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Prosthetic leg

Toughening Skin to Withstand the Pressure of Prosthetics

Biomedical engineers look to fix the patient, not the artificial limb.

People who have had their legs amputated often use prosthetics to improve mobility and independence. But 75% of those wearing such prosthetics encounter problems such as skin tears, ulceration, and blisters.

The problem is that the prosthetics rub against skin (called stump skin) unequipped to deal with rubbing or supporting too much weight. Stump skin breaks down, causing blistering and ulcers, which can lead to infection and a lot of pain. This means that many patients wearing lower prosthetic legs often find more relief from taking off their prosthetics than they do from wearing them.

To sidestep this issue, engineers from Imperial University, UK, want to re-engineer the skin where the prosthetic contacts the patient. They hope to make it more resistant to friction and rubbing by making it thicker—and thus, better at bearing weight and mechanical force. The researchers want to model the new skin after the thick, tough skin from the sole of the foot as a template for sturdier stump skin.

The bottom of the foot is covered with plantar skin; this skin is unique to the soles of our feet. It is particularly thick and padded, which lets it stay intact despite having to bear so much weight and rub against shoes or the ground. Plantar skin is genetic; it does not develop as an adaptation to pressure, a la callouses.

The researchers modeled the plantar skin and found it behaves differently than regular skin under pressure. The outermost layer of sole skin, the stratum corneum, plays the largest role in protecting the skin from tears and blisters. This layer is much thicker in sole skin than other skin types.

But the thicker skin did not protect itself from ulcers. The major factor in that role was the way strong structural proteins, called keratin and collagen, are arranged. The epidermis on the sole, which is the layer beneath the stratum corneum, contains far more keratin—as well as different types of keratin—than other skin, which helps the skin resist breaks and tears. And the collagen is arranged in much thicker “bundles”, and the collagen fibers are also thicker.

These factors make plantar skin tougher and more resistant to injury. The researchers knew their goal was to engineer skin that had thicker collagen fibers, thicker bundles of collagen, and different types of keratin.

One method of doing this might be to incorporate genetic material into the patient’s stump skin so it becomes thicker and changes its make-up. This might be done using sole skin-grafts.

Another method would be to alter and manipulate the genetic material in the patient’s stump skin to morph it into the desired characteristics. This approach relies on the physiology of skin that gives it the potential to grow in different ways. For example, doctors could inject fibroblasts, cells which trigger collagen production. This could alter the type of keratin produced, leading to thicker skin layers over time.

The third approach has researchers taking plantar skin cells into the lab and growing thick layers of them. These layers would then be grafted onto the patient’s stump.

This is a different approach to biomedical engineering in that most researchers try to improve the prosthetics rather than improving the interface between patient and technology.

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