Perspective—Five Sensor-Centric Grand Challenges in Soft Robotics

Soft robotics, offering precise actions in complex environments, stands at the brink of transformative advancements across diverse fields. To realize this potential, the field must address five key challenges: creation of soft power and control mechanisms, emphasis on sustainability, cultivation of advanced intelligence, and the imperative for standardization. This perspective argues for solutions grounded in sensory feedback systems, aiming to fortify the foundation of soft robotics, ensure its sustainability, enhance adaptability in robot intelligence, and set the stage for scalable robot production. Addressing these challenges, we aim to pave the way for a more inclusive era of soft robotic technology.

Soft robotics, drawing inspiration from the malleable and adaptable nature of biological systems, is rapidly emerging as a transformative force in various sectors.From medical interventions, where they assist in delicate surgeries and patient care, to deep-sea explorations and agricultural tasks, soft robots are proving their mettle by navigating environments that are challenging for traditional rigid robots.Their ability to mimic natural organisms allows them to seamlessly integrate and interact with their surroundings, making them ideal for tasks that demand high levels of adaptability and sensitivity.Soft robots are not only promising for intricate realtime tasks but are also experiencing a significant market surge.In 2022, the Soft Robotics Market stood at USD 852.28 Million and is projected to soar to USD 10,839 Million by 2030, with a CAGR of 37.42% from 2023 to 2030. 1 Complementing this, the global flexible sensors market, essential for robot adaptability, was valued at USD 5.34 billion in 2021 and is expected to reach USD 9.04 billion by 2029. 2 Additionally, the Biosensors Market, crucial for integrating biological responses, is set to grow from USD 27.41 billion in 2022 to USD 48.89 billion by 2030. 3 These market trends underscore the potential of soft robots and their anticipated cohabitation with biological beings.However, challenges persist.While researchers have developed numerous compliant mechanisms inspired by natural actuation, their broader application is limited due to material constraints, rudimentary actuation principles, and a lack of comprehensive integration of sensor technologies.The future of soft robotics demands innovative energy sources, revolutionary control systems, and a fusion of interdisciplinary domains, including material sciences and biotechnology.This article highlights five pivotal challenges that could steer researchers toward sustainable advancements in soft robotics.The first two challenges focus on the design of a fully compliant soft robotic structure.To truly emulate biological systems, the integration of flexible sensors and biosensors is essential, 4,5 offering realtime feedback and enhancing adaptability.These sensors can improve the efficiency of soft power sources and advanced control mechanisms, bridging the gap between artificial and organic entities.As the demand for soft robots intensifies, sustainability becomes a central concern.Here, biosensors play a crucial role, ensuring robots operate in harmony with the environment, balancing technological progress with ecological care.The evolution of robot intelligence is another challenge, where flexible sensors can significantly enhance the body-task complexity ratio, paving the way for more dexterous soft robots.Lastly, the challenge of standardization is paramount.To foster trust and consistency in the development process, it's vital to standardize key components, especially flexible sensors, biosensors, soft actuators, and control systems.
The field of soft robotics is undergoing a transformative shift, transitioning from specialized academic pursuits to tangible realworld applications.This rapid evolution underscores the importance of a structured, yet comprehensive, analysis of prevailing challenges and prospective pathways.Our article seeks to fulfill this exigency by elucidating the core impediments and exploring the pivotal role sensors could play in resolving them.By delineating these aspects, we not only aim to cultivate a deeper understanding but also foster interdisciplinary collaboration.Soft robotics, intrinsically, is an amalgamation of diverse scientific domains.Thus, a clear, coherent roadmap is indispensable.Through this perspective, we endeavor to provide both a current overview and a forward-looking vision, intending to channelize research endeavors and inspire pioneering breakthroughs in soft robotics.

