2.2. Characterization of an Inflatable Air Bladder

Air bladder is located in the prosthetic socket to compensate for the volume change of the residual limb. The residual limb should be centered on the prosthetic socket even if the volume of the residual limb changes. Therefore, as shown in Figure 4a, the air bladders were located to surround the residual limb. This arrangement reduces the movement of the residual limb in the prosthetic socket. The three air bladders were located on anterior and posterior tibia areas, which is the minimum number that allows the residual limb to be centered. Air bladder #1 and #2 at the anterior tibia have a size of 150 (L) × 70 (W) mm, and air bladder #3 at posterior tibia has a size of 70 (L) × 50 (W) mm. Using TPU (Thermoplastic Polyurethane) material with a thickness of 0.5 mm, the air bladder was made to fit the residual limb as shown in Figure 4b.

(a) The locations of the air bladders in the prosthetic socket; (b) the appearance of the air bladders.

Figure 5 shows the mechanical properties of the proposed air bladder. In Figure 5a, changes in the height of the air bladder were investigated according to the different internal pressure. For air bladder #1(or #2) and #3, the heights of the air bladders were changed linearly as the internal pressure increased, having slopes of 0.248 and 0.344, respectively. The maximum height was about 50 mm and 35 mm for air bladder #1(or #2) and #3, respectively. In Figure 5b, changes in the internal pressure (dotted lines) and height (full lines) of the air bladder were investigated according to the weight applied externally. As the pressing weight increased, the internal pressure started to increase from the initial pressure of 11 kPa. The small air bladder #3 had a larger slope in internal pressure than that of air bladder #1(or #2). As the pressing weight increased, the heights of the air bladders decreased and kept their heights at the saturated point. The air bladder #1(or #2) showed the internal pressure of 80 kPa at 60 kgf weight load, and the air bladder #3 showed 80 kPa at 20 kgf. Here, the maximum allowable weight was determined as a percentage of the air bladder in the entire socket, assuming that a person with a weight of 90 kg wore the prosthesis.

The mechanical properties of the air bladder. (a) Changes in the height of the air bladder according to the internal pressure; (b) changes in the internal pressure (dotted line) and height (full line) of the air bladder as the applied weight varies, when initial pressure in the air bladder is 11 kPa.