Material considerations for peripheral nerve interfacing
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Introduction In the last decade, the development of advanced robotic limbs has revolutionized the simple hook prosthesis powered by body movements currently used by many amputees, with multifingered lightweight devices capable of up to 22 degrees of freedom and populated with force and movement sensors that more closely resemble the operation of the human hand.1,2 Despite such progress in prosthetic devices, providing the user of the robotic limb with natural control and feel remains a formidable challenge. Most robotic prostheses are controlled through command signals recorded from remaining limb muscles (electromyogenic signals) that are measured by surface electrodes, amplified, and converted to mechanical signals.3,4 In order to provide a more natural control and feel of the prosthetic hand, Kuiken and collaborators developed a surgical strategy that transfers residual arm nerves to pectoral muscles (a method known as targeted muscle reinnervation), from where electromyogenic activity can be recorded and used to control the robotic hand.5 In addition, recent data from animal studies indicate that transferred sensory nerves can also reinnervate the skin over the pectoral area, suggesting that this technique can provide sensory feedback to amputees and contribute toward the development of
closed-loop control systems.6 Alternatively, prosthetic devices can be controlled by signals obtained through microelectrode arrays implanted in the brain premotor cortex from where neural activity is known to correlate with intention of movement.7,8 In human volunteers, this so-called brain-machine interface (BMI) technology has proven successful in allowing paralyzed patients to control a hand prosthesis by thought,9,10 and recent animal studies suggest that sensation might be conveyed to the user via electrical micro-stimulation of the sensory cortex.11 However, the invasive craniotomy surgery required for cortical interface placement and the lack of modulation from other areas in the brain or spinal cord needed for context-dependent control and sensory discrimination limit their use.12,13 In contrast to BMI, placing electrodes in the peripheral nerves of amputees offers a readily accessible portal to the bidirectional flow of information between the nervous system of the user and smart robotic prosthetic devices. Motor commands initiated by the user are transmitted from the motor cortex in the brain to the midbrain, cerebellum, and ventral motor neurons in the spinal cord for integration and coordination, ultimately traveling through the peripheral nerve where they can be recorded by peripheral nerve interfaces (PNIs) (Figure 1). Conversely, sensations from the limb such as motion, pressure, and temperature
Young-tae Kim, University or Texas at Arlington/U.T. Southwestern Medical Center, Arlington, TX 76019, USA; [email protected] Mario I. Romero-Ortega, Bioengineering Department, University of Texas at Arlington/U.T. Southwestern Medical Center, Arlington, TX 76019, USA; [email protected] DOI: 10.1557/mrs.2012.99
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