Conclusions and outlooks

Soft actuators and robots enabled by fluid-powered, soft active materials, and chemical reactions have been successfully used in rehabilitation and assistance for hands, wrists, ankles, joes, backs, lower and upper limbs. They can provide the advantages of low cost, ease of manufacture, and high actuation performance and have become a research topic of intense research interest in the field of soft robotics. The soft characteristics of actuators and devices make it possible to improve safe interactions between humans and devices. Not only can the soft actuators and robotic devices be used for medical rehabilitation to provide effective and professional treatment, but they can also be used for assisting in daily activities such as walking, lifting and climbing. While the applications of rehabilitation platforms and assistive devices have previously focused on rigid robotic systems, recent advances in soft actuation have opened up new opportunities for creating new soft rehabilitation and assistive devices.  This review has described the current state-of-the-art technologies in this area to demonstrate the nexus between materials, mechanisms, actuation, and applications. We have summarized the latest actuation mechanisms using in soft rehabilitation and assistive devices, including pneumatic and hydraulic fluid-powered actuation, electrical motor actuation, chemical reactions and soft active material-based actuation, which includes dielectric elastomers, shape memory alloys, magnetoactive elastomers, liquid crystalline elastomers, and piezoelectric materials. The advantages and challenges of these devices have been discussed and there are a number of opportunities as to make these devices efficient, portable, wearable, and user-friendly, which are now described.
(1) To realise wearable and portable assistive devices, there is a need to develop compact, safe and efficient power supplies, which can be readily integrated into devices and act as a reliable power source. Assistive devices should fulfil critical safety requirements prior to their practical applications on the human body. For fluid power pneumatic and hydraulic systems, the size of the power supplies has limited the development of untethered assistive devices. For electric field-driven systems, the need for high applied voltages has led to safety concerns for the direct use of devices on humans. Although significant effort has been paid to the development of compact, safe and efficient power sources, the range of currently available power supplies is still limited in terms of their sizes and power mechanisms. In this regard, the design and development of compact, high power density and safe power units for assistive devices will be particularly important.
(2) There is a need to design new and effective soft actuators which can be attached, embedded, or integrated into rehabilitation and assistive devices, such as soft wearable suits. The use of a distributed network of soft pneumatic actuators to develop assistive devices for upper and lower bodies is highly promising, in particular for providing a large range of motion. Clothing-based rehabilitation and assistive devices are able to achieve high degrees of freedom and complex movements, benefiting from the direct guidance and attachment to human bodies. Advanced human-machine interaction and interface technologies can further enhance the functionality of the current state-of-the-art devices. More innovative soft actuators will emerge when actuation performance and mechanisms are combined in synergy with advanced materials with tailored properties.
(3) The state-of-the-art control strategies of soft actuators and robots for rehabilitation and assistance represent a technical challenge. Control strategies are relatively simple as it is challenging to develop accurate models of soft systems, as well as human users. Advanced control algorithms and modelling techniques provide opportunities to enable current control systems to provide a rapid and accurate response, a high degree of robustness, and adaptability. In addition, control parameters of such devices are usually determined or adjusted by experience or use a trial-and-error approach for individuals. Automated customised approaches for the determination of the control parameters will be highly useful in the field and therefore artificial intelligence (AI), such as machine learning, can be used for the design of control systems that are tailored to the needs of individuals.
(4) The muscle-like mechanical properties and biocompatibility of soft active materials provide significant opportunities for the development of next-generation soft rehabilitation and assistive applications at a range of scales; ranging from the applications in medical treatment to devices for daily use. The actuation mechanism is one of the challenges for widely applying these materials. For example, high-voltages and electric fields are usually needed for soft dielectric elastomer actuator-based actuators, which can be a safety concern for rehabilitation and assistive devices. However, with the development of new materials, it is believed that suitable and safe actuation and power mechanisms will be developed, which will significantly expand the applications of smart materials in rehabilitation and assistance. Flexible piezoelectric materials such as polyvinylidene difluoride (PVDF) have been successfully used as sensing elements in rehabilitation and assistive devices. The advantages of flexibility, accuracy, and cost-efficiency of PVDF makes it ideally suited for integrating into these devices for motion sensing and detection. With the development of soft rehabilitation and assistive devices, it is believed that flexible piezoelectric materials can be applied more widely in the field.
