Robotic Mirror Therapy Exoskeleton

Hemiplegic cerebral palsy (CP) is a type of unilateral CP that causes paralysis on only one side of an individual’s body. While there is no cure for a disorder such as hemiplegic CP, there are several technological solutions available that can assist these children in maintaining balance and improving muscle movement. However, such methods do come at a cost, one that not all people are able to afford. Our team’s project for the Summer 2022 STIP program is focused on creating a robotic mirror therapy-based exoskeleton for children with hemiplegic cerebral palsy – the second most common form of CP that affects the sensorimotor function of the limbs on one side of the body. The overall design of the exoskeleton includes one arm that is motor-based and the other arm that is sensor-based. Our team chose robotic mirror therapy as the subject of our exoskeleton because it shows promise in assisting individuals with hemiplegic cerebral palsy, helping to improve the motor functions pertaining to the impaired side of their body. For this project, our team utilized cost-effective materials, including 3D printing parts, to create our robotic mirror therapy exoskeleton. This is to ensure that all people in need of such an exoskeleton would easily be able to obtain and afford one without struggle.

Cerebral palsy is the most common motor disability to occur in childhood often characterized with poor muscle movement and balance.

According to US studies, about two to three children out of every 1,000 are born with cerebral palsy averaging to about 10,000 babies born each year that will develop CP.

One type of unilateral CP is known as hemiplegia which is caused due to damage of the spinal cord or brain. Hemiplegic CP is usually characterized by paralysis on only one side of an individual’s body.

While there is no cure for a disorder such as hemiplegic CP, there are several ways that individuals can work to strengthen and gain better control of their affected muscles.

One way involves mirror therapy, a commonly used form of physical therapy utilized by recovering stroke patients.

While on a fundamental level some may consider mirror therapy to be meant for different types of physical therapy, it does show promise in helping individuals with hemiplegic cerebral palsy as it helps to improve the motor functions pertaining to the impaired side of their body.

However, simply simulating the motor function movement of an individual’s impaired side can only do so much. What CP patients need is a way to successfully train their impaired muscles through actual action, using repetitive, passive rehabilitation exercise.

Our team’s project for the STIP program involves creating a robotic mirror therapy exoskeleton that will drive the impaired arm of children with hemiplegic CP in response to the movement of their unimpaired arm.

The arm that our team has created allows for the planar flexion and extension of the participant’s elbow. The left arm is the “sensorized” arm that is being driven by the participant’s good arm, while the right arm is the motorized arm that mirrors the “sensorized” arm and moves the impaired arm with the help of a motor.

Both devices have potentiometers to sense the arm position, and a single microcontroller that bridges them together. The design of the device primarily consists of an armrest to hold the participant’s arm that would rotate at the elbow to extend the arm away from the user’s body. A hub under the elbow includes the necessary components to accomplish the rotation – bearings to provide smooth rotation, a potentiometer to sense the current arm position, as well as a motor in the slave device to acuate the motion.

An atmega328p based Arduino microcontroller was used to control the system of the exoskeleton. The motors were driven by a L298N H-bridge motor driver that has a limit of 48V with a peak current of 4A, and can be controlled by a 5V input signal. The motor driver also allows for current sensing, which can be utilized for overcurrent protection.

To assemble the arms together, our team used V-slot linear rails to allow for a flexible system where components such as forearm rests, handles, or other supports can be mounted to the arm assembly without having to redesign the entire assembly.

The control code for the microcontroller was written in C++. Running in a loop, the code would check the position of the potentiometers in both devices, check the current drawn to the motor, then calculate the desired input to the motor. If the desired position was outside of reasonable bounds, or the current to the motor was too high for too long, the motor would simply be stopped to prevent damage to the system.

Overall, the final design of the exoskeleton did function as was expected, successfully being able to move both arms together, mirroring the movements of each other. Our group had hoped to be able to add a wrist supination and pronation movement to the exoskeleton, but due to time constraints we were only able achieve the planar flexion and extension during the course of the summer. However, we do have CAD models demonstrating our group’s design for the wrist movement, showing that it is a viable option for future research, easily being able to be attached to our group’s current prototype of the exoskeleton thanks to the flexibility of the V-slot linear rails used for the arms design.

In the end, while our exoskeleton does successfully work and did have minimal costs, there is much that can be improved for future research. Both through design and types of materials, we can continue to improve and create more efficient exoskeletons for children with hemiplegic CP.