Patent of the month: Composite orthotic support

Materials World magazine
,
30 Apr 2017

Dr Jennifer Unsworth of intellectual property firm Withers & Rogers describes a process aiming to control the resin content to improve the properties of composite medical supports.

While the use of composite materials is well established in some sectors – most notably aerospace – these specialist materials are not yet used to the same extent in other applications, such as the manufacture of medical devices, including orthotics and prostheses. 

During the design of orthotic and prosthetic devices it is important to ensure that the fit is perfect and that the functional design is optimised. This sometimes requires remodelling to change the shape or stiffness of the device.

A plaster of Paris model, or ‘tool’, corresponding to the patient’s anatomy is created and used as the starting point for the manufacture of most orthotic and prosthetic devices. Remodelling the device can require multiple curing cycles. 

Although the technical characteristics of composite materials are clearly advantageous in such applications, their use may be limited by the inability to process and re-process them in a cost-effective manner.

Infusion composite manufacturing methods can be used in the development of orthotic or prosthetic devices. This involves using a polyvinyl acetate (PVA) bag, which is wetted and applied to the tool. The bag is then dried and shrunk to the right shape. Vacuum suction is used to tighten the bag over the tool, while dry carbon fibres are placed over the surface of the tightened bag. A second PVA bag is then placed over the carbon fibres, which are infused with resin. Finally, a chemical catalyst is applied to cure the composite by heating it in an oven. 

It is not possible, however, to accurately control the resin content of the product using these methods. This means it is not always easy to ensure that the final product has the desired properties. 

To avoid the need to infuse the carbon fibres with resin, composite fibres that are already impregnated – pre-preg fibres – may be used, allowing the resin content to be controlled. However, the PVA bags used in each of these methods are fragile and often destroyed during curing. If an additional curing step is required, the entire process needs to be repeated.

Based in the UK, Orthotic Composites Limited is developing methods to enable composite materials to be used in established manufacturing processes, allowing orthotic and prosthetic devices to be manufactured more efficiently. The company has developed a method and apparatus for making orthotic and prosthetic devices, which is the subject of the recently granted UK patent GB2535612.

This method removes the need for PVA bags altogether. Instead, a thermoformable polymeric material, such as polypropylene, is applied to the tool and retains its shape without the use of a vacuum. 

Pre-preg fibres can then be placed on specific regions of the polymer-covered tool to make the device according to a particular configuration. For example, pre-preg carbon fibres may be placed on the sole of the foot, on a portion of either side of the foot, alongside the back of the leg and/or extending around a portion of the calf.

The fibres are manipulated so that they form a 3D sheet, and the tackiness of the uncured resin ensures that the pre-preg holds its shape. Additional layers of pre-preg can be added, if required, to obtain the desired thickness. The tool is then covered by a vacuum-permeable breather fabric, placed in a heat-tolerant vacuum bag and cured in an oven or autoclave under vacuum pressure. 

The curing process does not damage the protective polymer layer, so the tool can be used in multiple curing cycles without any modifications. In addition, the resin does not stick to the protective polymer layer, so the composite device can be removed with relative ease. 

To view the patent, visit bit.ly/2o9BNL6