Biomedical engineering is a vast field with a multitude of concentrations and research initiatives. While the technicians affiliated with clinical engineering and a number of other concentrations focus mainly on pre-existing technologies, researchers enjoy the exhilaration of innovating the new. Biomedical engineering has grown exponentially since its acceptance as a field less than a century ago, to the extent that today there is not a branch of medicine untouched by the problem-solving skill set of the engineer. The objective of this section is not to make the reader aware of every cutting-edge technology in development today but rather to introduce a sample of these new adventures.

Prosthetics

Prosthetics are one of the oldest innovations of biomedical engineering. The assistive technology field, prosthetics especially, became a true engineering discipline in itself in the period following World War II, when an unprecedented number of veterans returned home alive, but disabled, due to advances in medicine.

Prosthetics are defined as any “internal or external device(s) that replace lost parts or functions of the neuroskeletomotor system” and may be either orthopaedic or externally controlled. Externally controlled devices may be powered by the body itself through myoelectricity or a separate power supply. Neural prosthetics represent the newest field in prosthetics and one of the fastest-developing topics in biomedical engineering today.

Orthopaedic Prosthetics

In designing a “replacement” limb for the human body, an engineer is buried under an obscene amount of considerations and design constraints. The appendage must be functionally sufficient, a design unique to each individual, depending on the activities to be accomplished. It must be comfortable, aesthetically pleasing, convenient, and simple in attachment. Prosthetics and orthoses seeking to imitate the human body piece by piece tend to have a great amount of difficulty in development and implementation. Instead, the general application of the device should always be considered, with the user in mind. An example of this design strategy can be found in the flex foot, a prosthetic foot with no real resemblance to the natural appendage. Instead of struggling to recreate the biomechanics of the ankle, tarsals, metatarsals, and phalanges of the lower leg, designers created a prosthetic with a single contact piece, no joint, and consisting of only one material. The Cheetah Leg shown in Figure 1.11, is one type of such a prosthetic and has

FIGURE 1.11 Paralympic sprinter Oscar Pistorius with a prosthetic leg. Designing for overall function, as opposed to mirroring the human body, is often the more practical approach.

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 compete at a level that became subject to controversy. In 2008, the South African sprinter battled 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 definition 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 tetanisation, or the smooth motion contraction of muscle. Stimulation below this frequency results in isolated twitches and muscle fatigue. Electrodes may be implanted transcutaneous (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 provide 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 activating different “pedals,” the user can rotate the wrist or grasp or release an object. Sensory feedback, a constant issue with mechanical prosthetics, is provided via a pressure sensor on the fingertips, which feedback 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 modelling and physiology. The American Board for Certification in Orthotics, Prosthetics, and Pedorthics provides guidelines for certification as a licensed prosthetist. Those in the field are required to complete an accredited

FIGURE 1.12 Dean Kamen’s Luke Arm, the most advanced neural prosthetic to date, which uses myoelectric signals. Clinical trials are presently underway

undergraduate program in prosthetics or a graduate program specializing in the field with an appropriate undergraduate degree. Neural prosthetic development involves a team of members from various backgrounds, including biomechanics, electronics, and mathematical modelling. While prosthetics provide a strong example of the evolution of assistive technology and present a number of interesting design innovations, in recent years the field of biomedical engineering has shifted from a focus on mechanical systems to biological and organic solutions. Whereas decades ago the primary objectives of the biomedical engineer consisted of device design, modern feats are more likely to involve biochemistry and gene therapy than screws, nuts, and bolts. Two prime examples of this shift in focus are tissue engineering and stem cell research.