A Review of Soft Robotic Actuations for Minimally Invasive Implantable Devices and Sensors

2.1 Fluidic Actuation

Fluidic soft actuators utilize pressurized media – typically air, water, or biocompatible fluids – to generate shape transformation and motion. The implant is delivered in a compact form and expands to its functional configuration inside the body. This mechanism enables minimally invasive surgeries and precise organ targeting.

The most common strategy for a deployment system is an origami-inspired folding mechanism, in which we can reduce its two-dimensional device size via a traditional Japanese art-inspired folding pattern. A landmark work by Coles et al. demonstrated an origami-inspired fluidic actuator that deploys a large-area ECoG array through radial expansion once inside the skull (Fig. 1(a)) [10]. This approach enables the integration of stretchable electronics and sensors onto folded soft substrates, ensuring effective cortical interfacing across wide areas. Origami techniques are particularly advantageous due to their programmable kinematics, compact initial volume, and ability to support multimodal device integration such as embedded electrodes, strain sensors [11], and microfluidic channels.

Fig. 1.

Representative soft robotic actuation strategies; (a) Origami-inspired fluidic actuation for minimally invasive large-area ECoG placement. Adapted from Ref. [10]. (b) Shape-actuating implant for minimally invasive spinal cord stimulation. Adapted from Ref. [12]. (c) Fluidic actuator deploying an electrocorticography (ECoG) array from a compact form. Adapted from Ref. [32]. (d) Ferromagnetic soft continuum robot with programmable magnetic deformation. Adapted from Ref. [16]. (e) Helical magnetic robot for navigation through complex vasculature. Adapted from Ref. [17]. (f) Ultrasound-triggered shape recovery of implanted shape-memory devices. Adapted from Ref. [25]. (g) Skin-inspired sensory robots for physiological monitoring and control across vascular, cardiac, digestive, and bladder systems. Adapted from Ref. [8].

Beyond the origami mechanism, an unfurling motion that can be seen in a rolling paper has also been demonstrated by the fluidic actuation. Woodington et al. developed a shape-actuating electronic implant that combines thin, flexible electronics with integrated fluidic channels as a spinal cord stimulator (SCS) (Fig. 1(b)) [12]. The device can be rolled into a compact cylindrical shape to fit within a standard percutaneous needle and then unfurled in situ into a paddle-shaped configuration. This approach merges the clinical benefits of paddle-type devices with the minimal invasiveness of needle-type implants. It paves the way for fluid-driven sensory and stimulation implants that offer a large functional footprint while reducing surgical burden and enabling deployment under local anesthesia.

Although folding- and unfurling-based mechanisms have demonstrated promising applications in minimally invasive deployment, they are fundamentally constrained by the requirement that the entire folded device must be inserted through an incision prior to expansion. As the surface area of the final interface increases, the folded device inevitably becomes thicker, raising the risk of tissue damage during insertion even before deployment occurs. To overcome this limitation, Song et al. introduced a bio-inspired fluidic actuation strategy known as ‘eversion’, which mimics the growth pattern of tree roots [13]. In this approach, the folded substrate is stored within a loader positioned outside the incision, and only the everting tip is advanced through the opening to progressively deploy the surface area (Fig. 1(c)). This configuration ensures that the insertion depth remains constant, in contrast to origami- or unfurling-based mechanisms, where the compression depth scales with the final device size. The critical advantage of this design has been validated through an electrocorticography (ECoG) electrode array, which achieved successful in-vivo recording of somatosensory evoked potentials (SSEP) in a minipig during acute surgery via a small craniotomy.

In addition to the application for central nervous systems like the brain or the spinal cord, fluidic systems have also been applied to various other organ systems. In the gastrointestinal (GI) tract, soft actuations have enabled applications such as site-specific drug delivery or localized physiological sensing. Integration of the fluidic actuations with various physiological sensing systems allows navigating through the highly deformable and wet GI environment for the above application. [14,15].

In summary, fluidic actuation enables fast and precise mechanical transformations with relatively high force density, offering a simple yet powerful means of achieving minimally invasive functionality.

2.2 Magnetic Actuation

Magnetic actuation enables wireless control of soft implants by applying external magnetic fields to magnetically responsive materials embedded within the device. Magnetic actuation combined with position-sensing mechanisms, such as fluoroscopy, CT, or magnetometers, allows high controllability of the soft magnetic robot motions with high precision.

Thread-shaped ferromagnetic soft continuum robots developed by Kim et al. have shown significant potential of such magnetic actuation for minimally invasive soft bioelectronics. These robots are fabricated from elastomeric matrices embedded with hard-magnetic microparticles, enabling programmable magnetization profiles along their bodies [16]. This design allows complex, continuous deformation in response to uniform magnetic fields, producing a wide range of 3D shapes and motions—including bending, twisting, and gripping—without requiring the integration of multiple actuators (Fig. 1(d)). These capabilities make ferromagnetic soft continuum robots particularly suitable for minimally invasive interventions in deep or confined anatomical regions.