(5) Soft MAEs and LCEs are very new to the field of soft rehabilitation and assistive devices. There are few applications and prototypes, and the majority of work to date is at the concept stage. However, the advantages of MAEs and LCEs have shown promising results in medicine, biomedical and healthcare. Future research directions can include (i). the design and development of compact, portable MAEs assistance systems for individuals at home. This will expand the areas and provide sustainable development for independent living and healthcare. (ii). the design of advanced control systems to accurately control the rehabilitation and assistance devices to suit individual needs. The state-of-the-art control systems for MAEs and LCEs are based on the feedforward control structure. Effective sensing elements and feedback control systems would result in a breakthrough in the area, which can accelerate the development of assistance devices. (iii). The output force of the MAEs and LCEs are relatively small, compared to the fluid-powered systems. The creation and fabrication of new materials can directly benefit the area for more possible applications, for example, lower and upper limbs rehabilitation and movement support.
(6) The use of chemical reactions is new to soft actuators and robotic devices for rehabilitation and assistance, but it is a highly promising approach as a human and environmentally friendly actuation mechanism. The storage of actuator chemicals is one of the challenges for the implementation on rehabilitation and assistive devices, which require safe and consistent energy supplies. In addition, the control of the chemical reaction and energy release can also be challenging for assistive devices, which requires accurate and robust control to provide a comfortable user experience. However, this is still an emerging area to explore, and should not be limited with regards to the creative and effective design for rehabilitation and assistive devices, which can combine new actuation mechanisms, materials, sensing and control technologies.
(7). Soft rehabilitation and assistive devices are complex and highly dynamic systems. Unlike individual soft actuators and robot, they are a combination of a variety of actuators, sensors and a dedicated control system, which make it difficult to directly model system dynamics and predict device performance. The understanding of people’s needs is crucial to the design of the devices. In addition, although soft devices provide benefits of safer human-device interaction, the difficulty in precise control of the devices provides research challenges. The creation of soft intelligent rehabilitation and assistance devices for healthcare treatment and daily activities assistance is rapidly needed.
While soft actuators and robotic devices for rehabilitation and assistance are in their infancy stage, there continues to be significant scope and opportunities for research and development in this emerging new research area. With a broad multidisciplinary research effort in the pursuit of new soft actuators and systems, it can be envisioned that these new emerging fields will achieve exciting applications in the future and continuously improve our quality of life.

Acknowledgements

We thank the support from The Leverhulme Trust for the Leverhulme Research Fellowship RF-2020-503\4, the UKRI Innovate UK The Sustainable Innovation Fund, No.79502, the University of Bath Alumni Fund F1920A-RS02 and the University of Bath International Funding Schemes 2020. C.Y and X.L thank the support from the China Scholarship Council PhD studentship (201706150102) and the China Scholarship Council visiting Scholar fund (202006150085).

References

[1] S. Bauer, S. Bauer‐Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger, R. Schwödiauer, Advanced Materials. 2014, 26(1), pp.149-162.
[2] D. Rus, M.T. Tolley, Nature. 2015, 521(7553), pp.467-475.
[3] M. Cianchetti, C. Laschi, A. Menciassi, P. Dario, Nature Reviews Materials. 2018, 3(6), pp.143-153.
[4] T. George Thuruthel, Y. Ansari, E. Falotico, C. Laschi, Soft Robotics. 2018, 5(2), pp.149-163.
[5] S. Coyle, C. Majidi, P. LeDuc, K.J. Hsia, Extreme Mechanics Letters. 2018, 22, pp.51-59.
[6] S. Chen, Y. Cao, M. Sarparast, H. Yuan, L. Dong, X. Tan, C. Cao, Advanced Materials Technologies. 2020, 5(2), p.1900837.
[7] A. Chortos, J. Liu, Z. Bao, Nature Materials. 2016, 15(9), pp.937-950.
[8] D. Chen, Q. Pei, Chemical Reviews. 2017, 117(17), pp.11239-11268.
[9] T. Someya, M. Amagai, Nature biotechnology, 2019, 37(4), pp.382-388.
