Manufacturing for the Amputee
Body-part replacement challenges manufacturing engineers and designers
By Robert B. Aronson
Medical problems of Baby Boomers and treatment of war casualties have focused attention on prosthetic devices, a long-established field of medical manufacturing. These are the "artificial" arms and legs that help restore the amputee's capabilities.
According to the Amputee Coalition of America, there are approximately 2,000,000 people with limb loss in the US (excluding fingers and toes). More than 185,000 new amputations are performed each year. The vast majority, 80% or more, are the result of vascular problems, chiefly diabetes. With this malady, circulation to the lower extremities is often blocked, resulting in deterioration that ultimately requires amputation. War wounds and traumatic accidents account for around 20%, with a minor number the result of birth defects.
Despite their long history of use, two factors still slow advances in prosthestics.
- Limited market: Those manufacturing prosthetic devices cannot take advantage of the cost reduction in the same way that volume-production manufacturers can.
- Manual labor: Most prosthetic devices have to be tailored to a specific individual, so much manufacture requires hand labor. In addition, the prosthetics must be fitted to the user by a highly trained professional.
And because of body changes, the fitting has to be done more than once.
These problems are mitigated somewhat by the use of off-the-shelf components in some simple designs, and the funding of R&D in this area by government agencies, chiefly the work done at the Walter Reed Army Medical Center (Washington, DC).
The Veteran's Administration is a major provider of prosthetic equipment. In fiscal year 2007, the VA supplied prosthetic equipment valued at $869 million. This included 8058 legs ($46 million), 380 arms ($2.5 million), and 86,945 wheelchairs, scooters, and accessories.
Prosthetic development has accelerated in recent years and opened opportunities for manufacturers. Among the more pressing needs is software. The goal for all of the newer designs is to more closely copy the motion of a natural limb. This requires analyzing natural motions, determining what motions the prosthesis should replicate, and translating that information to a program.
For example, Siemens (Elk Grove Village, IL) provides CNC, motor-and-drive packages, as well as specific software suites for both machine builders and manufacturers of medical devices, ranging from surgical instruments to orthopedic and orthotic devices. As an example, the company's Virtual Production software simulates a part program in the manner of any postprocessor, but it has the added benefit of including the relevant machine-tool kinematics into its calculations. This becomes extremely valuable, according to the company, for exotic materials or oneoffs, where the surface finish as well as time-to-part reduction are equally critical. Virtual Production can be performed in three steps.
- The first step: Analysis of the data quality as provided by the part program.
- The second step: Execution of the part program by the numerical control (NC) where set-point positions on the NC output are evaluated and velocity can be optimized and controlled.
- The third step: Simulation based on characteristic features of position-controlled drives, including machine dynamics.
Among the other needs:
- Sensors. Advanced prosthetics use a multitude of accelerometers, strain gages, pressure sensors, and current sensors. And more with functions yet to be determined will be needed. But in all cases, they must be accurate, rugged, low cost, and frequently, quite small.
- Power. Batteries power many prosthetic devices and the need is for high power density and small size. At the same time, the powered elements should have minimal current draw.
- Materials. Prostheses have unique requirements for mechanical performance in the areas of strength, weight, wear resistance, cost, and flexibility. Plus, user comfort is a major issue, often requiring special fabrics.
CAD/CAM innovations have contributed greatly to prosthesis manufacture, particularly in socket making. While many of the parts can be standardized, making the socket for the user's stump is a critical, individualized process. In the past, this was done based on a series of manual measurements, then craftsmen did their best to hand-form the socket.
Now CAD/CAM systems do much of the work by borrowing equipment from reverse-engineering manufacturing techniques. The stump is scanned by a digitizer, and the data converted to a CAM file. This file may then be modified by a technician to meet special needs, or used directly. In either case, the final file goes to a lathe-like copying machine. The resulting prosthesis is available sooner, and requires far less manual work. In addition, the user can be scanned in a remote location and the data sent to a central location for part manufacture.
