Soft MAEs can perform a range of modes of deformation, including elongation, contraction and bending with a magnetic field. They are highly promising in medicine and healthcare, such as drug delivery, biomedical applications and disease treatment. However, to date, there are few applications in rehabilitation and assistance since the output force is small and the actuation systems need to be designed to be less bulky and more versatile in their deformation.
Liquid crystalline elastomers (LCEs)
Liquid crystalline elastomers (LCEs) are materials that combine the entropy elasticity of polymer elastomers with the self-organisation of liquid crystals [90]. LCEs can reversibly change shape to generate motion in response to a range of stimuli such as temperature, pH variations, light, electric, or magnetic fields, which enables them to be used as soft actuators. Ferrantini et al. reported a biocompatible acrylate-based light-responsive LCE that can be activated continuously for long periods of time with a low light intensity within a physiological environment [91]. The materials are able to act as artificial muscles and reproduce the cardiac twitch contractions. The mechanical properties of the LCE can also be modulated by light intensity, stimulation frequency and duty cycle to fit different contraction amplitude/time courses of human muscle. To prove this concept, actuation tests were performed on a mouse muscle. The results showed the LCEs can improve muscular systolic function, with no impact on diastolic properties, which indicated LCEs are promising in assisting cardiac mechanical function and developing a new generation of contraction assist devices.
LCEs are new to the field of soft rehabilitation and assistive devices, and the actuation mechanism is more complex than other systems such as fluid-powered and electrical driven systems. They can reversibly change shape to generate motion in response to a range of stimuli such as temperature, pH variations, light, electric fields, or magnetic fields. However, the dynamics and responses of LCEs are difficult to control accurately. The state-of-the-art technology uses a feedforward control mechanism to achieve the desired motion. There are few soft rehabilitation and assistive device prototype in the field, although the concept of the cardiac contraction device [91] is a highly promising start in the field by using LCEs to assist the cardiac treatment and further applications can emerge from improved system design and control.
Piezoelectric materials
Piezoelectric materials are materials that produce an electric charge when they are placed under mechanical stress (sensor) or provide a strain in response to an applied electric field (actuator). Polyvinylidene fluoride (PVDF) is one of the most promising piezoelectric materials for soft robot applications, as it possesses superiority in an ability to be formed in thin-film form, low weight and high mechanical flexibility. However, in actuation mode, the strains are low, the forces capability is small, and they require high driving electric fields. For soft rehabilitation and assistive applications, they are therefore widely used as a sensing element by converting mechanical energy into the electrical signal and show excellent performance such as high sensitivity (volts per applied stress), increased flexibility, lightweight, and self-powered characteristics since a charge is generated by the direct piezoelectric effect which is directly proportional to the level of applied force. Asadnia et al. developed a highly stretchable, self-powered and ultrasensitive strain sensor for fingers using PVDF nanofibers [92]. Using aligned PVDF nanofibers, gold electrodes were fixed on a liquid-crystalline polymer substrate using a conductive carbon tape. A soft assistive glove with two sensors mounted on the middle and index fingers was fabricated for recognition of finger motion. The sensing feedback provided by the soft gloves provided the potential to open up new avenues in rehabilitation engineering and brings a sense of touch to myoelectric limbs. Alluri et al. developed a simple, large-scale fabrication technique to form spherical piezoelectric composite (BaTiO3 nanoparticle/Ca-alginate) beads using an ionotropic gelation method [93]. A high output (82 V, 227 mA) was generated during the application of a low mechanical pressure of 1.70 kPa; in addition to providing a sensing functionality, the voltage/current was reported to be sufficient to drive low power electronic devices. The device made from this novel piezoelectric material was able to act as a self-powered wearable flexion sensor for decoding right arm finger flexion/extension movements. This novel piezoelectric material is non-invasive, robust, cost-effective. It does not require external power for wearable assistive devices, unlike piezoresistive sensors that require an applied current to measure a change of resistance with stress. For gait rehabilitation, Ahmad et al. designed a novel PVDF pressure sensor based on d33-mode polarisation for sensing gait motion, where d33 corresponds to a force applied in the polarisation direction of the piezoelectric material [94]. In contrast to the d31-mode of polarisation, where