Engineers love solving design problems. And especially so when the challenge is personal.
Bob Radocy had a challenge more personal than most. An automobile accident resulted in loss of his left hand. The conventional prosthetics he was fitted with, including the familiar split hook, were not satisfactory in providing him with the active lifestyle he desired. So, combining his background in drafting, engineering, biology, and recreational therapy with research, he developed a prosthetic mechanism for his personal use that resulted in the formation of his own company, TRS (Boulder, CO).
The firm specializes in upper limb technology for missing hand and hand dysfunction, including a specialized line of prosthetics for many sports and outdoor activities. Radocy emphasizes "the devices are high performance to complement the other hand, not just assist it, and be totally functional."
The result of Radocy's R&D is the patented Anatomically Design-Engineered Polymer Technology (ADEPT) system. This prosthetic mechanism provides feedback in the form of resistance to the user, giving a proportional feel to using the device, making it "natural" to learn its use (see sidebar).
ADEPT is the basis of the latest-generation Lite-Touch biomechanical hands for persons three years and older. Like most engineering designs, the hands represent a series of design compromises. While the ADEPT mechanism was easy to learn, its two contoured closing halves did not have the appearance of natural hands--a critical factor in dealing with children's prosthetics. Not just the patients, but many parents, too, had reservations about the appearance. "The parents always want another hand, says Radocy. "They are not going to get another hand, but we have to try and come as close as we can. It's like asking someone what a blind person needs. The usual response is 'sight,' but if you ask a blind person, he or she will probably say 'To be able to read.' Thus the challenge was to provide ADEPT's simplicity and efficiency in a cosmetically acceptable package
After urgings from fellow prosthetic professionals and caregivers at a conference several years ago, Radocy laid out the concept for Lite-Touch on the flight back. He notes that copying exactly the human hand would result in a large mass of fingers to move or work around. And any mechanism with a cosmetic glove over it would have increased resistance due to energy absorption from glove elasticity. Radocy says "I don't think building a mechanical hand can be done well with engineering efficiency (energy output to input) more than 35%." He puts the simple, reliable, ADEPT at 90% efficiency.
His solution was to have the two halves of the ADEPT gripper not only emulate the index finger and thumb functions of the hand, but also to mold them to look like a combined index and middle finger on one side and a thumb on the other. The key was to create the illusion of a complete hand while eliminating the bulk and complexity of a full-size ring finger and pinky. The ring and pinky fingers are "folded" and sculptured into the palm to look real but not interfere with prehension (wrapping around) or grasping action from any angle to an object. The latter fingers are not full sized but are more of relief features to allow larger objects to be held within the inside surface of the hand.
Design features. While the concept proved workable, the choice of materials was critical to a reliable, long-life mechanism. A nylon skeleton provides basic strength and an overmolded polyurethane gives a compliant and durable surface for grasping. "We had experience with polyurethane surfaces and structures. So we looked at different durometer types for conformity in grasping. We had to balance strength and compliance to give a natural feel. Too soft a material wouldn't have the structural integrity and abrade off. Plus we needed abrasion, chemical, and stain resistance. We couldn't find anything in a thermoplastic that approaches the characteristics of urethane."
Radocy also cites molding technology advances that allowed the chosen dispenser-poured urethanes to be molded in complex shapes, with undercuts and facets, producing the desired hand-like appearance. "Two-piece molds with flexible thermoplastic inner walls, as used previously in art applications, were now used for prosthetics," he adds. Also aiding in mold extraction of the complex shape is the flexibility of the hot urethane workpiece.
While CAD is used in most design engineering today, TRS does not have an in-house CAD capability. Although used in his designs, Radocy instead relies on CAD resources at the University of Colorado and his medical engineering subcontractor ProtoMED (Denver), which specializes in medical models. "I could learn CAD, but I wouldn't use it enough to stay proficient without needing to re-learn it for each design," he says. TRS does the basic design, finishing, and assembly, while also subcontracting production manufacturing.
A grip on the future. Lite-Touch is designed for children three years and older. For children as young as six months, Radocy is developing a gripping mechanism which is also hand-like, but does not have the cable mechanism. Somewhat like an automatic coupler on a toy train, the patient would exert force on an object with the device and it would "snap" its fingers into place around it, effecting a grip. The child would then remove the object with the other hand when done. TRS also has cupped, free-flexing hands, without grasping features, that rebound to their original shape after loading for cosmetics, infants, and some sport uses.
As to the efficacy of the Lite-Touch in helping young patients, Byron Backus, certified prosthetist at Shriners Hospital for Children/Twin Cities in Minneapolis, offers "We've used quite a number of them and patients prefer it over other terminal devices." He adds, "They like the look and function, with feedback through the harness system. The advantage is it works like a human hand, a voluntary-closing terminal device which uses shoulder muscles rather than hand muscles." Like a hand, relaxing the muscles releases the object. Backus does acknowledge that some patients, particularly older, long-time users of voluntary opening devices that stay closed without effort, prefer those mechanisms.
Radocy says that chief competition to the ADEPT-based products are electromechanical arms which require artificial feedback and come in at three to five times higher cost than the roughly $3,000 to $5,000 price-tag on arms equipped with his devices. In the future, he sees competition to his prosthetics coming from microsurgery, so much so that, he says, "Medical science will satisfy functionality ahead of engineering, with techniques to link nerve endings, making functional limb transplants possible. Farther out, probably not in my lifetime, will be genetic-based regeneration of limbs."
For more information on Lite-Touch Bio-Mechanical Hands, Contact Bob Radocy, TRS, 2450 Central Ave., Unit D, Boulder, CO 80301-2844; Tel: (800) 279-1865; Fax: (303) 444-5372; Internet: www.oandp.com/trs.
Adapting ADEPT: how Lite-Touch works
Bob Radocy used his Anatomically Design-Engineered Polymer Technology (ADEPT) lever, cable, and harness system as the basis for the Lite-Touch hands for young children. This system interacts with the user's upper limb and shoulder muscles much the way actual human hands respond to nerve input, providing proportional feedback in the form of increased resistance to the muscles actuating the cables. The exact layout varies, depending on how much of avestigial arm the user has. ADEPT is a voluntary closing mechanism, being proportional throughout its range of motion and in applying gripping force, giving the user a full range of control. Traditional prosthetics, such as split hooks, work with more of a reverse loop--proportional to activate (open) but once closed they apply a constant, preset force to an object with no resistance feedback to the user. A drawback is that the force may be inappropriate to the task, being the same, whether trying to grip a brick or eyeglasses. †