Once the prosthesis is fitted the work isn't over. The stump shrinks as the body reacts to the initial trauma and as the user ages, so modifications have to be made.
Prosthetic arms can be inert, bodypowered, or bionic. The inert type has only a cosmetic function and is not activated. A body-powered prosthesis uses some existing body motion to activate a device. The most common is the shoulder harness used to activate an arm or hand. Bionic prosthesis devices, the newest and most complex of these devices, come in two categories. One version uses a built-in computer. Signals activate or modify various mechanical motions so motion of the prosthesis more closely follows actual motions. The most advanced systems of this type are activated by signals picked up from the wearer's existing nerves, which are amplified and used to activate some mechanical function.
A prosthesis still widely used for arm replacement uses body power. The design has a halter that fits to the shoulder opposite the missing hand or arm. A cable runs from the halter to the artificial hand, and operates when the user flexes the shoulder with the harness.
Despite the mechanical simplicity of this design, advances in materials have been responsible for many improvements. For example, the systems offered by TRS Inc. (Boulder, CO) utilizes an ultrahigh-molecular-weight polymer called Spectra from Allied Chemical (Morristown, NJ). "It's used for the arm's cable," explains Bob Radocy, company president and CEO. "It has an inherent lubricity and a strength equivalent to stainless steel.
"Thanks to this material and other aerospace-developed materials, chiefly resins, the artificial arm's life has been extended from a few years to decades," Radocy explains.
A recent company development is a cable lock, which functions much like a rope cleat. It allows the user to lock the gripper in place rather than relying on body force to hold it in position.
In situations where no hand function other than holding is required, much simpler designs are practical. "We have arms specifically for golf, holding a hammer or wrench, or lifting weights," says Radocy. "These are tailored to the user's needs and take advantage of the latest materials for strength and light weight."
Prostheses for lower extremities (full leg, lower leg, and foot) may be totally mechanical, mechanical with a computer assist, or bionic.
The Rheo knee made by Ossur (Reykjavik, Iceland) is a motor-powered prosthesis for above-knee amputees. It makes use of artificial intelligence to make the prosthesis perform more like an actual leg.
A magnetorheologic-fluid actuator minimizes fluid drag, restores a more natural motion, and reduces fatigue. This "fluid" is a suspension of oil and small magnetic particles. In the absence of a magnetic field, it performs as a fluid. But within a magnetic field the fluid can be gradually "hardened." That is, it can be totally rigid or have a viscosity related to the magnetic field. The foot design takes advantage of the fluid's damping characteristics.
With this design, the knee is capable of "learning and responding" to individual walking styles and adapts to changes in pace, load, and terrain. Functions of a microprocessor, integrated sensors, and a magnetorheologic-fluid actuator combine to control the knee's movements, producing a rapid response throughout every stage of the gait cycle. Sensors on the foot accurately measureing motion, load, and position of the limb at a rate of 1350 times per sec.
This response offers customized levels of resistance (knee flexion). Disturbances in the walking path are recognized automatically, and stance support is instantly activated to provide against inadvertent stance release.
Power comes from a rechargeable battery that has a usefuel life of about 48 hr.
For each stride, gait-pattern recognition algorithms in the foot detect and identify when a user is walking on flat or sloped surfaces and up or down stairs, as well as when he/she is stationary or sitting down. A simple calibration process evaluation is needed initially, then the control modifies itself based on the wearer's unique way of walking.
"A computer-controlled prosthetic foot that can move toe-up 25 and toe down 45, which is the human range of motion, is more difficult to design and implement than a computer-controlled knee," explains Chris Johnson, engineering director for College Park Industries (Fraser, MI). Knees have a "home" position when fully extended, and can be held against that stop. With feet, however, terrain and body-position changes result in no typical home position, and the stops of the limits of motion are not often hit.