Soft Power Sources
Future need for soft power sources.-Theindispensability of a robust and sustainable energy source is incontrovertible.Despite six decades since the inception of industrial robotics, mobile robots still struggle with the endurance of their power sources.Current powerhouses of energy like lithium polymer and ion batteries, though formidable, necessitate frequent recharging and pose significant environmental challenges upon disposal.To capitalize on the compliant behavior of soft robots, power sources such as pneumatic, hydraulic, and electrostatic energy, in addition to conventional lowvoltage DC voltages, are employed.Despite hydraulic and pneumatic energy sources mimicking the natural actuation found in many organisms, their dependence on bulky, energy-intensive air compressors and fluid pumps compromise system efficiency.Highvoltage power sources have been used to actuate robots designed with Electroactive Polymers (EAPs) 6,7 and Dielectric Materials (DEMs). 8Recently, Hydraulically Amplified Self-healing Electrostatic (HASEL) actuators have emerged, which closely mimic the muscle movement of biological entities.HASEL's structure, composed of small fluid-filled pockets electrostatically compressed to generate high-pressure regions, creates motion similar to biological muscle structure when arranged in series. 9However, their high-voltage actuation requirement detracts from their desirability in a completely soft system context.In summary, while innovations like HASEL actuators offer promise, the field still grapples with the challenge of finding sustainable and efficient power sources.The current reliance on bulky equipment and high-voltage actuation remains a limitation in realizing the full potential of soft robots.
Potential pathways for sensor integrated soft power sources.-Thepath to crafting a soft power source may hinge on the intricate integration of biology, soft robotics, and advanced sensing mechanisms.While the actuation mechanisms of soft robots draw inspiration from the morphology of biological beings, the role of sensors in monitoring and optimizing energy extraction methods becomes paramount.The vision of creating self-sufficient soft power sources, capable of intelligent energy extraction from the environment at the cellular level, is intertwined with the development of sensors that can monitor and regulate this process.Such power sources would not only necessitate laboratory cultivation using living cells but also require advanced sensors integrated within cell-based soft robots for efficient energy absorption and utilization.
Recent research has illuminated the potential of light-triggered actuation, where sensors monitor the integration of supercapacitors and thermos-responsive liquid crystalline polymer networks, facilitating the direct conversion of light into motion. 10In other research, electronics-free soft pneumatic circuits powered by mini CO 2 cylinders have been explored, hinting at a future where sensors could regulate artificially cultured soft pumps.Flexible organic solar cells, 11 equipped with sensors to optimize energy capture, present another avenue.Their low flexural rigidity makes them ideal candidates for powering soft robots.Chemical methods, such as gas generation via fuel decomposition, can be enhanced with sensors to provide real-time feedback for pneumatic actuation. 12The study of thermomechanical properties in artificial muscles, powered through fuel combustion, 13,14 becomes more efficient with integrated sensors monitoring the combustion process.Furthermore, MXenebased micro-supercapacitors, combined with triboelectric nanogenerators, have shown promise in powering wearable devices. 15While they possess low energy density, the integration of sensors can optimize their performance, especially when dispersed throughout a soft robot's structure.In conclusion, the integration of sensors into soft power sources offers promising avenues for improving energy efficiency and autonomy in soft robots.From light-triggered actuation to flexible solar cells, sensor-driven innovations are paving the way for more self-sufficient and responsive energy solutions in soft robotic systems.

