The Opportunities and Challenges for the Rising Star of Soft Robots

Robots tend to be designed to replace human beings, so as to efficiently finish some tasks in a repeatable or dangerous situation, bring us huge economic value and good services [1,2,3,4,5,6,7]. Conventional robots, which refers to rigid/hard robots, are built by connecting rigid members made of metals or plastics at concrete joints, which are driven by electromagnetic motors [1,2,3]. They have been extensively used in industrial production [1,3], medical surgeries [8,9], and daily life [1], including industrial robots, Da Vinci medical robots, autonomous vacuum cleaners, etc. However, these rigid robots, with limited compliance and adaptability, risk manipulating fragile objects and interacting with humans [1,3]. Systematic designs and complex robot controls are required to guarantee the precision of actions and mitigate the safety concerns of robots [1,3], which may introduce great instability and high costs, due to the overall complexity. Therefore, the field of robots is eager to be further revolutionized. In recent years, soft robots have gradually entered the vision of scientists and engineers. They are primarily composed of soft materials, with Young’s modulus in the range of soft biological materials [3]. When compared to rigid robots, they have special advantages, including autonomous functions without feedback controls, high compliance, large deformation capabilities and freedom, as well as unprecedented adaptability and agility [1,2,3], enabling them to easily operate in unstructured terrains, confidently grasp fragile objects, and safely interact with humans when actuated by the matched driving sources [10]. The innovation core of soft robots lies in materials performance, programming structural changes, functional integration, and special applications. Nowadays, soft robots are still at an early stage of research and application; more advanced works are anticipated to promote the development of this field.

Soft materials are the main elements for constructing the body of soft robots. Silicon elastomers [11,12,13], liquid crystal elastomers [14], responsive films with layered structures [15,16], hydrogels [17], and ferrofluids [18] have been widely accepted for the fabrication of soft robots. Even so, new construction materials are still pursued to enrich the family of soft materials. By introducing structural design elements (shapes, cavities, etc.), anisotropic deformation elements (magnetic programming, molecular arrangement, folds, rigid and fibrous matrix arrangements, etc.), and functional elements (special structures, drugs, electronics, etc.) into soft materials, some dexterous and skillful soft robots can be completely fabricated. Deformation strategies and structural designs that determine the overall deformation results are also critical for soft robots. Developing new strategies and designs always open up new research directions for soft robots. Aimed at different soft materials, several fabrication techniques have been proposed to fabricate soft robots [1]. Creating new techniques tends to bring huge breakthroughs for soft robots. Moreover, as a promising research direction to improve the intelligence of soft robots, consideration of multifunctional integration is undoubtedly required in the fabrication process [19,20]. Additionally, the efficient actuation of soft robots based on different controlling methods (gas, fluids, light, temperature, electric field, magnetic field, etc.) enable their successful applications [10]. However, new or jointed controlling methods lack research, holding great potential for improving the action capabilities of soft robots.

Depending on the compliance, adaptability, and mechanical properties of soft robots, they possess great potential for biomedical applications, apart from simply grasping or manipulating fragile objects, which include medical surgeries, drug delivery, artificial organs, prostheses, and assistive wearable devices [8,9]. Particularly, the biomedical applications of untethered, small-scale soft robots are appealing, and a wide exploration space exists. Owing to their small size and soft nature, these soft robots can navigate in hard-to-reach, tortuous, narrow regions inside the biological body, and physically adapt to the tissues without damaging them [8,9]. Soft robots have exhibited unique capabilities in the treatment of some diseases (gastric ulcers [19], blood clots [21], tissue repairs [22], etc.), but there are still some challenges to perfectly match these diseases to soft robots. More special diseases should be investigated to find the ultimate biomedical applications of soft robots. By cooperating with doctors, the durability and reliability, and allergic, contact, and immediate and long-term immune responses to soft robots need to be considered [8].

Therefore, the research on soft robots is still in a period of rapid rising, according to the development laws of other successful scientific fields, and more chances need to be grabbed to contribute to this field. These aspects of construction materials, fabrication methods, deformation strategies, multifunctional integrations, control methods, and specific applications for soft robots should be carefully taken into account, promoting soft robots from the research stages to practical applications.