"The human leg and foot form a system with a vast array of sensors connected to a massive parallel-processing supercomputer—the human brain. Every part of the leg and foot caan be tracked and moved in space, while a flood of sensory information is being processed. The complexity of the system is incredible," says Johnson. "When someone loses their foot, much information and control is lost. With our prosthetic, which we call iPed, we are trying to achieve a higher level of control than is currently available with a conventional prosthetic foot. The task is daunting, but we are making great progress."
Five mechanical fingers are moved by signals detected on the user's body in the iLimb hand from Touch Bionics (Boston). Each is separately driven through a small dc motor and a worm-and-wheel gear arrangement.
In use, the fingers can be wrapped around an object and the drives are designed to stall when pressure reaches about 20 lb (88 N). There are no invasive sensors. All signals come from surface-contact electrodes. A microprocessor in the hand sorts out the signals and sends commands to the proper finger.
When the hand is fitted, special instruments are used to find the muscles with the necessary signal type and intensity. The user controls the speed and force of the hand. To meet the strength required for a mechanical finger, the hand uses a special version of DuPont's Zytel.
Among the challenges during a 20-year R&D program was creating a software program that would allow the fingers to be controlled separately. The company currently offers a unique product called PRODGIT that replaces one or more fingers.
"After the technology was proven on a laboratory scale, the next main challenge was reducing the elements so that the package was no larger than an actual human hand," explains Hugh Gill, director of technology and operations at Touch Bionics.
"We are currently working on even smaller designs suitable for adult females and children. In addition, we have an advanced prototype of a prosthesis for the upper arm," Gill concludes.
The prosthesis sockets on the prosthetic into which the stump fits are a major concern, and remain a problem area. Sockets have to be strong enough the support the user, light enough to be easily moved, and comfortable. Normally, the stump is held in place by friction between the skin and prosthesis and/or vacuum. As mentioned earlier, the issue is further complicated by the fact that the stump changes size with use and the wearer's age.
One of the more advanced solutions, now in the research stagte, is a socket lined with a fabric that automatically reconfigures to the the stump.
A major area of prosthesis research is osteointegration. In this process, the prosthesis is attached to the user's bone, and the bone grows to secure it in place. The major problem with this method is that the prosthesis actually passes through the skin, and this opening can be a major source of infection.
Wheelchairs are another common way of assisting amputees, as well as those suffering from leg paralysis. The first device that looked something like a modern wheelchair was reportedly built in 1595. Today's commonly used wheelchairs take advantage of strong, lightweight materials and precise wheel bearings. Motorized versions use small, high-torque motors powered by rechargeable batteries.
And a major subcategory of this mobility assist, which has experienced major growth in recent years, is "sports" wheelchairs. They are manufactured for just about every sport imaginable, from racing through rugby. (There are 147 wheelchair rugby teams in the US.)
Building these wheelchairs combines craftsmanship and NC manufacturing techniques. This is necessary because customers have unique needs based on the sport, and the user's size, age, and capabilities.
"We started out as a supplier of aerospace parts, then gradually migrated into wheelchair manufacture," explains John Box, president, Colours in Motion (Corona, CA). "However we hang onto many aircraft manufacturing processes, chiefly in our selection of materials and assembly.
"The craftsmanship comes in when forming the chair's frame," Box explains. "This establishes the rider's position and reach." High-strength tubes are manually bent to individual specifications. Company engineers found that automatic bending machines were too slow because of the setup necessitated by variation among the chairs, and the need for precise adjusting that only a craftsman can provide. Therefore, manual bending is more practical in this case.
The type of sport determines other features. For example, a racing chair is designed chiefly for speed, and a chair for tennis stresses maneuverability.
Once the frame design is established, the rest of the assembly can be handled by conventional machining and welding. "There are no super-precise requirements. For us a 0.0002" [0.005 mm] spec is about as critical as we need," Box concludes.
Many of the major advances in medical equipment are not new developments but improvements of existing designs. One long-term challenge with equipment using X-rays or nuclear elements was containment of the radiation.
