Recent Trends in Soft Actuators for Biological Applications

Soft actuators are gaining attention in biological applications due to their flexibility, adaptability, and ability to mimic natural movements. Unlike traditional rigid actuators, these systems offer safer interactions with biological tissues, making them useful for medical devices, prosthetics, and soft robotics. Their potential to improve patient outcomes and enhance biomedical research has driven significant advancements.

With ongoing developments in materials, actuation methods, and fabrication techniques, researchers are refining the performance and biocompatibility of soft actuators. Understanding the latest trends highlights emerging possibilities and challenges in their application.

Material Classes

The performance of soft actuators is heavily influenced by their materials, which must balance flexibility, durability, and responsiveness to external stimuli while maintaining biological compatibility. Advances in polymer science, nanocomposites, and biohybrid materials have expanded available options, each with distinct advantages.

Silicone elastomers, such as polydimethylsiloxane (PDMS), remain widely used due to their high elasticity, biocompatibility, and ease of fabrication. Their ability to undergo large deformations without permanent damage makes them suitable for repeated actuation, such as in artificial muscles and soft robotic grippers. Their chemical stability allows for long-term use in physiological conditions, though their relatively low mechanical strength limits load-bearing capacity. Researchers are addressing this through fiber embedding and composite formulations.

Hydrogels are particularly useful for applications requiring direct interaction with biological tissues. These water-rich polymers mimic the mechanical properties of soft tissues, making them ideal for drug delivery systems, tissue engineering scaffolds, and biohybrid actuators. Recent advancements in double-network hydrogels have improved toughness and resilience, addressing traditional hydrogel limitations. Stimuli-responsive hydrogels, which change shape or volume in response to pH, temperature, or ionic concentration, enable controlled actuation in biomedical devices.

Liquid crystal elastomers (LCEs) can undergo reversible shape transformations when exposed to external stimuli like heat or light. Their anisotropic molecular alignment enables programmable deformation patterns, making them useful for precision motion control. Studies have explored their potential in soft robotic systems for minimally invasive surgery. However, challenges related to synthesis complexity and response time remain areas of research.

Inorganic nanocomposites, including graphene-based and carbon nanotube-reinforced polymers, enhance the mechanical and electrical properties of soft actuators. These materials improve conductivity, enabling electrically driven actuation while maintaining flexibility. Research has shown that incorporating graphene into elastomer matrices significantly enhances electromechanical response, making them suitable for wearable haptic feedback devices and bioelectronic interfaces.

Actuation Mechanisms

Soft actuators rely on external stimuli such as air pressure, electric fields, or magnetic forces to achieve controlled deformation. Each approach offers distinct advantages in response time, energy efficiency, and adaptability.

Pneumatic Approaches

Pneumatic actuation is widely used in soft robotics for its simplicity and ability to generate large deformations. These systems consist of elastomeric chambers that expand or contract when pressurized air or fluid is introduced. Their flexibility makes them ideal for biomedical applications, such as assistive exoskeletons and soft prosthetic hands.

Recent advancements focus on improving efficiency and miniaturization. Traditional pneumatic actuators require bulky external pumps and tubing, limiting practicality in wearable or implantable devices. Researchers are integrating microfluidic networks and soft valves for localized pressure control. A study demonstrated a compact, untethered pneumatic actuator using embedded microchannels for more portable operation. Bioinspired designs, such as fiber-reinforced actuators mimicking muscle structures, enhance precision and durability.

Dielectric Elastomer Systems

Dielectric elastomer actuators (DEAs) deform when an electric field is applied across a soft, insulating polymer membrane between two compliant electrodes. This electrostatic force enables rapid, reversible actuation. DEAs are attractive for biomedical applications due to their high energy density, lightweight nature, and muscle-like movements.

A key challenge has been durability and operational stability. Traditional elastomers can suffer from electrical breakdown or mechanical fatigue. Researchers are developing self-healing elastomers that recover from minor damage, extending device lifespan. Nanostructured electrodes, such as carbon nanotube or graphene-based coatings, improve conductivity while maintaining flexibility. These developments have enabled applications like soft robotic grippers for delicate tissue manipulation and implantable pumps for controlled drug delivery.

Magnetic-Based Methods

Magnetically responsive soft actuators embed magnetic particles within an elastomeric matrix to achieve motion when exposed to an external magnetic field. This allows for wireless control, making them particularly useful for minimally invasive medical applications such as targeted drug delivery and micro-scale surgical tools.