Soft Control Mechanisms
Current challenges and future need for soft control mechanisms.-Exercisingcontrol over a soft robot's actuation, stiffness, and adhesion necessitates precision in input energy management.Numerous research efforts have harnessed basic physical principles, such as fluid pressure differentials, electrostatics, and magnetic fields, to induce motion and stiffness in soft actuators.As seen in pneunets and McKibben actuators, the degree of bending and linear motion is proportional to the input pressure.Similarly, the bending of dielectric actuators made of electroactive polymers depends on electrostatic charge amount, which is indirectly reliant on highvoltage input supply.Approaches like granular and laminar jamming, used for stiffness control, rely on regulating input vacuum pressure, 16,17 while the stiffness of magnetorheological fluid-based jammers depends on the input magnetic field intensity.Electroadhesive strip and gecko adhesive pad adhesion properties are proportional to electrostatic charge and applied normal pressure, respectively.9][20] In conclusion, despite these strides, the long-term sustainability, maintenance-free operation, and advantageous lifecycle of future soft robots depends on their capacity to self-heal and intelligently control their dynamics at the cellular level.While numerous control mechanisms have been explored, the future of soft robotics hinges on combining biology, AI, and flexible electronics to ensure durability and advanced adaptability.
Harnessing chaos: sensor-guided control mechanisms in soft robotics.-Thecontrol characteristics of soft robots are enhanced when sensors are seamlessly integrated into their structure, unlocking their full potential.One innovative approach involves using dead plant tissues as a foundation for soft actuators, with sensors monitoring and optimizing their performance.Anisotropic cellulose composites can be 3D printed in layers, and their swelling and shrinkage behavior, when coupled with embedded sensors, can be harnessed to craft responsive soft actuators. 17Biological entities have the innate ability to sense pain and self-repair, ensuring continued functionality.To emulate this, soft robots need sensors that can detect damage and initiate self-healing processes.Nonautonomous self-healing (SH) polymers, which boast high mechanical strength and come equipped with ultraviolet emitters or heaters, can be utilized to develop sensor-integrated soft skins. 15ddressing actuation control at the macroscopic level without sacrificing flexibility is vital.Shape memory alloys, when embedded within a soft structure and paired with sensors, can regulate intricate motions, reminiscent of a plant-inspired robotic tendril, making them ideal for slender continuum robots. 19The future lies in additive manufacturing, particularly in crafting smart soft skins with integrated flexible sensors and self-healing capabilities.3D printing sensory fiber networks onto these skins, guided by real-time sensor feedback, can significantly enhance the flexibility and reflexes of soft structures. 20Therefore, by integrating sensors within soft robots, we can mimic biological responses and enhance control.From harnessing plant-inspired actuation to embedding self-healing mechanisms, the synergy of sensors and soft materials paves the way for more adaptive and responsive robotic systems.

Sustainability
Future prospects: the imperative of sustainability in soft robotics.-Theburgeoning demand and expansive applications of secure human-robot interaction systems underscore the need for soft robots in rehabilitation, space exploration, minimally invasive surgeries, and future home assistance devices.As we edge closer to mass production of soft robots, a safe and eco-friendly disposal mechanism becomes increasingly crucial to maintain our ecosystem's equilibrium.Materials frequently used to develop soft actuators, such as certain grades of silicone, plastics, and metal alloys, are non-degradable.As such, it's vital to glean insights from nature's creations and design our soft systems to mitigate the disruptive ecological balance caused by the hazardous materials employed in current technologies.In conclusion, the pinnacle of soft robotics should be the design of machinery with a self-sustaining lifecycle, drawing on biodegradable energy sources and ultimately decomposing at the end of its useful life.Achieving this, however, requires a paradigm shift in the field of soft electronics, coupled with an integration of biotechnology and artificial intelligence.
Solutions for crafting sensor integrated sustainable soft robots.-Figure 1 provides insights into potential solutions for bolstering the sustainability of soft robotics, with sensors playing a pivotal role in optimizing and monitoring their performance.The initial step towards sustainability is the adoption of eco-friendly, biodegradable materials such as elastomers, hydrogel, and plastics.Biodegradable polyurethanes (bio-PUs), already recognized for bioimplants, emerge as strong contenders for soft robotic structures, with sensors ensuring their optimal functionality.Rapidly degradable polyesters like poly(polyol sebacate) (PPS) can be utilized for large-scale production, while materials like poly(glycerol sebacate) ECS Sensors Plus, 2023 2 046601 (PGS), poly(diol citrates) (PDCs), poly(1,8-octanediol citrate) (POC), and poly(octamethylene maleate (anhydride) citrate) (POMaC) are synthetic, biodegradable elastomers.Their impressive mechanical attributes and stretchability, when paired with sensors, make them ideal for soft robotic systems.
However, the sustainability of soft robotics isn't just about the body structure.Control systems, too, must incorporate biodegradable electronics, with sensors ensuring their efficiency and longevity.Metals like calcium, magnesium, zinc, and iron, essential to biological nutrient structures, can be integrated into electronic circuitry.Emerging innovations highlight bio-semiconductors 21 with sensors monitoring their time-limited functionality.These semiconductors, though in their infancy, present a hopeful avenue for crafting soft, recyclable control systems.Polyamide films, being thin and biodegradable, are apt for flexible circuits with integrated sensors.Recent breakthroughs spotlight recyclable, enzyme-based conductive composites for 3D printing sensor-monitored circuits on diverse materials.These composites, degradable in warm water, facilitate the reuse of fillers for new composite creation. 22A blend of reduced graphene oxide (r-GO) and eco-friendly polylactic acid, monitored by sensors, can also be harnessed for 3D printing electrical circuits. 23Therefore, sustainability in soft robotics relies on the integration of biodegradable materials with sensor systems.From elastomers to flexible circuits, the incorporation of ecofriendly components, monitored by sensors, offers a promising future for sustainable and efficient soft robotic applications.

