Following up on the success of cochlear and retinal prostheses for people who have lost sensory function, neuroscientists see a limitless horizon for related devices that are able to read electrical and chemical signals from the nervous system to stimulate capability and restore quality of life in persons suffering injury and disease.
In the future, according to researchers, the devices – known as neural prosthetics – will help epileptics, persons with treatment-resistant depression and chronic pain, victims of Alzheimer’s disease, wounded war veterans suffering post-traumatic stress disorder and traumatic brain injury, persons with speech disabilities, and individuals who have sustained spinal cord injury and loss of limbs, among other applications in the research pipeline.
But before neural prosthetics can advance, engineers will be called on to make innovative use of materials to design and fabricate devices that allow sustained electronic functioning in the harsh environment of the human body, without causing tissue infection and other serious adverse conditions. Research efforts have focused on enhancing the performance of various types of materials used in neural prosthetics, in addition to developing interface technologies that enable the micro devices to be safely implanted in human tissue for long periods.
Researchers at Lawrence Livermore National Laboratory, Livermore, CA, are making gains with thin-film flexible polymer materials. In experiments with auditory prosthetic applications, the microelectrodes comprising the neural interface are embedded in the polymer, which allows the device to move naturally and conform to live tissue in which it is implanted. These materials have mechanical properties that mimic neural tissue more closely than traditional micro wires used in current cochlear and deep brain stimulating implants.
“Among the engineering challenges associated with neural prosthetics is the biocompatibility of the implant,” says Dr. Sarah Felix, a lead research engineer at Lawrence Livermore and also a member of ASME. “Research suggests that polymer is more compatible with the human body than the silicon in conventional neural probes used in neuroscience studies.”
Livermore's artificial retinal implant has an array of 240 thin-film microelectrodes inside a microelectronics package. Image: LLNL.gov
Conventional rigid neural devices are believed to cause micro tearing because the neural tissue is softer than the device. According to Felix, the flexibility of a thin-film polymer probe mitigates this problem. However, the flexibility also makes polymer devices difficult to implant. Felix’s solution is to temporarily attach a rigid stiffener.
“For the polymer neural interfaces, we attach the device to a needle-like stiffener using bio-dissolvable polyethylene glycol (PEG) to enable extraction of the stiffener after surgical insertion,” says Felix, who holds a Ph.D. in mechanical engineering from the University of California, Berkeley. “An innovative bonding process enables accurate alignment of the device to the stiffener.”
A novel feature of the design is a shallow channel running lengthwise, which allows the even distribution of the PEG or other bio-adhesive during assembly and implantation. Felix’s team used the method to implant unique dual-sided polymer electrode arrays into brain tissue, and these arrays successfully recorded neural signals.
A Promising Future
Lawrence Livermore believes its devices and surgical methods can be expanded for future applications in deep brain and spinal cord simulation, which will enable physicians to advance neural prosthetics to the next level of human health and rehabilitation. In fact, Lawrence Livermore is currently developing neural implants that will restore auditory, motor, and bladder function; aid speech; and control depression and epilepsy.
Each year, the National Institutes of Health spends $6.5 million on neural prosthetics R&D, and today several of the most prestigious medical research institutions in the U.S. – Case Western University and the Massachusetts Institute of Technology among them – are engaged in promising clinical studies.
Many medical scientists believe the sky is the limit for neural prosthetics, but ultimately it is the engineering community that will need to design and fabricate devices that enable the realization of the promise of neural modulation for patients and their families.
Says Felix: “There exist many engineering considerations with neural prosthetics, particularly in the interface of the device with human tissue. Engineers must think about a complete range of issues, from electrode materials and the lifetime of the implant to electronics and signal processing. This will be an intriguing pathway of multidisciplinary scientific and engineering development for many years to come.”
Research suggests that polymer is more compatible with the human body than the silicon in conventional neural probes.
Dr. Sarah Felix,
Lawrence Livermore National Laboratory