[10] A.J. Bandodkar, J. Wang, Trends in biotechnology, 2014, 32(7), pp.363-371.
[11] W. Zeng, L. Shu, Q. Li, S. Chen, F. Wang, X.M. Tao, Advanced Materials, 2014, 26(31), pp.5310-5336.
[12] A. Nag, S. C. Mukhopadhyay, J. Kosel, IEEE Sensors Journal, 2017. 17(13), pp.3949-3960.
[13] I. Díaz, J.J. Gil, E. Sánchez, Journal of Robotics, 2011. 2011, 759764,
[14] H.S. Lo, S. Q. Xie, Medical Engineering & Physics, 2012, 34(3), pp.261-268.
[15] N. Aliman, R. Ramli, S. M. Haris, Robotics and Autonomous Systems, 2017, 95, pp.102-116.
[16] C. Y. Chu, R. M. Patterson, Journal of Neuroengineering and Rehabilitation, 2018, 15(1), pp.1-14.
[17] Z. M. Tsikriteas, J. I. Roscow, C.R. Bowen, H. Khanbareh, iScience, 2020, p.101987.
[18] H.R. Lim, H.S. Kim, R. Qazi, Y.T. Kwon, J.W. Jeong, W.H. Yeo, Advanced Materials, 2020, 32(15), p.1901924.
[19] Q. Lyu, S. Gong, J. Yin, J. M. Dyson, W. Cheng, Advanced Healthcare Materials, 2021, p.2100577.
[20] Q. Xu, X. Gao, S. Zhao, Y. N. Liu, D. Zhang, K. Zhou, H. Khanbareh, W. Chen, Y. Zhang, C. Bowen, Advanced Materials, 2021, p.2008452.
[21] V. Sanchez, C. Walsh, and R. Wood, Advanced Functional Materials, 2021, vol. 31, no. 6.
[22] R.H. Gaylord, S. Heights, 1958, U.S. Patent 2,844,126.
[23] J.R.Allen, A. Karchak, R. Snelson, abstract in Mechanical Engineering, 1962, August: 52–53.
[24] T. J. Engen, The Journal of Bone and Joint Surgery. British volume, 1965, 47(3), pp.465-468.
[25] V. L. Nickel, D. L. Savill, A. Karchak, J. R. Allen, The Journal of Bone and Joint Surgery. British volume, 1965, 47(3), pp.458-464.
[26] B. Tondu, Journal of Intelligent Material Systems and Structures, 2012, 23(3), pp.225-253.
[27] K.A.Shorter, G.F. Kogler, E. Loth, W.K. Durfee, E. T. Hsiao-Wecksler, Journal of Rehabilitation Research & Development, 2011, 48(4).
[28] Y. L. Park, B. R. Chen, D. Young, L. Stirling, R. J. Wood, E. Goldfield, R. Nagpal, IEEE/RSJ International Conference on Intelligent Robots and Systems, 2011, pp. 4488-4495. IEEE.
[29] C.M. Thalman, H. Lee, IEEE International Conference on Robotics and Automation (ICRA), 2020, pp. 1735-1741. IEEE.
[30] M. Wehner, B. Quinlivan, P.M. Aubin, E. Martinez-Villalpando, M. Baumann, L. Stirling, K. Holt, R. Wood, C. Walsh, IEEE international conference on robotics and automation, 2013, pp. 3362-3369. IEEE.
[31] C. Thakur, K. Ogawa, T. Tsuji, Y. Kurita, IEEE Robotics and Automation Letters, 2018, 3(4), pp.4257-4264.
[32] F. Z. Low, R.C. Yeow, H. K. Yap, J. H. Lim, IEEE International Conference on Rehabilitation Robotics (ICORR), 2015, pp. 589-593. IEEE.
[33] R. Ezzibdeh, P. Arora, D.F. Amanatullah, Arthroplasty Today, 2019, 5(3), pp.314-315.
[34] D. Baiden, O. Ivlev, IEEE 13th International Conference on Rehabilitation Robotics (ICORR), 2013, pp. 1-6. IEEE.
[35] O. Ivlev, IEEE International Conference on Rehabilitation Robotics, 2009, pp. 1-5. IEEE.
