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A research team from the Massachusetts Institute of Technology’s (MIT) Department of Materials Science and Engineering has recently developed nanowire-coated fibers for optoelectronic probing of spinal cord circuits.
Polina Anikeeva’s team developed a stretchable and flexible probe that can be used in developing tools that are useful for monitoring and manipulating neural dynamics in the spinal cord. The team used elastic polymers, coated with micrometer-thick meshes of conductive silver nanowires (AgNWs), which allowed these probes to be both stretchable and electrically conductive1.
Spinal cord injuries in multiple trauma patients are frequently associated with the loss of organ function and/or the loss of voluntary limb control2. Diagnosing and treating such patients requires optoelectronic devices to modulate and record the electrophysiological activity of specific neurons that are affected2.
The low elastic modulus of 0.25 to 0.3 megaPascals (MPa) of the spinal cord requires these engineered probes to be both elastic and bendable, similar to the nature of the spinal cord, in order to withstand repeated strains, while still being able to record electrophysiological activity during spinal cord deformations.
The optogenetic modulation capabilities of genetically identifiable neural populations in the rodent models have been successfully used in neuroscience. These models could therefore serve as an indispensable tool for the discovery of neural pathways that are important for recovery following spinal injury2.
To fabricate a probe that is suitable for both optical neuromodulation and electrophysiological recording, Anikeeva’s team used thermal drawing, a process involving heating, drawing/stretching under stress and cooling a material, to produce a flexible optical fiber. Polycarbonate (PC) was used as the core of the optical fiber, and cyclic olefin copolymer (COC) was used for cladding of the optical fiber2.
To facilitate an accurate neural recording, a 1 micrometer (mm) thick conductive layer of AgNWs with a diameter of 70 nm and a length 40 nm was deposited over the COC cladding by a process called dip coating. Dip coating required the use of different concentrations of isopropanol (IPA) solutions after treating the fibers with oxygen plasma to improve adhesion and enhance the uniformity of AgNW mesh layers2.
To minimize direct contact of the AgNW with tissues, and to also prevent surface oxidation and mechanical degradation, the entire structure is further encapsulated in polydimethyl siloxane (PDMS) 2. As a result of this design, these hybrid probes maintained low optical transmission losses in visible range showed promise in extracellular recording under strains exceeding mammalian spinal cords.
The results showed that the nanofibers with then COC cladding (PC/COC/AgNW) showed an increased transmission at a wavelength (λ) of 473 nm as compared to the nanofibers without the COC cladding (PC/AgNW) at the same wavelength2. The PC/COC/AgNW probes showed good flexibility with a maintained transmission under extreme deformations that were carried out by bending the fiber to 90° and 180°2. Transmission electron microscopy images revealed that the resistivity of AgNW mesh is directly proportional to the concentration of dip-coated solutions at concentrations above 6 mg/ml2. Furthermore, impedance values of similar orders of magnitude were recorded in fiber probe measurements ranging from 1 cm to 10 cm, suggesting the scalability of this fabrication approach to larger animals2.
Impedance measurements over five extension and release cycles of the stretchable neural probes showed that these fibers are stretchable and bendable. To test the functionality of this probe in animal models, the neural probe was implanted into the spinal cord of mice and the mechanical, optical and electrical properties allowed for acute recordings of spontaneous neural activity through sensory-evoked potentials2.
The simultaneous recording of optically-evoked potentials and the optical control of hind limb muscles suggests that these fiber may potentially be used in monitoring and controlling neural activity to promote recovery following spinal injury2.
These novel optical fibers facilitating the simultaneous electrical recording and optical stimulation could open up new possibilities in neuroscience, especially in avenues benefitting the monitoring and recovery of patients with spinal cord injuries1,2. The group of MIT researchers believe that these implantable fibers could allow scientists to stimulate specific targets in the brain to monitor electrical activity1,2.
References
- Chandler, David L. "Rubbery, Multifunctional Fibers Could Be Used to Study Spinal Cord Neurons and Potentially Restore Function." Phys.org. 3 April 2017. Web. https://phys.org/news/2017-04-rubbery-multifunctional-fibers-spinal-cord.html.
- Chi Lu et al. Flexible and stretchable nanowire-coated fibers for optoelectronic probing of spinal cord circuits, Science Advances (2017).
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