Intelligence
Future need for intelligent soft robots.-Thenotion of intelligence is fundamental to the survival of all biological organisms.The human brain, with its impressive operational capabilities, is often held as the gold standard for intelligence.It is the aspiration of every soft roboticist to imbue their creations with a comparable level of cognitive capacity.An intelligent soft robot possesses numerous advantages.The ability to alter its form, adapt to environmental conditions, multi-task, and safely and seamlessly interact with users can only be achieved through the incorporation of innate intelligence.As the complexity of a soft robot's form augments, so should the complexity of its tasks, mirroring the evolutionary adaptations observed in biological organisms.Moreover, the evolution of intelligence in soft robots should be progressive and autonomous, necessitating the encoding of initial design directives at the microlevel, akin to the role of the genome in living organisms.The challenge of complexity can only be addressed through innate intelligent behavior.At present, soft robots are designed with a specific application in mind.However, with the integration of inherent intelligence, the scope of their applications can be significantly broadened, thereby justifying the cost and lifecycle of a soft system.
Sensor driven cognitive advancements in soft robotics: pathways to intelligent systems.-Byintegrating advanced sensors with machine learning and computer vision, soft robots can achieve selforganization and learning behavior, enabling them to adapt and refine their structures for evolutionary purposes. 24This data-driven approach, guided by real-time sensor feedback, could be pivotal in crafting genuinely intelligent soft robots.The ability to morph and adapt to diverse applications can be realized by merging computer vision, sensor data, and 3D printing technology.This synergy allows the robot to evolve internal structures based on task requirements.A manifestation of this idea is evident in rigid robotics, where tool fabrication using synthetic adhesives, monitored by sensors, has led to efficient object manipulation. 25Flexible sensors, strategically embedded across the robotic "skin," can amplify reflex actions and calibrate inbuilt actuators through model-based learning. 26Yet, achieving profound intelligence demands intricate computational prowess.A visionary goal in this domain is the crafting of a soft, flexible "brain" structure, equipped with sensors, to oversee the robot's motor functions.
Recent studies on microfluidic amplifiers, which magnify low fluid pressure input signals for pneumatic soft robots, hint at a promising direction. 27With sensors guiding their operations, these amplifiers could lay the foundation for soft controllers, culminating in the creation of soft, programmable computers.In summary, harnessing sensors, machine learning, and fabrication technologies is pivotal for advancing intelligent and adaptive soft robots.Sensors remain crucial in refining and directing these robotic functionalities.Figure 2 encapsulates this vision, showcasing potential strategies and highlighting the key materials, all underpinned by sensors, to elevate the intelligence of soft robots while preserving their innate flexibility.

