The large strain, high compliance and fast response of DEAs provide the advantages for the development of soft rehabilitation and assistive devices. However, most of DEA-based devices remain in the concept stages and there are few prototypes for real-life use. This is because the drive mechanism for DEAs normally needs a high electric field, and therefore drive voltage, which can cause human-device interaction safety issue, in addition to the bulky size of the devices. Since the actuation material is a polymer the force provided by an individual DEA (1 – 10 N) is normally not sufficient to assist or support human movements. A variety of DEA structures, such as stacked and multilayer DEAs [75, 77, 83], folded DEAs [78] and a spring-roll DEA structure [81] are used to realise larger output force. For example, a maximum output force of a PVC gel-based wearable assistive device with a multilayer DEA structure can achieve 94 N, which can support the hip joint. However, it was high in weight and bulky in terms of size. To bring DEAs to the real assistive devices, there are four key challenges (i) the safety consideration of using high electrical field, and high drive voltages, when actuating DEAs. (ii) the need of compactness for the devices (iii) the improved performance of DEAs to generate larger force (iv) the current state-of-the-art DEA-based devices are based on feedforward control. Effective, efficient and robust control systems are needed to control the devices to suit rehabilitation and people’s needs.
Magneto-active elastomers (MAEs)
A magneto-active elastomer (MAE) is a composite made from an elastomeric matrix and magnetisable fillers [85]. In the presence of a magnetic field, soft MAEs are able to induce elongation, contraction, bending, and a range of other modes of deformation [86]. The magnetisation profile, actuation signal, and overall shape of filler materials are the major parameters that affect MAE deformation. To date, MAEs have been developed for soft robots targeted for biomedical devices and drug delivery, while there are fewer reports of MAE-based devices for rehabilitation and assistance due to the relatively low output force, actuation mechanism and control strategies. Nadzharyan et al. proposed a concept of a retinal fixator for retinal detachment surgery using MAEs, which consisted of a silicone elastomer matrix and embedded magnetic iron macroparticles [87]. The elastic modulus of the MAE was 104 kPa, while the stress and relative strain at break was 0.3 MPa and 141%, respectively. The fixator applies an MAE, which is used as a patch that is placed inside the eye on the retina surface to act as a seal. Permanent magnets covered with medical silicone are attached to the sclera to locate and fix the inner MAE patch, as shown in Figure 10a. The two components can be closely attached using the magnetic force to fix the retina to the underlying tissues. The relationship between the attraction force and their separation distance in a flat 'patch-magnet' system was studied by Finite Element Analysis and experimental validation. An enhanced theoretical model which includes the 'patch-magnet' system with a range of possible geometric configurations was also developed and used for investigating the effects of the change in the geometry [88][89]. The results showed that small changes in the geometry have strong effects on the interaction force and magnetic pressure. Figure 10b shows the relationship of an average pressure achieved from the measured interaction force and the surface area of the MAE sample with the distance between an MAE and the magnets. Force measurements were performed for both flat and curved configurations. The results showed that small size systems can produce considerable attraction force and can be potentially applied for retinal detachment surgery. It will be of interest to exploit such MAE force generation and fixture mechanisms to other soft systems.