Prosthetic Sockets and Exoskeletons

from Sara Bertsche | Sep 17, 2014

We are familiar with simple versions of them, worn by ambitious captains of the oceans with shady intentions. Be it Hook with his iron hook, or Ahab or Long John Silver with their wooden legs – literature has repeatedly employed artificial limbs as a device to add a highly visible symbol of the uncanny or grotesque to the appearance of figures with damaged bodies and minds. Which might be a reason why the artificial replacement of natural limbs still has an alienating effect on people. Yet for centuries medicine and prosthetics have endeavored to develop functional replacements for arms, legs or hands that work as perfectly as possible. Now new procedures enabled by digital manufacturing are giving doctors and therapists reason to hope their techniques can be perfected. In particular, 3D-printing is opening up surprising options – from improved methods to frightening future utopias.

Replacement and perfection

Since time immemorial researchers and developers have been motivated by the desire to compensate for some loss sustained or improve existing capabilities using technical devices. Today, prostheses driven by microprocessors permit complex series of movements, even if restoring sensations and feelings will remain a future vision for the time being.
While scientists time and again cause a stir with their visions of perfect, biomechanical people, the reality of life for people who rely on prosthetic limbs tends to be rather sobering. In his lecture “The sore problem of prosthetic limbs” at a TED conference (which can be viewed on David Sengeh, a Biomechatronics engineer, reports of circumstances in his home country of Sierra Leone, in which the effects of the ten-year civil war can still be felt. Many people lost an arm or leg in combat or had limbs amputated as punishment – the number of victims is estimated to be around 20,000. Sengeh has observed that even those who could afford a prosthetic limb often do not wear them. This frequently has to do with the socket of the prosthetic shaft. If it is not fitted exactly onto the remaining stump it will exert pressure, create wounds and cause pain. This then limits the wearer’s mobility, and the patient stops wearing the prosthetic limb, originally designed to make life easier.
As every stump has a different shape the sockets for the limbs are mainly made using casting, and the manufacturers follow set “assembly guidelines”. This involves body weight, stump length and existing remaining muscles being used to calculate average values, which are then used in manufacturing the prosthetic socket. However, in practice this method is far from ideal as the guideline values only apply to two to three percent of all patients. The alternative, a custom fit, is very expensive, and something very few victims can afford.

Tailor-made thanks to 3D-printing

New production techniques based on digital manufacturing look set to help remedy this situation. In 3D-printing David Sengeh has found an inexpensive method of helping amputees acquire tailor-made prosthetic limbs. Using magnetic resonance imaging (MRI), the anatomy of the stump can be exactly recorded. Subsequently, Finite Element Modeling (FEM, a calculation method for simulating solid bodies) is employed to create a digital blueprint, which serves as a basis for the prosthetic limb. Fed with this data the 3D-printer can now print out an individually-formed socket, which is precisely modified to the nature of the individual stump and only exerts pressure on those places through reinforced walls, where pressure is needed.
The material used is a thermoplastic biosynthetic, which is made into the pre-calculated shape using fused deposition modeling. Together with Hugh Herr, Professor of Biomechatronics at MIT, David Sengeh has recently started to test the prosthetics in the MIT Media Lab. The results are highly promising and have prompted much positive feedback from patients. Also a researcher at MIT, architect and designer Neri Oxman is addressing the impact of digital fabrication on architecture and product design. For example, she has developed the “Carpal Skin”, a 3D-printed glove that is perfectly adapted to the patient’s anatomy and provides material of different thicknesses to only exert pressure where it is required. It is used for patients suffering from carpal tunnel syndrome, where nerves get crushed in the wrist. And the glove is impressive in aesthetic terms, too – Oxman often draws inspiration from biological structures. There are definitely excellent prospects of 3D-printing being put to very good use and for the benefit of patients in the area of prosthetics. And this not only applies to prosthetics worn outside, but also for implants. At Washington State University, for example, research work is being done to find a suitable bone replacement, which is needed particularly in dental medicine and to treat osteoporosis. With a ProMetal 3D-printer an implant is printed from silicon, zinc, calcium and phosphate, which then serves as a structure for new bones to grow.

