One of the biggest challenges for neurotechnology is the design of devices that are tolerated well by brain tissue, without sacrificing functionality and implantability. This study examined which design choices mitigate tissue damage and improve longevity, by varying probe features implanted in the cerebral cortex of mice. We report on a systematic, quantitative analysis of neuronal and inflammation markers across cortical depth. We implanted a total of 103 stiff silicon or flexible polyimide probes in 32 mice, varying their thicknesses and widths, which were either attached to the skull or not. A new, automated workflow to quantify immunohistochemical data examines: 1) the tissue loss caused by the implant, 2) the cortical neuronal density, and 3) the immune response expressed by astrocytic and microglial reaction. Flexible polyimide probes exhibited a clear advantage, with fewer lesions and weaker immune responses than stiff silicon probes. Furthermore, we observed a weaker influence of the shank cross-section. A cortical depth profile of immune reactivity revealed focal reactions at the device entry points in the superficial cortex and at the cortex-white matter boundary. This study gives important insights on optimizing device design parameters as well as surgical insights for improved tissue integration of intracortical electrode arrays.

This paper reports a novel, cost-effective process for the fabrication of ultrathin silicon (Si) shuttles applied as insertion tools for highly flexible polyimide (PI) neural implants. The process exploits the so-called etching before grinding (EBG) process established to realize Si-based neural probes of the Michigan style. In this study, EBG is combined for the first time with a subsequent deep reactive ion etch (DRIE) process applied on the wafer-level. The innovative approach allows to realize insertion shuttles with a base thickness > 50 µm using wafer grinding and to reliably thin down the slender shuttle shanks (width ≥ 35 µm) to thicknesses as small as 15 µm using DRIE. The backgrinding liquid wax applied during wafer grinding enables the safe release of the delicate shuttle structures from their carrier wafer using isopropanol. Flexible, 15-µm-thin neural probes made from PI are precisely aligned and temporarily bonded to the custom-designed insertion shuttles applying polyethylene glycol (PEG) and reliably deployed into cortical tissue.

Minimally invasive biosensing using microneedles (MNs) is a desirable technology for continuous healthcare monitoring. Among a wide range of MNs, porous MNs are expected to be applied for sampling of interstitial fluids (ISF) by connecting the internal tissue to external measurement devices. In order to realize a continuous measurement of biomarkers in ISF through porous MNs, their integration with a microfluidic chip is a promising approach due to its applicability to micro-total analysis system (μTAS) technology. In this study, we developed a fluidic system to directly interface porous MNs to a microfluidic chip consisting of a capillary pump for the continuous sampling of ISF. The porous and flexible MNs made of PDMS are connected to the microfluidic chip fabricated by standard microelectro-mechanical system (MEMS) processes, showing a continuous flow of phosphate buffered saline (PBS). The developed device will lead to the minimally invasive and continuous biosampling for long-term healthcare monitoring.

Objective. Optogenetics involves delivery of light-sensitive opsins to the target brain region, as well as introduction of optical and electrical devices to manipulate and record neural activity, respectively, from the targeted neural population. Combining these functionalities in a single implantable device is of great importance for a precise investigation of neural networks while minimizing tissue damage.Approach. We report on the development, characterization, andin vivovalidation of a multifunctional optrode that combines a silicon-based neural probe with an integrated microfluidic channel, and an optical glass fiber in a compact assembly. The silicon probe comprises an 11-µm-wide fluidic channel and 32 recording electrodes (diameter 30µm) on a tapered probe shank with a length, thickness, and maximum width of 7.5 mm, 50µm, and 150µm, respectively. The size and position of fluidic channels, electrodes, and optical fiber can be precisely tuned according to thein vivoapplication.Main results.With a total system weight of 0.97 g, our multifunctional optrode is suitable for chronicin vivoexperiments requiring simultaneous drug delivery, optical stimulation, and neural recording. We demonstrate the utility of our device in optogenetics by injecting a viral vector carrying a ChR2-construct in the prefrontal cortex and subsequent photostimulation of the transduced neurons while recording neural activity from both the target and adjacent regions in a freely moving rat for up to 9 weeks post-implantation. Additionally, we demonstrate a pharmacological application of our device by injecting GABA antagonist bicuculline in an anesthetized rat brain and simultaneously recording the electrophysiological response.Significance. Our triple-modality device enables a single-step optogenetic surgery. In comparison to conventional multi-step surgeries, our approach achieves higher spatial specificity while minimizing tissue damage.

