# Touch, Texture and Haptic Feedback: A Review on How We Feel the World around Us
## 4 Haptic Feedback
### Kinesthetic Feedback
#### 4.1.2 Exoskeleton-Based Force Feedback System  
Exoskeleton-based force feedback systems have seen increased interest over the past few years due to the increasing popularity of VR systems. Unlike grounded force feedback systems, wearable devices are not limited to a constrained area as they require the user to be able to move around freely while still being able to experience haptic feedback on a more general range. Wearable devices aim to provide the body with natural degrees of freedom. The device must be able to conform to the user without impairing their movement. Like grounded force feedback systems, wearable force feedback systems utilize actuators which are attached directly onto the body to either admit or impede their movement. Different types of actuators have been used and developed and include string-based systems and pump-based systems. The fundamental principle behind these devices remains largely the same with those of grounded force feedback systems as they also aim to either admit or impede movement of the body. One of the most common actuator types used in wearable feedback devices are string-based actuators. Cables attached to motors are fixed around key areas within the body such as joints and fingertips which control movement and are controlled via motors which adjust the amount of force onto the string causing them to tense up in response to an applied force. String-based actuators generally require simple design and construction with fast and reliable manipulation and can easily be scaled up, making them popular for many DIY projects. [PERSON] et al. developed a Bowden cable actuator for use in force feedback exoskeletons in the arm [116]. Within a Bowden transmission, a cable is guided inside a sheath and connected on both ends to the motor and robotic joint in a pull-pull configuration. [PERSON] et al. developed the ExoTen glove [78] which utilizes a twisted-string-based system. Two strings are attached to the ends of a rotative DC motor and a moving element. As the DC motor rotates, the length of the strings is shortened which generates a linear motion onto the moving element. [PERSON] et al. also made use of a Bowden cable-based actuator system in the design of their 7-DOF arm-based exoskeleton. The design of the device is based on three groups of joints based on the main arm divisions. A set of actuators was placed on the shoulder joints, then elbow joint for forearm movements, and finally the wrists for hand movement. The cable was then attached to the fixed actuator base and rotary joints to facilitate movement [117]. Pump-based actuators are also used in wearable devices. In these systems, a pump is used to convert electrical energy into fluid or gas pressure which is used to drive the system. Key challenges in these types of systems generally involve the size of the actuation system and the overall bulk of it all, due to requiring thick wires and a pump system for these to function. [PERSON] et al. utilized a micro-hydraulic system to contract and relax artificial muscles for force feedback applications [98]. In this research, they made use of McKibben muscles wherein an expandable tube is wrapped in a braided veil while filled with working fluid, allowing the volume and pressure within the tube to be controlled. [PERSON] et al. designed a force feedback glove using pneumatically powered artificial muscles [118]. The glove includes four sets of pneumatic artificial muscles, each designed for a specific range of movement such as flexion, extension, pronation, and supination which mimic how human muscle works by contracting and relaxing based on the air pressure provided by the valves. [PERSON] et al. developed a soft pneumatically powered exoskeleton glove with 20-DOF movement [119]. In this research, two pairs of artificial muscles were attached to each finger to represent flexor and extensor movements of the fingers allowing for multiple postures for a total of 20-DOF on all five fingers. The study also made use of McKibben muscles to more closely mimic human muscle movement.