TPE Compounds Enhance Functionality of Prosthetic Fingers

Inspired by veterans in need, an inventor called on RTP Co. for help in developing a prosthetic device that aims to mimic natural hand movement
Material selection was crucial in enabling a prosthetic hand device to perform movements and function like natural fingers.

Following the trauma of losing a limb or digit, many patients want nothing more than a return to normalcy. Luckily, advancements in leg, arm, and other prosthetics over the years have helped to restore their confidence and improve their quality of life.

But progress in prosthetic hand and finger design has lagged behind that of other prostheses, according to Matthew Mikosz, president of Partial Hand Solutions (Southington, CT). “While there were a number of prosthetic hand designs on the market, no functional mechanical fingers had yet been developed,” he says.

Troubled by the high volume of veterans with hand and finger injuries and the lack of natural function in existing prosthetics, Mikosz decided to take matters into his own hands. From his basement, he developed a prototype of a prosthetic finger device, dubbed M-Fingers, designed to feel and operate as naturally as possible.

To achieve this goal, however, Mikosz needed help. He found it in RTP Co. (Winona, MN), a compounder of custom-engineered thermoplastics. The company assisted in material selection based on several criteria set forth by Mikosz. It was imperative, for example, that the base material featured good structural integrity to ensure that the fingers would not easily break. In addition, Mikosz required materials that would create a chemical bond between the hard structural fingers and the urethane finger tips. He also sought a material that would allow the fingers to firmly grip objects.

Offering strength and stability to the M-Fingers, RTP’s 2300-series glass-filled rigid thermoplastic polyurethane ultimately was selected for the inner structure of the fingers and multiposition thumb. These parts were then overmolded with an RTP 1200-series thermoplastic polyurethane elastomer.

During the development process, however, Mikosz found that the standard thermoplastic urethane that was employed for the fingertips was too slippery and unable to grasp objects as desired. To solve this problem, RTP modified the material to enhance its tackiness and optimize it for the application, thereby providing each finger with dexterity to independently and gently conform to whatever it grasps.

The mechanical fingers are actuated by wrist flexions and even feature molded-in fingernails for picking up small objects. “With so many M-Fingers being used by soldiers in rehabilitation, it was very important that RTP’s materials provide both structural stability and, at the same time, the ability to move and operate the prosthesis smoothly,” says Chris Budnick, general manager of Vanguard Plastics (Southington, CT), which performed the molding operations for the M-Fingers.

“They really brought their plastics knowledge and expertise to the table, and provided great materials,” Budnick says of RTP’s role in the process. “The M-Finger design is truly amazing and the feedback has been very positive.”