Design of a Carbon Fiber Ankle Foot Orthotic With Optimal Joint Stiffness

Abstract

An Ankle Foot Orthosis, or AFO, is a device used in human rehabilitation to support lower extremities. The device is similar to a brace that is capable of maintaining the stability of a joint while storing some elastic energy to help lifting the legs at the ankle. The main challenge for this design is to determine the optimal stiffness of the device and how such stiffness comes from the distribution of the stiffness at the joints. Furthermore, the device must be stiff enough for athletic play, but be comfortable for clinical scenarios. We performed a set of experiments on a commercial AFO made of carbon fiber. The angles at the ankle and toes were measured during self-paced walking together with the interaction force between the shin and the AFO. Based on such measurements, we estimated the total rotational stiffness of the device and the distribution of stiffness at each joint. A 3-point bending test following ASTM D7264/D7264M-15 standard was used to evaluate the stiffness of the material produced by a Mark II 3D printer capable of printing long fiber composite. After the optimal printing parameters were selected to obtain the desired stiffness of the material, a finite element analysis was performed to design the AFO geometry to match each single joint stiffness. This customized procedure can provide the design specification of a device so to produce the best athletic performances still maintaining comfort.