For a soldier wounded in combat or civilian hurt in a car crash, pain does not end in the emergency room. For many amputees the physical hardship has only just begun. An amputation is immune to time and rehabilitation; it results not only in a physical loss but also in the loss of the victim’s ability to carry out normal life as he or she once knew it. Most will go on to receive prosthetics of some sort, but as inanimate limbs, a prosthetic is inherently limited in its potential to restore varied function.
However, the field of robotics and neuroprosthetics is rapidly dawning on a new era of technological innovation in the field of robotic limbs, independently and artificially-powered limbs capable of interacting with our nervous system to produced coordinated movement. In a recent article published in Science Translational Medicine leaders in the area discussed where the field is now and the challenges that remain to move forward, especially for robotic leg development.
Modern robotic technology is superseding traditional prosthetics as an option for paraplegic and amputated patients. Traditional devices operate on a few simple principles, primarily Newton’s third law: for every action, there is an equal and opposite reaction. While prosthetics cannot generate their own energy, they afford our muscles a source of counterforce by providing resistance. Prosthetic feet are usually modified versions of springs that push back up when we generate downward momentum by putting our weight on the ground. Prosthetic knees are usually designed as rotational dampeners. When our thigh exerts torque on the dampener, the resistance it provides allows us to propel forward.
While this prosthetic model is certainly useful for an amputee, it comes with serious drawbacks. Unlike the actual musculoskeletal system prosthetics cannot adapt to their environment. Most are made to function optimally when walking on flat surfaces and thus are ill-suited for other types of terrain such as stairs and uneven surfaces. Since the user must “swing” their prosthetic at an even level, walking must be done in a bio-mechanically inefficient manner; this puts stress on other parts of the body, particularly the hip. These cannot self stabilize, causing amputees to be highly susceptible to accidental falls.
Robotic limbs are being designed as an attempt to shore up the shortcomings of the prosthetics mentioned above. They are intended not to merely aid in the mechanics of walking but also serve as a replacement for the neuromuscular system that was lost. Muscles often occur in antagonistic pairs that allow us to stretch and flex. Instead, in robotic limbs, electric motors are used to imitate this function, and batteries are used to power them.
Similar to how our bodies use special sensory organs embedded in our muscle fibers and tendons called proprioceptors, these robo-limbs are also equipped with sensors to detect and measure angle, angular velocity and torque of our limbs. This allows the limb to calibrate the force that it needs to apply. In addition a three-axis gyroscope and accelerometer are used as an imitation of our our vestibular system, a sensory organ located in our inner ear responsible for measuring spatial orientation and helping us balance. All this information is relayed to and processed by a microcontroller akin to our own peripheral and lower central nervous system.
Since robotic limbs are self powered units, they cannot be swung along like traditional prosthetics. Instead robotic limbs must move through concerted movement with the rest of our bodies via communication with our central nervous system. Several approaches to interpret and signal information to the CNS are being developed simultaneously. The three techniques primarily regarded in the field are (in order of increasing invasiveness):
Physical Sensor Interface: Sensors on the surface of the limb measure movement of the prosthetic and then kick in the limb’s own power supply to complete the movement. While this does not technically communicate with CNS directly, the initial movement of the prosthetic by the body is often a reliable indicator of intended movement.
Surface or Implantable Electromyography (EMG) Interface: This system can measure electrical activity from the muscle of the residual limb using electrodes. Electrodes can also be implanted into the peripheral nerves allowing for more sensitive motor commands and the potential to relay information back to the CNS.
Direct Implantation of Electrodes into the CNS: While this method might actually be less precise than the EMG, it would allow for greater measurement of intentions for your next move.
The development of these interlaying systems represents the greatest challenge in the late stage development of these devices. However, the payoff is potentially tremendous. With limbs capable of self power and stabilization and information processing amputees will be able to walk quicker and safer. Movement will require less strain on their bodies, and amputees will be able to adapt to variety of situations, preventing further injuries and suffering.
