Mechanisms boosting material embodied intelligence to realize self-healing soft robots

The recent introduction of self-healing soft materials in robotics is a major step towards sustainable next generation robots. By manufacturing soft robots out of these smart materials, we integrate a self-healing ability and increase the physical intelligence of these systems. However, the embodied intelligence in the material level needs to be augmented by incorporating assistive mechanisms in the system level with minimized control, enabling healing of damage in different sizes and in diverse working conditions. These assistive mechanisms can provide damage detection, damage closure, healing stimuli providing, health monitoring, or a combination of the previous. In this paper, we present two different mechanisms for an autonomous healing of damages; (i) Embedding a healable heater in a self-healing soft actuator to increase the temperature required for an efficient healing, while it allows detecting the damage and monitoring the health of the system. (ii) Incorporating shape memory alloy wires in a self-heling soft bending actuator, with simultaneous sealing through contraction and heating abilities. Apart from assisting in the healing action, both mechanisms play a part in the actuation of the bending robots as strain limiting elements. These assistive mechanisms will overcome the limitation on the material level, leading to robots that can self-heal in applications outside of laboratories and factories.


Introduction
Despite the remarkable advancement of soft robotics systems, they have not yet been integrated into a large variety of industrial and commercial applications to their full potential [1].One reason that hinders their wide adoption is the inherent vulnerability of their soft matter that reduces their lifetime.In addition, this limited lifetime and the non-recyclability of the polymeric materials have negative environmental effects [2].To tackle these issues, soft robots can be made out of recyclable and healable materials to recover their functional performance after an incurred damage [3,4].
Self-healing in soft robots is in general highly beneficial, whether the process of healing is carried out completely autonomously or by human intervention.Considering that many applications of soft robotics systems are where humans have limited access to, e.g., robots for earth or space exploration, the importance of having a fully autonomous healing system becomes more significant.This autonomous healing can originate from the selected self-healing material, e.g.materials that can be healed at room temperature [5][6][7], or provided by integration of some assistive mechanisms [5].However, to increase the robustness of autonomous self-healing processes, assistive mechanisms will be unavoidable.This robustness refers to the ability to perform multiple damage-healing cycles, to heal different types and sizes of damages, to heal at different environmental conditions and to heal without any human intervention.
A fully autonomous healing procedure, starts with an autonomous detection of damage [8][9][10].Secondly, as synthetic material cannot be grown, damage closure (sealing) is essential for healing and should be performed autonomously as well [11,12].Thirdly, depending on the material, providing a stimulus, in the form of heat or light, is needed for triggering the healing or for accelerating it [13,14].Finally, the system should monitor and re-evaluate its health after healing, e.g.defining the healing efficiency, to make sure that the healing was successful enough to continue operation.Here, we provide two different mechanisms, integrated in self-healing soft bending actuators, to facilitate and assist the autonomous healing in the robots.

Embedded healable heater
A self-healing heater, made of a hybrid composite of self-healing Diels-Alder based material, carbon black and nanoclay, has been developed [15].The addition of the two fillers makes the material electrically conductive, while maintaining its self-healing ability.These two unique properties, healing and conductivity, greatly enhance the embodied intelligence of the material.Due to the Jouleeffect, the material heats upon applying a current, generating the required temperature increase to healing autonomously.Furthermore, the heater is intelligent enough to self-target the heat towards the damaged region and provides localized heating/healing.Via resistance tracking in normal operation, both deformation as well as damage can be detected, while monitoring the resistance during and after healing allows to monitor its health and recovery.Moreover, we showed that the material is also able to self-sense the change in temperature.
As seen in Figure 1a, the heater is embedded in a soft self-healing PneuNet bending actuator to assist in the healing procedure of the robot.Illustratively, by passing electrical current through the heater, it heats up, and can provide enough stimulus for the healing of the robot [15].Note that some selfhealing materials need to be thermally triggered for either enabling or accelerating the healing action.Self-sensing of the temperature is very helpful in this regard.Overheating in some self-healing materials, may cause loss of the structure of the robot due to crossing of a solid-liquid transition (degelation).As a result, controlling the temperature is important and will enhance the robustness of the system, successfully healing without human intervention in diverse working conditions.As a damage sensing element, tracking the resistance of the heater which is changed upon damage provides Fig. 1.Integrated assistive mechanisms for autonomous healing.(a) An embedded healable heater in a PneuNet bending actuator that provides stimulus for healing, detects the damage and monitor the health of the robot [15].(b) Embedded shape memory alloy wires to make an SMA wirereinforced bending actuator with the ability of damage closure and providing stimulus for healing [16].information about its occurrence, however not its location.However, damage leads to heat concentration at the corresponding location.This can be monitored by a thermal camera and used to localize the damage [15].As a health monitoring system, comparing the resistance of the heater after a damage-healing cycle with the initial amount before the damage can be a criterion for health monitoring.Additionally, a successful healing procedure will distribute the heat generation along the heater, not just concentration at the damage location [15].Consequently, generating the heatmap after healing provides information about the performance of the healing process.All these contributions level up the intelligence of the system and remove human assistance from the healing practice in the robot.Being stiffer than the material of the actuator, the heater limits the strain of the robot locally, which improves the bending performance of the actuator.It is worth mentioning that there is a strong interfacial bonding between the heater and the actuator as they are made from a self-healing polymer and composite that benefit from the same chemistry in their polymer network structure.This expands the application of self-healing material for soft robotics and can provide a solution for multi-material robotics design where delamination and interfacial debonding is a serious problem.

Embedded shape memory alloy wires
Like in most biological systems, damage can only be healed properly if it is first closed (assuming that the damaged area is clean).In the human body, large wounds must be closed by stitching and bone fractures should be realigned and fixed.In self-healing soft robots, in case of a puncture or a small damage, closure may happen by the elastic response of the material, whereas large damage need to be closed by external forces.In figure 1.b, an SMA wire-reinforced self-healing soft bending actuator is shown [16].The embedded SMA wires are pre-strained and able to recover their permanent shape if heated above their transition temperature [17].They can be quickly heated to that temperature by passing electrical current through them.Being integrated into the actuator system, the contraction of the wires will contract the chamber and close the damage.Furthermore, the produced Joule heating that activates the wires will also transfer to the chamber and heat it up simultaneously.As such, the wires can not only close the damage, but also provide the stimulus for heating and healing of the actuator [16].Upon cooling, the wires can be re-elongated by the elastic forces of the chamber or upon actuation of the robot, making the damage closure possible for multiple cycles.Based on the length of the wires and the amount of pre-strain, they can close gaps as large as 2 mm [16].For healing, usually the material is heated for a few minutes.Continuation of the contraction forces of the embedded wires during heating ensures that the damage remains closed throughout the healing procedure, which is in contrast to when the robot is manually closed and heated up via an oven.In many cases, the damage opens while the specimens or robotic systems are heated up in the oven, due to different thermal expansion of the material or object in different directions.Another advantage of this system is the ability to perform in-situ healing, meaning that there is no need to disassemble the actuator from the working station and take it to a rest position.Thanks to the forces and heat of the SMA wires, the system has a higher level of autonomy in the healing procedure.Finally, the strong SMA wires reinforce the chamber radially and longitudinally which is crucial for the bending motion of the chamber when pneumatically inflated, preventing it from undesired expansions and ballooning.At the same time, they support the robot against very serious injuries, e.g.being cut it two.