* Wonseok Shin and DongWoo Nam contributed equally.
Email: bmahn@Kitech.re.kr
Abstract: Pneumatic active ankle foot orthosis (AFO) for drop foot correction has the advantages of inherent compliance and remote force control. However, pneumatic AFOs that use heavy stationary air compressors as the energy source have limitations for outdoor use. Although a portable air-compressor-powered pneumatic AFO has been developed recently, it is difficult to operate in practical applications owing to the bulky design of the AFO and excessive power sources with overheating issues. In this study, the AFO system was optimized to improve wearability. The weight of the AFO was decreased from 720 to 600 g. A Bluetooth module was installed instead of a 1.2-m cable between the master and slave boards. The efficiency of the portable pneumatic actuator increased 12.4%, whereas its volume decreased 11%. The internal temperature was reduced from 100°C to 40°C using two cooling fans. Throughout the optimization process, the wearability of the AFO system was improved for real-life use.
Introduction: Drop foot is a gait abnormality caused by neurological disorders, such as stroke and multiple sclerosis [1–4]. Drop foot causes disruption of the motor control pathway between the spinal cord and lower limb, and it causes the absence of ankle dorsiflexion [3, 4]. The swing phase is impeded by front foot drops, resulting in falling or compensatory hip movements [5, 6]. Although ankle foot orthosis (AFO) has emerged to correct abnormal gait patterns, most commercial AFOs have limitations of restrictive adaptiveness [2] from the passive structure [7]. To overcome these limitations, active AFO has been developed [8–16]. The most frequently used types of actuator for AFO are cable driven and pneumatic driven [8]. Although cable-driven actuators have been widely used for remote force transmission [10] because of their lightness and compact size, they cannot overcome the limitations of uncomfortable wearability owing to the tension problem of the cable [11]. Pneumatic-powered actuators can be an alternative to solve the limitations of cable actuators, but they have the limitation of tethered heavy external energy sources [11] or small-volume air tanks [12]. To reduce the volume of the actuator, Kim et al. developed an AFO with a double-crank untethered pneumatic actuator [13]. The developed portable air compressor can power the pneumatic AFO, satisfying the required flow rate and pressure level. However, an unstructured power source with a pneumatic component embedded jacket has thermal issues and high noise levels, which make the user uncomfortable.
In this study, the AFO system was optimized by minimizing the number of electronic parts, including the printed circuit board (PCB), pump design, and additional cooling fans. To improve the wearability of the AFO, a bulky plastic foot cast was replaced with a carbon insole, and a BOA Fit System was loaded instead of a Velcro strap. The wearability of the system was also increased in three ways: untethering the actuator from the AFO, reducing the size of the motor and gear box of the actuator, and adding cooling fans to reduce the internal temperature.
AFO improvements: The prototype of the AFO [15, 16] was fabricated using thermosetting plastic parts, as shown in Fig. 1(a). The plastic structure can be worn like a foot and calf cast. The frame of the AFO consisted of several aluminum parts. Two pressure sensors (I2A Systems, Daejeon, Korea) were attached under the cast to measure the ground reaction force. However, as mentioned above, the AFO could not be hidden in the shoe because of the large volume of the cast. The AFO was improved in the form of an insole to reduce the prototype volume, as shown in Fig. 1(b). The insole is thin and compact; therefore, it can be inserted into the shoe easily. The total thickness of the insole was similar to that of commercial insoles. The number of frame parts was reduced and consisted of titanium instead of aluminum. Braces are a necessary part of the AFO to ensure stable fixation on the leg of the wearer.
The prototype AFO has only one strap-type brace on the calf, which causes unstable and uncomfortable wearability during walking, and more brace was added above the