using traditional methods of micromachining based on lithography, an alternative to thin films, laser-patterned silicone/metal foil microelectrode arrays, is also presented. Characterization as well as application examples of these devices are also presented.

Extracellular matrix (ECM)-based materials used for neural interfaces allow for prolonged effective interaction between foreign devices and the neural cells in vivo because they mimic the natural environment. This review will highlight studies that have demonstrated how ECM-based materials can benefit neural interfacing applications as neural electrode coatings, scaffolds, and nanoparticle (NP) coatings. The performance of neural electrodes can degrade from inflammatory response as indicated by the signal obstruction between neurons and electrodes from glial scar formation during prolonged implantation. ECM materials can mitigate an inflammatory response because they are naturally biocompatible and biodegradable. Scaffolds composed from ECM-based materials have the characteristic length scale and biochemical cues that promote directional neural cell growth. ECM-based scaffolds can also be utilized as drug delivery vessels to infuse the neural tissue with neural growth factors and anti-inflammatory cytokines. The NPs currently employed for drug delivery and imaging use various ECM-based coating materials that shield them from the neural cells because many types of NPs are cytotoxic. As demonstrated by these three neural interfacing applications, ECM-based materials are very promising candidates for the development of next-generation neural regeneration and therapeutic devices.

Ultrafast differential transmission spectroscopy was employed to study the carrier dynamics in InGaN/GaN multiple quantum wells with high inverted hexagonal pits density due to threading dislocation. By monitoring the temporal evolution of the excitonic absorption spectrum, a reduction of the quantum-confinement Stark shift was observed due to the photo-induced in-well field screening at low carrier densities and excitonic absorption quenching at high carrier densities. By comparing the differential absorption spectra at various injected carrier densities, the in-well field screening effect was distinguished from excitonic bleaching.

The recording of neural signals with microelectrodes that are implanted into the cortex of the brain is potentially useful for a range of clinical applications. However, the widespread use of such neural interfaces has so far been stifled because existing intracortical electrode systems rarely allow for consistent long-term recording of neural activity. This limitation is usually attributed to scar formation and neuron death near the surface of the implanted electrode. It has been proposed that the mechanical property mismatch between existing electrode materials and the brain tissue is a significant contributor to these events. To alleviate this problem, we utilized the architecture of the sea cucumber dermis as a blueprint to engineer a new class of mechanically adaptive materials as substrates for “smart” intracortical electrodes. We demonstrated that these originally rigid polymer nanocomposites soften considerably upon exposure to emulated physiological and in vivo conditions. The adaptive nature of these bioinspired materials makes them useful as a basis for electrodes that are sufficiently stiff to be easily implanted and subsequently soften to better match the stiffness of the brain. Initial histological evaluations suggest that mechanically adaptive neural prosthetics can more rapidly stabilize neural cell populations at the device interface than rigid systems, which bodes well for improving the functionality of intracortical devices.

As biological science advances, there is a need for new technical tools to study biological matters. In neuroscience, new knowledge on the nervous system is discovered through biological experiments carried out under in vitro conditions. As experiments become more delicate, the technical requirements also increase. Recent advances in nano- and microscale technologies have increased the applicability of new emerging technology to neurobiology and neural engineering. As a result, many materials that were not originally developed for neural interfaces have become attractive candidates to sense neural signals, stimulate neurons, and grow nerve cells for tissue engineering. This article focuses on the material requirements for in vitro neural interfaces and introduces materials that are used to design various neural interface platforms in vitro.

In this work, different methods to estimate the value of thin film residual stresses using instrumented indentation data were analyzed. This study considered procedures proposed in the literature, as well as a modification on one of these methods and a new approach based on the effect of residual stress on the value of hardness calculated via the Oliver and Pharr method. The analysis of these methods was centered on an axisymmetric two-dimensional finite element model, which was developed to simulate instrumented indentation testing of thin ceramic films deposited onto hard steel substrates. Simulations were conducted varying the level of film residual stress, film strain hardening exponent, film yield strength, and film Poisson’s ratio. Different ratios of maximum penetration depth hmax over film thickness t were also considered, including h/t = 0.04, for which the contribution of the substrate in the mechanical response of the system is not significant. Residual stresses were then calculated following the procedures mentioned above and compared with the values used as input in the numerical simulations. In general, results indicate the difference that each method provides with respect to the input values depends on the conditions studied. The method by Suresh and Giannakopoulos consistently overestimated the values when stresses were compressive. The method provided by Wang et al. has shown less dependence on h/t than the others.