[36] D. Baiden, O. Ivlev, The 23rd International Conference on Robotics in Alpe-Adria-Danube Region (RAAD), 2014, pp. 1-6. IEEE.
[37] J. Fang, J. Yuan, M. Wang, L. Xiao, J. Yang, Z. Lin, P. Xu, L. Hou, Soft Robotics, 2020, 7(1), pp.95-108.
[38] A.Wilkening, H. Stöppler, O. Ivlev, IEEE International Conference on Rehabilitation Robotics (ICORR), 2015, pp. 729-734. IEEE.
[39] H. Li, L. Cheng, The 32nd Youth Academic Annual Conference of Chinese Association of Automation (YAC), 2017, pp. 860-865. IEEE.
[40] H. K. Yap, B. W. Ang, J. H. Lim, J. C. Goh, C. H. Yeow, IEEE International Conference on Robotics and Automation (ICRA), 2016, pp. 3537-3542. IEEE.
[41] Yap, H.K., Mao, A., Goh, J.C. and Yeow, C.H., 2016, June. Design of a wearable FMG sensing system for user intent detection during hand rehabilitation with a soft robotic glove. In 2016 6th IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob) (pp. 781-786). IEEE.
[42] Yap, H.K., Khin, P.M., Koh, T.H., Sun, Y., Liang, X., Lim, J.H. and Yeow, C.H., 2017. A fully fabric-based bidirectional soft robotic glove for assistance and rehabilitation of hand impaired patients. IEEE Robotics and Automation Letters, 2(3), pp.1383-1390.
[43] K. Shiota, S. Kokubu, T. V. Tarvainen, M. Sekine, K. Kita, S. Y. Huang, W. Yu, Robotics and Autonomous Systems, 2019, 111, pp.20-30.
[44] F. Cho, R. Sugimoto, T. Noritsugu, X. Li, IOP Conference Series: Materials Science and Engineering, 2017, 249(1), p. 012004. IOP Publishing.
[45] T. Abe,  S. Koizumi, H. Nabae, G. Endo, K. Suzumori, IEEE International Conference on Soft Robotics (RoboSoft), 2018, pp. 572-578. IEEE.
[46] T. Abe, S. Koizumi, H. Nabae, G. Endo, K. Suzumori, N. Sato, M. Adachi, and F. Takamizawa, IEEE Robotics and Automation Letters, 2019, 4(3), pp.2532-2538.
[47] B. W. Ang, C.H. Yeow, 2nd IEEE International Conference on Soft Robotics (RoboSoft) 2019, pp. 577-582. IEEE.
[48] P. Polygerinos, Z. Wang, K. C. Galloway, R. J. Wood, C. J. Walsh, Robotics and Autonomous Systems, 2015, 73, pp.135-143.
[49] P. Polygerinos, P., Galloway, K.C., Savage, E., Herman, K. O'Donnell, C. J. Walsh, IEEE international conference on robotics and automation (ICRA), 2015, pp. 2913-2919. IEEE.
[50] A. T. Asbeck, S. M. De Rossi, I. Galiana, Y. Ding, C. J. Walsh, IEEE Robotics & Automation Magazine, 2014, 21(4), pp.22-33.
[51] Y. Mengüç, Y. L. Park, H. Pei, D. Vogt, P. M. Aubin, E. Winchell, L. Fluke, L. Stirling, R. J. Wood, C. J. Walsh, The International Journal of Robotics Research, 2014, 33(14), pp.1748-1764.
[52] A. Atalay, V. Sanchez, O, Atalay, D. M. Vogt, F. Haufe, R. J. Wood, C. J. Walsh, Advanced Materials Technologies, 2017, 2(9), p.1700136.
[53] A. T. Asbeck, S. M. De Rossi, K. G. Holt, C. J. Walsh, The International Journal of Robotics Research, 2015, 34(6), pp.744-762.
[54] L. N. Awad, J. Bae, K. O’donnell, S.M.De Rossi, K. Hendron, L. H. Sloot, P. Kudzia, S. Allen, K. G. Holt, T. D. Ellis, C. J. Walsh, Science Translational Medicine, 2017, 9(400).
[55] Y. Ding, M. Kim, S. Kuindersma, C. J. Walsh, Science Robotics, 2018, 3(15).
