Plastics reach out to prosthetics

The human machine can sometimes go wrong or even break down permanently, resulting in disabilities of varying degrees. Some of these are irreparable because, despite all the progress made, modern medicine is still unable to restore sight to a blind person or hearing to a deaf person. Neither can it offer any way to "regrow" an amputated limb. To compensate for this, teams of doctors and engineers are developing prostheses that are constantly being improved, particularly thanks to polymers. They are certainly strong, rot-proof, easily mouldable and lightweight, but these are not their only advantages, far from it!

Focusing on polymers

According to the World Health Organisation, more than 250 million people in the world are visually impaired (visual acuity of less than 3/10ths in the better eye), with nearly 40 million suffering from total blindness. The development of a genuine ocular prosthesis, some even speak of a bionic eye, is therefore not a fad intended to help a handful of people. Many university and private research centres are focusing their research on curbing (or even stopping) retinal degeneration, one of the major causes of poor eyesight. The retina is the entry point for light and plays a crucial role in vision. When it is degraded, light does not pass through as well and the brain cannot transform it into an image.

Silicone-based polymers are perfectly biocompatible. It is therefore logical that they should be used in bionic eyes, which are still under development.

Among the teams at the forefront of this research is that of Professor Fan Zhiyong, a research professor in electronic and computer engineering at the Hong Kong University of Science and Technology (HKUST). Last year, the team unveiled the fruit of their work in the journal Nature: a bionic eye (the first of its kind) named EC-Eye, for "Electro Chemical Eye".  This artificial eye has the same proportions as the human eye and, above all, reproduces the functions of the eye's photoreceptor cells and the optic nerve. To achieve this, the eye comprises nanosensors connected to flexible nanocables made from a liquid metal introduced into a soft rubber sheath. The whole thing is encapsulated in a silicone polymer known for its biocompatibility. Although the initial results are encouraging, the road to a true bionic eye is still long, since for the moment its resolution is 100 pixels compared with 7 million for a fully functioning human eye. However, it can distinguish white letters on a black background. Work is still needed to miniaturise the nanocables, which are bulkier than the optic nerve.  Polymers will certainly be used to achieve this. The researchers are very optimistic and hope that their bionic eye will find concrete applications within five years, both in the field of medicine and in that of robotics.

Corneal implant: seeing a way with PMMA

Diseases or damage to the cornea (the transparent part of the eye in front of the iris and pupil) are also a major cause of blindness worldwide. Just over a year ago, a medical team in Israel transplanted a synthetic cornea into a 78-year-old blind man. This implant enabled him to regain his sight. His vision is not perfect but he is now able to recognise his family members and read text. Although corneal transplants are not new, they used to be organ transplants from recently deceased people. However, there are too few donated corneas worldwide for the number of people suffering from progressive blindness (only one cornea is available for every 70 needed). This artificial cornea is therefore a real breakthrough.

PMMA, a polymer well known for being transparent, is ideal for designing artificial corneas.

The corneal implant called CorNeat KPro is the first synthetic product implanted directly onto the surface of the eye. It was designed by CorNeat Vision, an Israeli start-up specialising in biometric implants, and consists of a lens made of PMMA, a perfectly transparent polymer, and a 100% artificial, porous and non-degradable skirt. The process used for implantation is relatively simple and takes less than an hour: the implant is placed on the eye and then sutured with three stitches of a non-absorbable double-armed thread. This 100% polymer cornea could give hope to nearly 20 million people worldwide.

Plastics are making a big noise

Unfortunately, there are no hearing aids available at the moment that can give hearing to people who are deaf from birth. However, small hearing aids are available to help people whose hearing is deteriorating (even very badly) to avoid being locked into silence. The way they work is that an amplifier, usually placed behind the ear, picks up the sound, amplifies it and sends it via a wire to the heart of the ear in a device that incorporates a mini-speaker placed in a silicone envelope for comfort.

The unique moulding capabilities of polymers and advances in electronics make it possible to create ever smaller, almost invisible hearing aids.

Although the technology has been perfectly mastered and has known very few notable evolutions in recent few years, research is now looking into miniaturising these prostheses in order to make them invisible. Some brands offer completely in-the-ear hearing aids. The microphone, amplifier and earpiece are no bigger than a chickpea. The whole thing is custom-moulded in a soft, sometimes foamed polymer or silicone to ensure not only a perfect fit in the ear canal but also, and above all, great comfort.

Sometimes these devices are no longer effective enough. It is then possible to consider a cochlear implant, a device that allows people with severe hearing loss to have better access to sound. This implant is placed in the cochlea (the part of the inner ear that ensures hearing) and stimulates the auditory nerve. It is certainly effective, but not very discreet, because although the implant is invisible, it is inseparable from an external part consisting of an earpiece and an antenna, which are quite visible. However, this technology is improving little by little. Using the process of nano-coating, a micron-thick layer of polymer, usually PTFE or vinyl acetate, is applied to the device under vacuum to protect it. The polymer penetrates all cavities and bonds to their surface. This provides complete protection. The perfectly insulated internal and external components make the devices resistant to moisture. This reduces maintenance requirements and extends the life of the unit.