the materials is polarised in the thickness direction, the d33-mode of polarisation applies the electric field between two conductors and polarised laterally. A round-shaped prototype with a diameter of 15 mm was able to achieve a sensing sensitivity of approximately 1.85 mV/kg for load ranging from 1 kg to 5 kg. On increasing the diameter to 20 mm, the sensor could be used for a load between 38 kg to 92 kg, which is suitable for gait rehabilitation system. Rajala et al. developed a piezoelectric polymer film in-sole sensor for plantar pressure distribution measurements. The sensor was made of PVDF with evaporated copper electrodes to collect the piezoelectric charge [95]. A 200 nm thick film of copper was evaporated on both sides of the PVDF substrate using a mechanical mask to produce the sensor electrode pattern. The sensor consisted of eight measurement locations: hallux, first metatarsal head (under both sesamoid bones), metatarsal heads 2-5 and heel. The average peak-to-peak pressure ranged from 58 kPa to 486 kPa and the sensitivity was approximately 28.5 ± 1.0 pC/N. The sensor can be used for rehabilitation and sport monitoring for its long-term and cost-efficient plantar pressure measurement. To identify gait patterns for lower limb rehabilitation, Ma et al. built an intelligent perception system with multiple sensors that included (i) surface electromyography (EMG) sensors for detecting human intention, (ii) photoelectric encoders for determining the angle of hip and knee joints, and (iii) a piezoelectric thin-film sensor to assess the interaction force between the body and the exoskeleton [96]. The interaction forces of both thigh and calf could be measured, and the results showed that the correct rate for gait recognition is up to 92%. Compared to the conventional method that detects gait patterns using a ground reaction force, the new gait recognition system was easy to fabricate and could be used for soft lower limb rehabilitation devices.
Flexible piezoelectric materials are often used as sensing elements in the applications of rehabilitation and assistance since the power levels generated by ferroelectric polymers are relatively low. Due to their need for high electric fields for actuation, it would be highly changeling to apply them as self-powered soft actuators for soft assistive devices directly; their low strain and stiffness also limits their use in actuation. However, the integration of flexible piezoelectric sensing elements with other actuation mechanisms is highly promising for developing intelligent soft rehabilitation and assistive devices.
Rehabilitation and assistance enabled by chemical reactions
The use of chemical reactions is new to soft actuators and robotic devices for rehabilitation and assistance, and the development of rehabilitation and assistive devices based on this intriguing actuation mechanism is still in its infancy. Technical challenges remain in the aspects of actuation output energy, safety and designs related to human-interaction devices. Hosono et al. developed a metal hydride (MH) alloy which can absorb and release 1000 times as much hydrogen gas as its own volume [97]. The reversible chemical reaction is based on an alloy (M) that absorbs hydrogen (H2) by forming a metal hydride (MHx), with the release of heat as shown in equation (1):
\(\left(\frac{2}{x}\right)M+H2\ \leftrightarrow\left(\frac{2}{x}\right)MHx+Q\) (1)
where Q is the quantity of heat involved in the reaction. The hydrogen pressure changes depending on the hydrogen content and temperature to allow actuator control. The conversion mechanism between thermal energy and mechanical energy of a typical MH alloy is shown in Figure 11a.
The reaction can proceed at a constant hydrogen pressure (plateau pressure), and a constant temperature. The metal hydride actuator consisted of two soft bellows which are made from laminated casted polypropylene film (CPP), aluminium (Al) and polyester (PET) films and two hydrogen supplies with metal hydride alloys [97]; see Figure 11b. The total thickness of the laminated film is approximately 0.1 mm. The soft bellows can realise flexion and extension motion by heating and cooling the metal hydride alloys, which exhibit high impermeability to hydrogen gas, flex durability and sufficient passive compliance. Taking these advantages, a toe joint rehabilitation device was developed using metal hydride actuators [98-99], as shown in Figure 11c. The actuator was able to achieve a maximum output force of approximately 200 N and a maximum velocity of 50 mm/min with a load of 5 kg. It can also operate 3500 cycles at 30 mm/min with a 2.5 kg load. A MH alloy used for a toe joint device was able to operate in the range of 0.1-1.0 MPa at a temperature of 20-80°C. The high impermeability to hydrogen gas of the soft bellows was examined by filling with hydrogen gas and monitoring the pressure change. The results showed approximately 0.7% pressure depression in the bellows after 240 hours, which validated the high impermeability of the materials used to construct the device. The device driven by the heating and cooling mechanism can provide gentle and slow motion of the toes, which is both human and environment friendly.