Advances in Exoskeletal Materials

Developments in prostheses and robot technology have advanced rapidly, leading to the manufacture of devices known as 'exoskeletons.' These rigid devices, usually constructed with plastic and metal, act as an external skeleton, providing support and mobility to someone with decreased muscle tone or activity. While these devices provide exciting capabilities to the wearer, they are inherently heavy and inflexible.

More recently, researchers have focused on developing flexible, lightweight exoskeleton devices. Researchers at Linkoping University and the University of Boras in Sweden published a study in Science Advances. "It is our dream to create exoskeletons that are similar to items of clothing, such as "running tights" that you can wear under your normal clothes. Such device could make it easier for older persons and those with impaired mobility to walk," says Edwin Jager, associate professor at Division of Sensor and Actuator Systems at Linkoping University.

Advantages of Lightweight and Flexible Exoskeletons

Current exoskeletons are bulky and rigid because they rely on the structure itself to support the wearer's body while being driven by motors or powered by pressurized air. The researchers at Linkoping University are working to develop what they describe as "textile muscles." The goal is to take advantage of the key benefits of fabric, which are its flexibility and lighter weight. They seek to create "artificial muscles that are silent, soft and compliant, with performance characteristics similar to those of skeletal muscle." 

Scientists also envision adding sensing yarns into the fabric to allow better control through a feedback system, thus optimizing the performance of the actuators. They foresee creating flexible exoskeletons with the fibers weaved with custom-designed spacing that enables specific movements.

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Techniques Used to Create Flexible Exoskeletons

Testing and development initially involved coating mass-produced fabric with an electroactive material, which gives the fabric the potential to act as a muscle. The special coating on the fibers of the fabric reacts to the application of a low electrical voltage, causing them to increase in length. The properties of the textile are further affected and controlled by its knitted or woven structure.

Ongoing research focuses on using the two most common textile processing methods, weaving and knitting, and exploiting their individual advantages. Weaving fibers together creates a type of rigidity because close contact of the warp and weft threads. Knitting fibers together forms interconnected loops, giving more flexibility, but also the potential to become loose and deformed.

By combining textiles with new advanced materials, such as electroactive polymers, they fabricated a new kind of textile actuators which they refer to as "textuators." The textuators scale up force by parallel assembly of single fibers, amplify the strain by using stretchable patterns, and can be effectively mass-fabricated. Laboratory testing has demonstrated the feasibility of integrating textuators into soft robotics, by creating textile muscles that exert enough force that enable a simple robot device to lift a small weight.

The Future of Flexible Exoskeletons and Smart Textiles

The research team is working to further develop this process based on the promising results they've seen by combining the natural movement and flexibility of fabrics with the technology guiding robotics and mobility.

Another key factor in producing the textuators in a more cost-effective way is using already existing textile production technology. "What's more interesting, however," says Jager, " is that it may open completely new application in the future, such as integrating textile muscles into items of clothing."

In other research, a new smart textile was developed from carbon nanotube and spandex fibers that can both sense and move in response to a stimulus like a muscle or joint. Scientists at the ARC Centre of Excellence for Electromaterials Science (ACES) have integrated intelligent sensors into a fabric sleeve device. For example, a knee sleeve that can be used to monitor the movement of the joint will be helpful for providing data that is valuable in creating a personalized training or rehabilitation plan for the wearer. 

Collaborations between research facilities and teams are key to bringing together the breadth of expertise represented in materials science discoveries and making it possible to create practical, working nanotextiles and flexible exoskeletons.

Conclusion

Recent material developments are enabling lightweight, flexible exoskeletons to be made out of knitted or woven fabrics. Such pieces could also double as monitoring devices or integrated into clothing with the goal of providing greater independence for the mobility impaired.