[56] B. T. Quinlivan, S.  Lee, P. Malcolm, D. M. Rossi, M. Grimmer, C. Siviy, N. Karavas, D. Wagner, A. Asbeck, I. Galiana, C.J. Walsh, Science Robotics, 2017, 2(2), p.eaah4416.
[57] L. Stirling, C. H. Yu, J. Miller, E. Hawkes, R. Wood, E. Goldfield, R. Nagpal, Journal of Materials Engineering and Performance, 2011, 20(4-5), pp.658-662.
[58] Y. J. Lai, L.J. Yeh, M. C. Chiu, Sensors and Actuators A: Physical, 2012, 173(1), pp.210-218.
[59] A. Hadi, K. Alipour, S. Kazeminasab, M. Elahinia, Journal of Intelligent Material Systems and Structures, 2018, 29(8), pp.1575-1585.
[60] S. Kazeminasab, A. Hadi, K. Alipour, M. Elahinia, M., Industrial Robot, 2018, 45(5).
[61] M. Hillman, Advances in Rehabilitation Robotics, 2004, pp. 25-44. Springer, Berlin, Heidelberg.
[62] D. Serrano, D.S. Copaci, L. Moreno, D. Blanco, IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), 2018, pp. 2318-2323. IEEE.
[63] A. Villoslada, A. Flores, D. Copaci, D. Blanco, L. Moreno, L., Robotics and Autonomous Systems, 2015, 73, pp.91-101.
[64] D. Copaci, D. Blanco, L. Moreno,  Proceedings of the Joint Workshop on Wearable Robotics and Assistive Devices, International Conference on Intelligent Robots and Systems, 2016, pp. 9-14, Daejeon, Korea.
[65] D. Copaci, A. Flores, F. Rueda, I. Alguacil, D. Blanco, L. Moreno, Converging Clinical and Engineering Research on Neurorehabilitation II. Biosystems & Biorobotics, 2017, 15, pp. 477-481. Springer.
[66] K. K. Wu, Foot orthoses: principles and clinical applications, 1990, vol. 26, Williams & Wilkins.
[67] M. G. Mataee, M. T. Andani, M. Elahinia, Journal of Intelligent Material Systems and Structures, 2015, 26(6), pp.639-651.
[68] F. Sadeghian, M. R. Zakerzadeh, M. Karimpour, M. Baghani, Journal of Intelligent Material Systems and Structures, 2018, 29(15), pp.3136-3150.
[69] C. T. Hau, D. Gouwanda, A. A. Gopalai, L.C. Yee, F.A.B. Hanapiah, 39th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC) 2017, pp. 946-949. IEEE.
[70] S. Pittaccio, L. Garavaglia, C. Ceriotti, F. Passaretti, Journal of functional biomaterials, 2015, 6(2), pp.328-344.
[71] P. Joudzadeh, A. Hadi, K. Alipour, 4th International Conference on Robotics and Mechatronics (ICROM), 2016, pp. 530-535. IEEE.
[72] S.J. Park, C.H. Park, Scientific Reports, 2019, 9(1), pp.1-8.
[73] L.J. Romasanta, M.A. López-Manchado, R. Verdejo, Progress in Polymer Science, 2015, 51, pp.188-211.
[74] M. Lidka, A.D. Price, A.L. Trejos, IEEE Canadian Conference on Electrical & Computer Engineering (CCECE), 2018, pp. 1-4. IEEE.
[75] A. Behboodi, S.C.K. Lee, IEEE 16th International Conference on Rehabilitation Robotics (ICORR), 2019, pp. 499-505. IEEE.
[76] Compliant Transducer Systems GmbH, http://www.ct-systems.ch/.
[77] M. Duduta,  E. Hajiesmaili, H. Zhao, R.J. Wood, D. R. Clarke, Proceedings of the National Academy of Sciences, 2019, 116(7), pp.2476-2481.
[78] F. Carpi, A. Mannini, D. De Rossi, Electroactive Polymer Actuators and Devices (EAPAD), 2008,  6927, p. 692705. International Society for Optics and Photonics.