Simultaneous large-scale recordings and optogenetic interventions may hold the key to deciphering the fast-paced and multifaceted dialogue between neurons that sustains brain function. Here we have taken advantage of thin, cell-sized, optical fibers for minimally invasive optogenetics and flexible implantations. We describe a simple procedure for making those fibers side-emitting with a Lambertian emission distribution. Here we combined those fibers with silicon probes to achieve high-quality recordings and ultrafast multichannel optogenetic inhibition. Furthermore, we developed a multi-channel optical commutator and general-purpose patch-cord for flexible experiments. We demonstrate that our framework allows to conduct simultaneous laminar recordings and multifiber stimulations, 3D optogenetic stimulation, connectivity inference, and behavioral quantification in freely moving animals. Our framework paves the way for large-scale photo tagging and controlled interrogation of rapid neuronal communication in any combination of brain areas.

This paper reports on the fabrication and characterization of two miniaturized optical tools with integrated bare laser diodes (LDs) and optical fibers targeting the chronic implantation in small animals. The smaller, so-called integrated device with a single LD measures only 780 micrometer x 2.5 mm. The second, modular device variant (3.1 x 5 mm2) carries optical fibers and LDs on two separate chips. The removable module carrying the LDs is aligned to the optical fibers on a common substrate, and fixed using small magnets. The use of bare LD chips as light sources guarantees enough optical power to stimulate electrophysiological activity in optogenetically modified neurons. Both design variants reliably achieved >70% coupling efficiency between LD chips and fibers.

This paper reports on the fabrication and characterization of thin silicon (Si) probes with integrated channels for minimally invasive drug delivery into the brain. Buried channels are realized in Si using plasma-free, vapor-phase, continuous-flow (cf) xenon difluoride (XeF 2 ) etching, with the microfluidic probes subsequently thinned down to 70 μm using the etching-before-grinding approach. While plenty of data are available in literature on pulsed XeF 2 etching, the characteristics of cf-XeF 2 processing, which provides a more controlled etch process, have to date not been widely discussed. To the best of our knowledge, this is the first study that reports on the detailed characterization of cf-XeF 2 etching for the fabrication of micro-channels in single-crystal Si. By using etch openings of different dimensions and varying the process parameters, process characteristics such as etch rate, aperture effect, etch anisotropy, and surface roughness are investigated. Flow and mechanical fracture tests demonstrate the suitability of the fabricated probes for in vivo applications.

Porous microneedles (MNs) are expected to be applied for diagnostic microfluidic devices such as blood glucose monitoring as they enable a pain-free penetration of human skin and the extraction of interstitial fluids. However, conventional microfluidic systems require additional steps to separate the liquid from a porous structure used for fluid extraction. In this study, we developed a microfluidic system with a hydrodynamically designed interface between a porous MN array and microchannels to enable a direct analysis of liquids extracted by the porous MN array. The microfluidic chip with an interface for the MN array was successfully realized by standard MEMS processes, enabling a liquid flow through the whole microfluidic structure. The porous MN array was fabricated by the salt leaching and molding method, which was integrated with the chip and demonstrated the successful extraction of liquids from an agarose gel-based skin phantom.

Continuous sampling of interstitial fluids from the human skin is a key technique for clinical bioassays using micro total analysis (μTAS) technologies. Here, we present a porous microneedle (MN) array to be connected to a custom-designed microfluidic system for the minimally invasive access to the dermis. The pores and surface of the porous PDMS array are covered with hyaluronic acid to enhance mechanical strength and to achieve a fluidic path of water inside the MN bodies. The fabricated MN array is capable to penetrate an agarose gel phantom and extracts water contained in the gel, indicating the fluidic connection between the MN array and the microfluidic system.