Flexible and stretchable polymer optical fibers for chronic brain and vagus nerve optogenetic stimulations in free-behaving animals

Fabrication and Characterization of PDMS Waveguides for Flexible Optrodes

With the growth of optogenetic research, the demand for optical probes tailored to specific applications is ever rising. Specifically, for applications like the coiled cochlea of the inner ear, where planar, stiff, and nonconformable probes can hardly be used, transitioning from commonly used stiff glass fibers to flexible probes is required, especially for long-term use. Following this demand, polydimethylsiloxane (PDMS) with its lower Young's modulus compared to glass fibers can serve as material of choice. Hence, the long-term usability of PDMS as a waveguide material with respect to variations in transmission and refractive index over time is investigated. Different manufacturing methods for PDMS-based flexible waveguides are established and compared with the aim to minimize optical losses and thus maximize optical output power. Finally, the waveguides with lowest optical losses (-4.8 dB cm-1 ± 1.3 dB cm-1 at 472 nm) are successfully inserted into the optogenetically modified cochlea of a Mongolian gerbil (Meriones unguiculatus), where optical stimuli delivered by the waveguides evoked robust neuronal responses in the auditory pathway.

Polydimethylsiloxane as a more biocompatible alternative to glass in optogenetics

Optogenetics is highly useful to stimulate or inhibit defined neuronal populations and is often used together with electrophysiological recordings. Due to poor penetration of light in tissue, there is a need for biocompatible wave guides. Glass wave guides are relatively stiff and known to cause glia reaction that likely influence the activity in the remaining neurons. We developed highly flexible micro wave guides for optogenetics that can be used in combination with long-lasting electrophysiological recordings. We designed and evaluated polydimethylsiloxane (PDMS) mono-fibers, which use the tissue as cladding, with a diameter of 71 ± 10 µm and 126 ± 5 µm. We showed that micro PDMS fibers transmitted 9-33 mW/mm2 light energy enough to activate channelrhodopsin. This was confirmed in acute extracellular recordings in vivo in which optogenetic stimulation through the PDMS fibers generated action potentials in rat hippocampus with a short onset latency. PDMS fibers had significantly less microglia and astrocytic activation in the zone nearest to the implant as compared to glass. There was no obvious difference in number of adjacent neurons between size matched wave guides. Micro PDMS wave guide demonstrates in vivo functionality and improved biocompatibility as compared to glass. This enables the delivery of light with less tissue damage.

Development of a synchronous recording and photo-stimulating electrode in multiple brain neurons

The investigation of brain networks and neural circuits involves the crucial aspects of observing and modulating neurophysiological activity. Recently, opto-electrodes have emerged as an efficient tool for electrophysiological recording and optogenetic stimulation, which has greatly facilitated the analysis of neural coding. However, implantation and electrode weight control have posed significant challenges in achieving long-term and multi-regional brain recording and stimulation. To address this issue, we have developed a mold and custom-printed circuit board-based opto-electrode. We report successful opto-electrode placement and high-quality electrophysiological recordings from the default mode network (DMN) of the mouse brain. This novel opto-electrode facilitates synchronous recording and stimulation in multiple brain regions and holds promise for advancing future research on neural circuits and networks.

A silk-based self-adaptive flexible opto-electro neural probe

The combination of optogenetics and electrophysiological recording enables high-precision bidirectional interactions between neural interfaces and neural circuits, which provides a promising approach for the study of progressive neurophysiological phenomena. Opto-electrophysiological neural probes with sufficient flexibility and biocompatibility are desirable to match the low mechanical stiffness of brain tissue for chronic reliable performance. However, lack of rigidity poses challenges for the accurate implantation of flexible neural probes with less invasiveness. Herein, we report a hybrid probe (Silk-Optrode) consisting of a silk protein optical fiber and multiple flexible microelectrode arrays. The Silk-Optrode can be accurately inserted into the brain and perform synchronized optogenetic stimulation and multichannel recording in freely behaving animals. Silk plays an important role due to its high transparency, excellent biocompatibility, and mechanical controllability. Through the hydration of the silk optical fiber, the Silk-Optrode probe enables itself to actively adapt to the environment after implantation and reduce its own mechanical stiffness to implant into the brain with high fidelity while maintaining mechanical compliance with the surrounding tissue. The probes with 128 recording channels can detect high-yield well-isolated single units while performing intracranial light stimulation with low optical losses, surpassing previous work of a similar type. Two months of post-surgery results suggested that as-reported Silk-Optrode probes exhibit better implant-neural interfaces with less immunoreactive glial responses and tissue lesions. A silk optical fiber-based Silk-Optrode probe consisting of a natural silk optical fiber and a flexible micro/nano electrode array is reported. The multifunctional soft probe can modify its own Young's modulus through hydration to achieve accurate implantation into the brain. The low optical loss and single-unit recording abilities allow simultaneous optogenetic stimulation and multichannel readout, which expands the applications in the operation and parsing of neural circuits in behavioral animals.

Understanding the Pivotal Role of the Vagus Nerve in Health from Pandemics

The COVID-19 pandemic seems endless with the regular emergence of new variants. Is the SARS-CoV-2 virus particularly evasive to the immune system, or is it merely disrupting communication between the body and the brain, thus pre-empting homeostasis? Retrospective analysis of the COVID-19 and AIDS pandemics, as well as prion disease, emphasizes the pivotal but little-known role of the 10th cranial nerve in health. Considering neuroimmunometabolism from the point of view of the vagus nerve, non-invasive bioengineering solutions aiming at monitoring and stimulating the vagal tone are subsequently discussed as the next optimal and global preventive treatments, far beyond pandemics.

New Era of Electroceuticals: Clinically Driven Smart Implantable Electronic Devices Moving towards Precision Therapy

In the name of electroceuticals, bioelectronic devices have transformed and become essential for dealing with all physiological responses. This significant advancement is attributable to its interdisciplinary nature from engineering and sciences and also the progress in micro and nanotechnologies. Undoubtedly, in the future, bioelectronics would lead in such a way that diagnosing and treating patients' diseases is more efficient. In this context, we have reviewed the current advancement of implantable medical electronics (electroceuticals) with their immense potential advantages. Specifically, the article discusses pacemakers, neural stimulation, artificial retinae, and vagus nerve stimulation, their micro/nanoscale features, and material aspects as value addition. Over the past years, most researchers have only focused on the electroceuticals metamorphically transforming from a concept to a device stage to positively impact the therapeutic outcomes. Herein, the article discusses the smart implants' development challenges and opportunities, electromagnetic field effects, and their potential consequences, which will be useful for developing a reliable and qualified smart electroceutical implant for targeted clinical use. Finally, this review article highlights the importance of wirelessly supplying the necessary power and wirelessly triggering functional electronic circuits with ultra-low power consumption and multi-functional advantages such as monitoring and treating the disease in real-time.