[79] A. Behboodi, C. DeSantis, J. Lubsen, S.C.K. Lee, 42nd Annual International Conference of the IEEE Engineering in Medicine & Biology Society (EMBC), 2020, pp. 4930-4935. IEEE.
[80] L. Saharan, A. Sharma, M.J. de Andrade, R.H. Baughman, Y. Tadesse, Active and Passive Smart Structures and Integrated Systems, 2017, Vol. 10164, p. 1016428. International Society for Optics and Photonics.
[81] H. Amin, S.F. Assal, IEEE International Conference on Mechatronics and Automation (ICMA), 2018, pp. 997-1002. IEEE.
[82] G. Kofod, Journal of Physics D: Applied Physics, 2008, 41(21), p.215405.
[83] Y. Li, M. Hashimoto, Smart Materials and Structures, 2017, 26(12), p.125003.
[84] S. Pourazadi, S. Ahmadi, C. Menon, Smart Materials and Structures, 2014, 23(6), p.065007.
[85] E.Y. Kramarenko, A.V. Chertovich, G.V. Stepanov, A.S. Semisalova, L.A. Makarova, N.S. Perov, A.R. Khokhlov, Smart Materials and Structures, 2015, 24(3), p.035002.
[86] G.V. Stepanov, E.Y. Kramarenko, D.A. Semerenko, Journal of Physics: Conference Series, 2013, 412(1), p. 012031. IOP Publishing.
[87] L.A. Makarova, T.A. Nadzharyan, Y.A. Alekhina, G.V. Stepanov, E.G. Kazimirova, N.S. Perov, E.Y. Kramarenko, Smart Materials and Structures, 2017, 26(9), p.095054.
[88] T.A. Nadzharyan, L.A. Makarova, E.G. Kazimirova, N.S. Perov, E.Y. Kramarenko, 2018. Influence of the geometry on magnetic interactions in a retina fixator based on a magnetoactive elastomer seal. Journal of Physics: Conference Series, 2018, 994(1), p.012002.
[89] Y.A. Alekhina, L.A. Makarova, S.A. Kostrov, G.V. Stepanov, E.G. Kazimirova, N.S. Perov, E.Y. Kramarenko, 2019. Journal of Applied Polymer Science, 2019, 136(17), p.47425.
[90] C. Ohm, M. Brehmer, R. Zentel, Advanced Materials, 2010, 22(31), pp.3366-3387.
[91] C. Ferrantini, J.M. Pioner, D.  Martella, R. Coppini, N. Piroddi, P. Paoli, M. Calamai, F.S. Pavone, D.S. Wiersma, C. Tesi, E. Cerbai, Circulation research, 2019, 124(8), pp. e44-e54.
[92] M. Asadnia, A.G.P. Kottapalli, J.M. Miao, M.S. Triantafyllou, 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 2015, pp. 678-681. IEEE.
[93] N.R. Alluri, S. Selvarajan, A. Chandrasekhar, B. Saravanakumar, J.H.  Jeong, S.J. Kim, 2017. Composites Science and Technology, 2017, 142, pp.65-78.
[94] K.A. Ahmad, M.F.A. Rahman, R. Boudville, A.A. Manaf, N. Abdullah, 12th International Conference on Sensing Technology (ICST), 2018, pp. 149-153. IEEE.
[95] S. Rajala, R. Mattila, I.  Kaartinen, J. Lekkala, IEEE Sensors Journal, 2017, 17(20), pp.6798-6805. IEEE.
[96] W. Ma, X. Zhang, G. Yin, 13th International Conference on Ubiquitous Robots and Ambient Intelligence (URAI), 2016, pp. 587-592. IEEE.
[97] M. Hosono, S. Ino, M. Sato, K. Yamashita, T. Izumi. T. Ifukube, Proceedings of the 3rd Asia International Symposium on 0echatronics,2008, p. 473.
[98] S. Ino, M. Hosono, M. Sato, S. Nakajima, K. Yamashita, T. Izumi, World Congress on Medical Physics and Biomedical Engineering, Munich, Germany, 2009, pp. 287-290, Springer, Berlin, Heidelberg.
[99] S. Ino, M. Sato, M. Hosono, T. Izumi, Sensors and Actuators B: Chemical, 2009, 136(1), pp.86-91.