Recent advances focus on enhancing responsiveness and versatility. By tailoring the distribution and orientation of magnetic particles, researchers have achieved complex, multi-directional actuation patterns. A study demonstrated a bioinspired soft actuator mimicking undulatory motion for efficient locomotion in confined spaces. Additionally, integrating shape-memory polymers with magnetic actuation enables programmable, reversible transformations, expanding potential applications in medical devices.

Manufacturing Methods

Fabrication techniques must balance precision, scalability, and material integrity. The choice of method directly influences mechanical performance, durability, and functional integration into biomedical devices.

Additive manufacturing has advanced soft actuator production by enabling complex geometries and material gradients. 3D printing technologies, such as direct ink writing and stereolithography, allow layer-by-layer deposition of elastomeric and hydrogel-based materials. Multi-material printing integrates conductive or magnetically responsive elements directly into soft structures, enhancing actuation capabilities. This approach has proven useful in developing biohybrid actuators incorporating living cells within printed scaffolds.

Casting and molding techniques remain widely used for their ability to produce actuators with uniform material properties. Soft lithography has been instrumental in fabricating microfluidic-based actuators, where precise channel architectures dictate controlled deformations. Advances in mold design, such as sacrificial templates and tunable elastomeric molds, have enabled intricate internal structures optimizing responsiveness. Embedded reinforcements, such as fiber networks or nanocomposite layers, improve mechanical robustness.

Laser machining and microfabrication techniques create actuators with fine structural details and high reproducibility. Precision laser ablation forms micro-scale features enhancing actuation dynamics, such as surface texturing for improved grip or perforations for controlled fluid flow. These methods also pattern conductive pathways onto elastomeric substrates, enabling stretchable electronics for real-time sensing and feedback control.

Biomimetic Inspiration

Soft actuators increasingly draw inspiration from biological systems, where nature’s designs offer adaptability, efficiency, and resilience. Organisms have evolved movement strategies relying on compliant materials and dynamic structures, providing valuable blueprints for actuators capable of complex, lifelike motions.

Cephalopods, such as octopuses, have influenced soft actuator design. These animals employ muscular hydrostats, structures allowing continuous deformation without rigid skeletal support. Engineers replicate this principle using fiber-reinforced elastomers mimicking directional stiffness, enabling soft robots to perform delicate gripping and precise manipulation. This approach is especially useful for robotic surgical devices.

Plant movements also provide models for responsive actuation. The Venus flytrap closes rapidly in response to mechanical stimuli, driven by differential turgor pressure changes. Researchers have translated this concept into actuators harnessing osmotic pressure variations, allowing controlled, energy-efficient movement. Such designs have been explored for drug delivery systems that release therapeutic agents in response to physiological cues.

Mechanical Behavior

The mechanical performance of soft actuators determines their ability to undergo repeated deformations while maintaining structural integrity. These systems must balance flexibility and strength, ensuring they withstand mechanical stresses without permanent deformation or failure.

Material selection and structural design play key roles. Polymers with high strain tolerance, such as thermoplastic elastomers, enable actuators to endure extensive elongation without rupture. Reinforced composites, incorporating materials like carbon nanotubes or aramid fibers, enhance durability by distributing mechanical loads. Researchers have also developed architectures inspired by biological structures, such as helical or kirigami-based designs, which allow efficient stretching and contraction.

Fatigue resistance remains a challenge, as repeated actuation can degrade materials. Cyclic loading induces microcracks or plastic deformation in elastomers, reducing lifespan. To address this, self-healing polymers enable actuators to recover from minor damage. Prestrain optimization techniques, conditioning materials to operate within specific strain limits, also extend longevity. These approaches ensure reliability in applications such as wearable assistive devices and bio-integrated robotics.

Biocompatibility Factors

Soft actuators must integrate safely with biological environments, avoiding adverse responses while maintaining functionality. Factors such as cytotoxicity, degradation behavior, and immune system interactions influence their suitability for medical applications.

Material biocompatibility is critical. Silicone-based elastomers, such as PDMS, are widely used due to their established safety and chemical stability. Hydrogels derived from natural polymers, like alginate or hyaluronic acid, mimic the extracellular matrix, promoting cell adhesion. However, degradation byproducts must be evaluated to prevent inflammatory responses. Bioresorbable materials that break down into inert compounds allow temporary actuation without requiring surgical removal.

Sterilization compatibility also affects feasibility. Many synthetic elastomers degrade under high temperatures or harsh chemical sterilants. Alternative methods, such as gamma irradiation or supercritical carbon dioxide treatment, preserve material properties while ensuring microbial safety. Advances in surface modifications, such as plasma treatment or biofunctional coatings, further enhance biocompatibility by reducing bacterial adhesion and improving hemocompatibility.