Lead has long been a problem with medical devices needing radiation shielding. It's a potential health problem for the medical staff, plus, it has disposal challenges.
Lead also has manufacturing drawbacks. For example, lead-encapsulated glass plates for protection against X-rays must be very thick, limiting usage and design options. Further, lead shielding can have potentially dangerous "hot spots."
Sabic Innovative Plastics (Pittsfield, MA) has a new line of thermoplastic materials with high specific gravity that may replace lead in many healthcare applications that call for radiation shielding. Medical equipment and devices that produce Xrays and gamma rays must be shielded to protect operators, clinicians, patients, and sensitive electronic components from tube leakage and room scatter. Sabic's LNP Thermocomp HSG radiation shielding enables X-ray shielding solutions without the use of known toxic substances, while providing greater design freedom and higher-volume manufacturing with lower total part cost, through the use of injection molding.
They are based on tungsten in nylon 6. This shielding compound has enhanced stiffness, strength, and impact resistance for demanding injection-molding applications.
The transition from machined and stamped lead to injection-molded engineering resins may help enable tighter tolerance specifications and greater part consistency, enhancing the performance and safety of X-ray equipment. Avoiding secondary operations required with lead, plus combining multiple components in one part, reduces total manufacturing time, system cost, and complexity.
The company is working to match the specific gravity of lead, and is working to include a flexible version of the material for specialized applications.
Although sources vary, 1853 is given by many as the year the first hypodermic syringe was invented. Its main use was the administration of opium as a painkiller. The first disposable needles made of glass were introduced in 1949, and the first plastic needles appeared in 1955. Now we have disappearing needles.
Researchers at the University of Kentucky and the Georgia Institute of Technology have demonstrated that patches coated on one side with microscopic needles can facilitate delivery of drug doses.
The study may help advance the use of microneedles as a painless method for delivering drugs, proteins, DNA, and vaccines into the body. It may also lead to a more integrated approach merging engineering with pharmaceutical technology.
By painlessly punching a series of microscopic holes in the outer layer of skin, microneedles may expand the range of drugs and vaccines that can be delivered.
In use, thumb-sized patches containing 50 stainless-steel microneedles, each about 620µm long, are applied to the skin. Microneedle administration also reduces the amount of drug required.
What's Going on in There?
Today's Healthcare information systems were mainly designed to manage acute illness, such as infections and injury, making them ill-equipped to cope with the growing requirement for pervasive monitoring of long-term conditions.
One answer has come from Toumaz Technology Ltd. (Abingdon, UK). Their Sensium platform uses a range of disposable, low-voltage, body-worn physical and biochemical sensors powered by low-cost "printed" batteries. It replaces currently used bulky monitoring systems.
"One of the main advantages is its use of lowcost, low-voltage, nonintrusive, wireless vital-signs monitors," explains Alison Burdett, director of technology. "As a result, the sensing system is much smaller and lighter weight than the bulky systems currently used."
The key physiological parameters of the patient can be continuously monitored. This includes heart rate, body temperature, respiration, and activity level. Data are sent to a base station, and further filtered and processed by application software.
The system delivers usable information rather than just raw data. On-chip signal processing is able to intelligently extract critical information from sensors, providing feedback to the patient and enabling healthcare providers to focus on the important issues.
The device includes a reconfigurable sensor interface and an RF-transceiver block. On-chip program and data memory permits local processing of signals. This capability significantly reduces the transmitted data. The user can be mobile but must stay within the range of the receiver.
Sensium has tiny, nonintrusive, locally intelligent systems on a chip that acquire, process, transmit, and receive data at nanowatt power levels. This means there are economies of scale that allow real-time, personalized care.
The units may be used for a single event, such as emergency-room examination, or carried by the patient for some days to aid long-term evaluation.
"We expect that in the future this system will be used to actuate devices within the body," says Burdett. "For example, the internal insulin pumps might be operated based on data collected by the network."
This article was first published in the May 2008 edition of Manufacturing Engineering magazine.