Cochlear implants need to be protected from moisture that can impair their performance. New polymer-based nano-coating technologies have made this possible.

All ears: polymers to the rescue

Although not strictly speaking a disability, some people are born without an outer ear or may have lost it as a result of  an accident involving burns, for example. Generally, this does not affect the hearing system but it is disfiguring.

Thanks to polymers, it is now possible to reconstruct a missing or damaged outer ear. In the first case, which is usually related to a congenital problem, the cartilage is missing and the outer ear is not supported and hangs down. The cartilage can be reconstituted from Medpor, a porous high-density polyethylene that is perfectly biocompatible and whose porosity allows for neovascularisation. Made to measure, it is placed and then wrapped in the skin of the auricle during a rather long surgical procedure. Note that this same material is also used in cosmetic surgery in the form of implants to rebuild a nose or a chin for example. In the second case, the entire outer ear (cartilage and skin) no longer exists. An outer ear, also made to measure, can be created using medical-grade silicone. This is a prosthesis and more specifically an epithesis. The procedure is much less invasive because the prosthesis is attached to the head by means of bone-anchored implants.

The reconstruction  of an auricle involves the use of a silicone-based polymer which, once custom-moulded, is dyed to match the colour of the skin.

Prostheses have plastics under their skin

There has been enormous progress in the field of upper limb prostheses in recent years. Today, they can pinch and grasp objects, giving amputees more independence. And thanks to 3D printing, it is now possible to manufacture polymer prostheses at lower cost (see interview with Ayúdame3D). However, until recently, no prosthesis could give back the sensation of touch. For an amputee, grasping an egg with their prosthesis can be problematic as without this sense, it often ends up being crushed by the forceps. In 2017, the first major improvement was made. It was the work of the American company Deka, which, after years of research, successfully brought to market a prosthetic hand named Luke in homage to the hero of the Star Wars saga. Connected to the nerve endings of the amputated limb by electrodes, this hand made of Teflon, silicone and polyester is capable of complex movements and adapts to the consistency of the object being manipulated. When the patient uses his or her muscles to transmit information to the prosthesis, the latter knows exactly whether the object it is handling is fragile or not.

Based on Teflon and silicone, this new artificial hand is able to recognise the degree of fragility of the object being grasped.

Polymers feel the heat

A first step, then, but not enough for a large number of laboratories around the world, which have set out to design a kind of artificial skin incorporating an embryonic nervous system to recover this essential sense.

Among the most recent publications is that of Stanford University in the United States, in collaboration with Seoul National University (South Korea), which claims to have developed an ultra-sensitive polymer tactile sensor (the rest is top secret) capable of detecting the slightest caress once it is connected to the nervous system. For the moment, the tests have been carried out on insects, on cockroaches to be precise, and have proved conclusive, according to the authors.

Designing artificial skin capable of feeling is the goal of many laboratories around the world. A challenge also taken up by polymers!

In Australia, at the Royal Melbourne Institute of Technology, work is being carried out on an artificial skin capable of feeling pain, heat and cold. It would be the first of its kind to be able to differentiate between a caress and a sting. It has been designed by creating elastic electronic circuits based on a flexible composite polymer that contains a conductive material based on carbon (also top secret). The circuits are fitted with pressure sensors and the whole thing is placed on a layer of biocompatible silicone as thin as a sticker. Admittedly, it is still only a prototype, but this skin could be used on prostheses within a few years. It could also be used as a non-invasive alternative to skin grafts.

Finally, although the work is older, another American-Korean group announced in 2014 that they had created a skin from polydimethylsiloxane, a transparent polymer. Incorporating a very dense network of silicon and gold nanowires, their skin is capable of sensing pressure, temperature, stretch and humidity. Since then, the team has not made any further announcements. This is not unusual because, as is often the case, although the initial results may be very promising, the road to the project’s completion generally takes several years or even decades.

As we have seen, arm and especially hand prostheses fulfil their essential functions and improve year after year. Leg prostheses are not to be outdone.

Polymeric materials such as silicone, carbon and glass fibre or epoxy resin have enabled real progress to be made. More comfortable, more flexible, and more dynamic, the new generation of prostheses almost make you forget about the disability, as they provide the user with an almost natural gait. To find out more, read the interview with Össur, one of the world leaders in the field: https://plastics-themag.com/Prosthetics:-from-comfort-makers-to-record-breakers

The carbon fibre-based Cheetah blade developed by Össur is now a regular on the podium at the Paralympic Games.