Future bio-inspired robots require delicate structures

The goal of mimicking animals in nature is to enable robots to adapt to unstructured and dynamically-changing environments that are full of uncertainties. Developing bioinspired robots requires us to parallel or surpass the functions and performances of biological systems with hierarchical structures: Cells are subordinate to tissues, tissues are subordinate to organs, and organs are subordinate to the whole body. We can even look deeper into a cell and treat it as a complex subsystem. The development of bio-inspired robots usually starts from observing animal behaviors. Typical behaviors are selected for imitation due to their extraordinary performances under certain circumstances. Next, the functions of muscles and skeletons are replicated by artificial actuators and mechanical structures. These artificial components play similar roles as biological tissues and organs but may differ significantly in working principles, material compositions, and structures. Subsequently, proper controllers are developed to regulate the motion of the actuators based on the feedback of sensors, enabling the robot to generate the right behavior at the right time. These controllers are usually borrowed from conventional robotic systems and may not fully reflect the structure of the biological nervous control system. Despite many years of efforts of scientists and engineers, the agility, efficiency, and intelligence of state-of-the-art bio-inspired robots still cannot parallel their biological counterparts. Why is there still a stark gap between bio-inspired robots and animals? Bio-inspired robots are abstractions of the biological system. Is it because we leave out so many details that we smear out various merits of the animals? In this opinion article, we argue that we must look deeper into the biological systems and value the critical role of delicate structures. We use the word “delicate structure” here to refer to the functional components much smaller than the biological system or the low-level organizations of neurons, muscles, and sensors subordinate to the central nervous system (Figure 1). Although these delicate structures seem minor compared to the biological system, their influence on the overall performance may be non-trivial. For example, although the heights of the sea bass scales are less than 1 mm, a decrease in friction drag up to 9.31% was observed (Muthuramalingam et al., 2019). It should also be noted that different delicate structures are closely related and can function synergistically tomeet the goal. For example, the fish needs to synergize sensing signals from massive proprioceptors (delicate sensing units) and contract or relax massive muscles (delicate actuators) in concert to achieve steady swimming. The massive delicate OPEN ACCESS