ACTUATING SOFT ROBOTIC EXOSKELETONS: THE POTENTIAL AND PRACTICALITY

Thomas Hinds and Rachel RoundWhat is Soft Robotics?

Therapy and Rehabilitation Applications

Sustainability

Soft Robotic ActuationActuators are movement-producing components, and in the scope of soft robotic exoskeletons, these soft mechanisms are a major component in wearable device. Soft robotic actuators mimic motion that is more natural for humans, in order to use them in robotics. These actuators offer high power-to-weight ratios, since they are made of lightweight materials and are not bulky. This feature enables robots to apply high forces in environments where humans can interact without risk of serious injury. Additionally, the flexibility of these soft materials being used offer adaptability and forgiveness in motion—qualities rigid devices do not have and are specifically desirable for exoskeletons. In the current field of soft robotics, there are three specific actuator types that are the most widely used. These three types are the McKibben actuator, fluidic actuator, and cable-driven actuator.

McKibben Actuator• Originally developed in 1950’s• Produces muscle-like expansion/contraction

from air bladder• Most versatile and commonly used actuator

in devices• Outside weaving prevents rupture• Inefficient due to large amounts of air

required to operate

Fluidic Actuator• Composed of inner expansion chamber and

outer sheath• More versatile than McKibben

• Chamber can be filled with any fluid• Greater efficiency allows device to have

greater mobility• Can produce non-linear movement• Heavy equipment required to store and

move fluid

Cable-Driven Actuator• Strong and flexible, able to transfer force

from motor to anchor point• Few components needed for storing power• Provides distributed and continuous action• Cables provide extreme flexibility• cables fit where other actuators can not• Friction between cable and outer tube

decrease efficiency

Comparison to Rigid DevicesSoft Exoskeletons Rigid Exoskeletons

• Typically made of lighter-weight materials like cloth or rubber.

• Stretchable material and soft joints with many degrees of freedom

• Low cost material and production

• Versatile with one suit fitting many body types

• Minimized danger in uncontrolled environments because of high adaptability

• Haptic feedback provides high reliability.

• Simplicity of design

• Typically made of heavy metals or plastics.

• Complex joints with limited degrees of freedom

• Costly sensors and extensive production prices of more than $50,000 dollars

• Must be custom built to fit various body types

• Can be dangerous to user and/or surroundings when impeded in uncontrolled environments

• Relies heavily on sensors for feedback

• Complex mechanisms

In terms of the sustainability of energy and renewability of materials, soft robotic exoskeletons are groundbreaking. Many of the materials used can be made from recycled resources, unlike previous exoskeletal designs which typically relied on custom materials. The soft materials that are implemented in soft robotic designs do not utilize these types of materials and have heightened capability to interact with humans and real-world environments more easily.Green sources of power and sustainability are also important factors when it comes to soft robotics. Power sourcing can come from a variety of different places. For the majority, soft robotics are powered by batteries, however, more research has been done to see how the dependence on battery power can be reduced by capturing the kinetic energy of motion.This can be used to harness the power of the device itself in order to help recharge the battery while it is still in use. Other research involves using the energy from the motors moving backward on the device due to users’ movement, when it is not receiving energy from the battery, to charge the battery.As more clean energy sources, renewable materials, and innovative ways of increasing power in devices like soft robotic exoskeletons becomes more clear, they can be utilized in devices like soft robotic exoskeletons, the potential for their advancement in the medical field is promising.

A soft robotic exoskeleton—usually made of fabric, rubber, or any other flexible material—is a wearable device with robotics incorporated into the design in order to assist the human body. The main objective engineers aim to solve with the development of this technology is to improve the interaction between the robot, the user, and their environment. Recent advances in soft materials, passive mechanisms, and non-linear models, have led to more use of soft materials in robotics worldwide. This increase is driven not only by new scientific paradigms, but also a need for better applications, such as biomedical, service, and rehabilitation.

Those in need of physical therapy are generally suffering from some sort of injury, disease, or other impairment, and most rehabilitative methods include assisted movement to regain motor function and the use of a limb or extremity. However, recovery time and degree of improvement is heavily dependent on the frequency of treatments. With the high cost of seeing a physical therapist on a frequent, regular basis and the lack of constant availability of physical therapists, the time it take for a patient to improve their condition can increase, and they may not see the best possible results. Conversely, a patient prescribed to use a soft robotic exoskeleton can expedite his or her recovery time, and ultimately receive more effective therapy. Because of the lowered costs associated with physical therapy administered via a soft robotic exoskeleton, treatment can be available to a much larger population.

A prototype for a soft robotic exoskeleton incorporating Bowden cables, designed for post-stroke shoulder rehabilitation