Nanowire fibres help spinal injury recovery

Materials World magazine
,
1 Jun 2017

An implantable nanowire fibre probe could improve the treatment of spinal cord injuries. Khai Trung Le reports.

Paralysing spinal cord injuries are among the most difficult to recover from, often with associated loss of organ function and/or voluntary limb control, and tools for studying neural pathways that contribute to loss and recovery of function are limited. Now a collaborative team at the Massachusetts Institute of Technology (MIT), the University of Washington, USA, and the University of Oxford, UK has developed a nanowire fibre that can be implanted into the spinal cord. The fibre is capable of bending and stretching with the spine without damaging tissue, and may help with exploration of spinal cord injuries and recovery.

The paper, Flexible and stretchable nanowire-coated fibers for optoelectronic probing of spinal cord circuits, published in Science, notes that ‘our understanding of and ability to treat these symptoms is currently limited by the tools available for monitoring and manipulating neural dynamics within the spinal cord’. However, current fibres remain brittle and stiff, and are liable to damage the delicate spinal cord tissue. The paper continues, ‘The spinal cord’s viscoelastic modulus of 0.25 to 0.3MPa poses engineering challenges to the design of optoelectrophysiological probes. Furthermore, repeated spinal cord deformatons during normal movement demand resilience to the implantable devices to bending and extension fatigue.’

Recognising the dual requirements of flexibility, durability and ability to record strain in spinal cord movement, the team opted for a transparent elastomer probe coated in a mesh of silver nanowires to provide conductivity. The fibre was drawn together with a polycarbonate and cyclic olefin copolymer cladding, which is dissolved after the process.

Polina Anikeeva, Professor of Materials Science at MIT, said, ‘The spinal cord undergoes stretches of around 12% during normal movement. The goal was to mimic the stretchiness, softness and flexibility of the spinal cord. We need biocompatibility and the ability to withstand the stresses in the spinal cord without causing any damage.’ Anikeeva describes the fibre as ‘really just a piece of rubber, but conductive.’ It can stretch by 20–30% without impacting on its properties. ‘They’re so floppy, you could use them to do sutures and deliver light at the same time.’

Larger animals are typically used when testing research on spinal cord injuries, as they can withstand the strain from more rigid wires used for recording. However, MIT graduate student Chi Lu said, ‘We’re the first to develop something that enables simultaneous electrical recording and optical stimulation in the spinal cords of freely moving mice. There are many different types of cells in the spinal cord, and we don’t know how the different types respond to recovery, or lack of recovery, after an injury. We hope our work opens up new avenues for neuroscience research.’

Although the team is hoping to eventually incorporate the fibres into combating spinal cord injury – with the paper noting, ‘these fibres may, in the future, be tailored to address fundamental questions in spinal cord or visceral organ neurophysiology’ – Anikeeva said their efforts were focused on biocompatibility and resilience. Likely changes include exploring other metallic nanowire materials that may be deposited onto polymer fibre surfaces and additional chemical stability through covalent cross-linking of the mesh.

The full paper can be read at bit.ly/2ryBf11