Standardization
The essential role of standardization in soft robotics.-Asthe field of soft robotics burgeons, the need for developing a comprehensive framework for standardization becomes paramount.This would facilitate a consistent approach towards design, fabrication, and performance evaluation of soft robotic systems, paving the way for comparative analyses and fostering collective advancements in the field.A linchpin in this framework would be the standardization of material selection.A comprehensive database encompassing material properties and behaviors under different operating conditions would serve as an indispensable resource, fostering informed decision-making in material utilization.Further, uniformity in fabrication processes -be it casting, molding, or 3D printingwould ensure replicability and consistency across diverse labs and manufacturers.Standardizing variables such as curing times, temperatures, and pressures would bolster this cause, fostering trust in the manufacturing practices of soft robotics.
The establishment of uniform control algorithms, complemented by open-source software platforms, could underpin advancements in soft robot control strategies.This would promote interoperability and ease-of-use, spurring innovation and collaboration in the field.Lastly, standardized testing protocols would provide robust and comparable performance metrics across soft robotic systems.Assessing parameters like actuation speed, resilience, force exertion, and energy efficiency under uniform testing conditions would make these evaluations more meaningful and universally applicable.In conclusion, by embracing standardization, we can catalyze the evolution of soft robotics, facilitating its transition from the lab to the real world.This leap towards standardization is a pivotal stride in the journey of making soft robotics a pervasive reality.
Sensor-guided standardization in soft robotics.-Thefuture of advanced soft robotic systems is deeply intertwined with the harmonious integration of design, manufacturing, and evaluation, all of which are underscored by the need for sensor-driven standardization.A significant step towards this goal may arise from the evolution of 3D printing technology, especially when fine-tuned for printing compliant composite materials.These printers, equipped with sensors, could be further optimized with temperature-regulated air circulation systems, ensuring precise curing of silicone and other elastomers during the print process.Drawing inspiration from established standards like G and M codes in manufacturing, there's a pressing need to integrate sensor data and extend such codes to include temperature parameters.This would offer a consistent manufacturing platform for soft systems worldwide.Beyond just manufacturing, the scientific characterization of elastomers, informed by sensors, becomes essential.This demands readily available and precise data on attributes like Young's modulus and hyperelastic material model parameters, reducing user-end testing and potential inaccuracies.
Simulation software, such as Abaqus and the SOFA framework, already incorporate hyperelastic material models to predict elastomer actuation can be tuned further based on sensor feedback.Integrating soft robot motion planning into universally recognized ECS Sensors Plus, 2023 2 046601 software like ROS (Robot Operating System) and MATLAB, with sensor data at their core, marks another significant stride towards standardization. 28Automation, enhanced by real-time sensor monitoring, is the linchpin for achieving unparalleled standardization.Analogous to advanced five-axis CNC machines, the next-gen technology will seamlessly accept CAD models and produce the final product with minimal human intervention.Therefore, sensor integration is crucial for achieving standardization in soft robotics, enhancing precision in design, manufacturing, and evaluation.Leveraging these sensors in tandem with evolving 3D printing and simulation software will set new benchmarks in manufacturing consistency and replicability.

Materials at the Core of Sensory Evolution in Soft Robotics
The foundational bedrock of soft robotics is undeniably its materials, shaping the robot and sensing its environment.Elastomers, hydrogels, and conductive polymers meld form and function.Hydrogels, sensitive to pH or temperature, serve as natural sensors.Conductive polymer materials, like poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) and polyaniline, ensure signal transmission.The rise of self-healing materials guarantees sensor performance longevity.Bio-hybrid materials, with living cells, react organically to stimuli.Piezoelectric materials convert stress into electric signals.Overall, the union of materials and sensors in soft robotics suggests future robots will be deeply attuned entities.

Conclusions
The burgeoning applications of soft robots underscore an imminent need for their widespread accessibility, augmented capabilities, and eco-friendly production.The five grand challenges delineated in this article chart a roadmap for the sustainable and intelligent evolution of future soft robots.These challenges encompass the absence of innovative soft power and control mechanisms, sustainability imperatives, the quest for embedded intelligence, and the call for manufacturing standardization.Sensor-driven solutions for soft power sources include flexible organic solar cells, triboelectric nanogenerators, and light-responsive polymers that harmonize with the malleability of soft robots.For optimal control, soft robots necessitate integrated sensor systems within their architecture.Self-healing polymers, anisotropic cellulose composites, shape memory alloys, and sensor-enriched fiber networks emerge as promising materials for crafting soft, controllable structures.Sustainability, a paramount concern, can be championed by leveraging biodegradable elastomers, sensor-monitored bio-semiconductors, enzyme-based composites, and polyamide films for constructing both soft structures and their associated circuits.Intelligence, coupled with adaptive morphing capabilities, can be realized through sensor-augmented computer vision, correlationbased learning, and integrated 3D printing techniques.Lastly, the standardization of soft robotic technologies hinges on advancements in 3D printing, the expansion of simulation tools with embedded sensor data, the democratization of scientific datasets, and the evolution of automated manufacturing methodologies.In essence, the sensor-centric approach emerges as the linchpin, ensuring the holistic and sustainable progression of soft robotics.

Figure 1 .
Figure 1.Pioneering pathways towards sustainable soft robotics: Harnessing biodegradable materials and technologies.

Figure 2 .
Figure 2. The confluence of design: Integrating materials, flexible sensors, and advanced AI to elevate soft robotic intelligence.