Exoskeletons promote healing

Those engaged in orthopedic research have also recognized the advantages of 3D-printing. And in the research center of the children’s hospital Alfred I. duPont in Wilmington, Delaware scientists are not content with just developing replacements for individual limbs, but are devoting themselves to creating new exoskeletons for the entire body. These are particularly important in treating children suffering from spinal cord injuries, muscle atrophy or multiple sclerosis. The “Wilmington Robotic Exoskeleton” was developed to support a patient’s arms, legs and torso in performing movements. What might sound futuristic and frightening in practice proves to be a support suit, consisting of 3D-printed elements, elastic bands and metal clips. It can either be buckled around the patient like a jacket or mounted onto the wheelchair. And in a departure from normal practice the sockets are not made of metal but ABS plastic, which gives them two decisive advantages: They are lighter and can be individually made as 3D-printed elements based on digital data. Customized parts better support the patient’s fine motor skills; and there are less signs of fatigue in the muscles. As exoskeletons made of this material are lightweight they can already be implanted in small children. Thanks to the greater freedom of movement they can interact better physically and cognitively resulting in a small measure of freedom. Moreover, the fact that 3D-printed exoskeletons are less expensive to produce than comparable models of metal is a special advantage for children. As they are still growing, their exoskeletons need to be replaced more often.

HAL, the man machine

Moreover, 3D-printing could help in the area of rehabilitation measures to push decisively the development of tailor-made, more precisely in the area of rehabilitation robotics. In the United States and Japan clinical studies are being performed with exoskeletons in patients with signs of paralysis. At Tsukuba University in Japan, in collaboration with the firm Cyberdyne Inc., the first robot suit has been developed with the pretentious name “Hybrid Assistive Limb” (HAL) – like the super computer in Stanley Kubrik’s movie “2001: A Space Odyssey”. It is a full-body exoskeleton, whose limbs are driven using special electric motors. Supported by “HAL 5”, a person can lift five times as much weight than with his natural physical strength. No more than science fiction? Not at all. The first series of experiments started back in 2012 in Japanese hospitals and will probably run until 2015. The robot suits are already being used at German’s “Bergmannsheil” University Clinic in Bochum as part of a research project. Scientists are hoping to attain new approaches to therapy, for example by stimulating cognitive activities in patients with spinal cord injuries. Should things prove successful, exoskeletons could also reduce the number of therapists or nurses when carrying patients or moving them to new beds. Here too digital manufacturing methods using 3D-printers are employed. So as to achieve maximum freedom of movement the socket elements must be as light as possible and custom-made.

Enhancing the human model

In the medical realm exoskeletons primarily serve to stabilize the bodies of people who are ill and weak. However, military researchers are also experimenting with such highly sophisticated technological supports. In this case, the idea is to compensate for completely different deficits and to improve the performance of healthy individuals, so as to lend them an edge as soldiers in combat – which raises the question of using digital technologies responsibly. DARPA, an agency undertaking high-tech research for the U.S. Defense Ministry, is currently conducting research projects on this. The “Exosuit” is designed to help soldiers carry large weights and lend them superior strength in combat. Allegedly the biggest problem facing development is maintaining an adequate energy supply over a long period of time. In other words, not only in medicine are improvements being made to the human model.
An improved soldier, due to the armored exoskeleton of the United States Army. Photo © Daren Reehl
The medical glove „Carpal Skin“, designed by Neri Oxman.
Photo © Neri Oxman
The exoskeleton Hybrid Assistive Limb, developed by Cyberdine, is tested until 2015 in 130 Japanese Institutions.
Photo © V. Daum / Bergmannsheil