Biomedical Engineering Reference
In-Depth Information
allowed paralympians like Oscar Pistorius to compete at a scale approaching that of able-
bodied athletes. Actually, the Cheetah Leg allowed Pistorius, a double amputee, to com-
pete at a level that became subject to controversy. In 2008, the South African sprinter bat-
tled courts for the opportunity to race with able-bodied athletes in the Beijing Olympic
Games. While Pistorius ultimately did not qualify, his efforts fueled a debate as to
whether his engineered prosthetics functioned better than a human leg, actually giving
him an advantage over runners in the standard Olympic Games.
Externally controlled prosthetics use external motors to power their operation. The C-Leg
is an example of such a device. This prosthetic leg has a microprocessor-controlled knee;
has force sensors throughout for angle, swing, and velocity; and lasts 25 to 30 hours without
charging. Uneven terrain is tackled with the C-Leg, as are changes in walking pace and
direction. In recent years, sensor and minimally sized motor developments have made
devices such as the C-Leg possible.
Neural Prosthetics
Neural prosthetics present one of the newest and perhaps most exciting concentrations
of biomedical engineering. These devices may be powered by the human body—that is,
they operate from electrical signals sent via electrodes from an external source to the
peripheral muscle neuron—or they may be powered externally. These systems that use
functional electrical stimulation (FES) to “restore sensory or motor function”are the defini-
tion of neural prostheses. These NPs have the potential to assist victims of spinal cord or
cervical column injury (SCI and CI), restoring function to the muscle and lower extremities.
Stimulation via electrodes must reach a threshold frequency to achieve tetanization, or
the smooth motion contraction of muscle. Stimulation below this frequency results in
isolated twitches and muscle fatigue. Electrodes may be implanted transcutaneously (on
the surface), percutaneously (stimulator outside the body connects to a stimulation point
inside), or implanted.
As opposed to the leg, where a series of fairly simple joints and large motor units pro-
vide sufficient function, the upper extremities prove a significant challenge in fine-tuned
control requirements. The incredible strength and flexibility of complex hand function are
difficult to reproduce. The newest in prosthetic design hopes to overcome some of these
challenges. The Luke Arm (Figure 1.12) is the brainchild of Segway inventor Dean Kamen.
The arm has just as many degrees of freedom as the human arm and is capable of lifting
above the user's head. The arm uses myoelectric signals originating from residual nerves
in the upper body. Fine-tuned control is assisted by controls in the user's shoe; by acti-
vating different “pedals,” the user can rotate the wrist or grasp or release an object. Sen-
sory feedback, a constant issue with mechanical prosthetics, is provided via a pressure
sensor on the fingertips, which feed back to a vibrating patch worn on the user's back.
Increased pressure is felt by the user by changes in vibration intensity. Clinical trials
are underway.
The design of prosthetics involves an intensive materials engineering background, as
well as an in-depth understanding of kinematics modeling and physiology. The American
Board for Certification in Orthotics, Prosthetics, and Pedorthics provides guidelines for cer-
tification as a licensed prosthetist. Those in the field are required to complete an accredited