In line with the development of thread-like soft robots mentioned above, Dreyfus et al. Further advanced this technique by enabling the forward and backward motions of the continuum robots [17]. The proposed thread robot consists of a rod-shaped inner core surrounded by a helical structure; the inner rod rotates in response to an applied magnetic field, while the helical structure enables forward and backward movement depending on the direction of the magnetic field rotation. Such reversible and tunable motion has demonstrated its potential to open doors to myriad applications that require precise navigation through intricated vasculatures in the brain, especially to treat aneurysms (Fig. 1(e)).

Beyond the thread-shaped soft continuum robots, soft magnetic microrobots such as the milli-spinner—a hollow cylindrical robot with helical fins—navigate tubular environments at speeds exceeding 0.2 m/s, powered solely by rotating magnetic fields. This platform offers on-the-fly drug delivery and mechanical interaction while preserving blood flow through its internal channel design [18]. Another remarkable application is in targeted kidney stone dissolution: researchers have developed thin, flexible soft robots embedded with urease and micromagnets that are guided magnetically through the urinary tract. Once positioned near the kidney stone, they locally release the enzyme to dissolve the stone, providing a minimally invasive alternative to traditional surgical methods [19].

Magnetic actuation allows untethered deployment and motion for soft robotic implants, which provides critical advantages over other actuation mechanisms for chronic in-vivo validation, especially enabling constraint-free movement of target animals [20]. Also, this feature eliminates the need for internal power sources and batteries for deployment motions [21], which are especially beneficial for highly confined anatomical spaces such as the cerebral vasculature or spinal canal. Importantly, the biocompatibility of magnetically loaded elastomers has been well-demonstrated, ensuring minimal immune response [22]. Eventually, these open venues for multi-functional and responsive implants that are both adaptive and safe for long-term use.

2.3 Thermal actuation

For implantable systems, thermal actuations are particularly attractive as a soft actuator, because they enable wireless, untethered, and programmable shape changes, reducing or even eliminating the need for bulky mechanical components or onboard power supplies [23]. In some cases, actuation can be triggered directly by physiological conditions—such as body temperature—allowing operation without batteries or complex controllers, thereby simplifying peripheral hardware for actuation [24].

Among them, here we mainly focus on discussing the use of SMPs, capable of triggering their pre-programmed shape morphing capabilities with respect to multiple external stimuli, such as lights and heat, including active and passive heating with human body temperature. In Fig. 1(f), the use of high-intensity focused ultrasound (HIFU) activates the recovery of SMP-based implants deep inside the brain, as demonstrated by Zhu et al. [25]. This non-contact thermal trigger facilitates precise shape morphing even in anatomically constrained or sensitive tissue regions. Since HIFU can be applied externally and focused on specific internal targets, it enables remote control of the implant without the need for physical access.

Leveraging the advantage of SMP-based shape morphing mechanisms mentioned above, Bae, J. Y et al. introduced a biodegradable and self-deployable electronic tent electrode that leverages the minimal actuator design enabled by a passive heating system for SMPs for ECoG [26]. The device is designed for syringe-based delivery in a highly compact form and unfolds into a three-dimensional tent shape once inside the cranial cavity. This enables large-area conformal contact with the brain surface while maintaining a minimally invasive footprint. Over time, the device naturally degrades, eliminating the need for retrieval surgery – a milestone for transient biomedical implants.

Light-based actuation of SMPs has also been explored to achieve precise, localized shape transformation. By incorporating photothermal fillers such as gold nanorods, carbon nanotubes, or graphene into SMP matrices, these systems can convert incident light into heat, triggering shape recovery with high spatial and temporal control [27,28]. This approach enables wireless, targeted actuation through optical fibers or minimally invasive endoscopes. Moreover, the ability to modulate light intensity and wavelength provides additional control over actuation speed and recovery profile, expanding the functional versatility of SMP-based implants.

Wubin Bai and colleagues further advanced the SMP-based shape-morphing concept by integrating skin-inspired sensor arrays with an active heating system for stimuli-responsive 3D SMP microarchitectures (Fig. 1(g)), enabling fully flexible, wireless electronic implants that combine mechanical adaptation with functional sensing and data acquisition [8]. These systems are built using mechanically guided assembly, allowing for programmable geometry and modular integration of electronics.

Additionally, SMP-based structures offer inherent advantages for implantable applications. Because the actuation originates from the deformation of the material itself, SMP systems require minimal peripheral hardware, eliminating the need for complex mechanical linkages or large onboard components. Eventually, these structures are compatible with bioelectric signal sensing, advancing toward fully functional and biocompatible soft robotic interfaces.

Table 1.

Representative soft robotic actuation strategies for implantable bioelectronic systems

2.4 Electrical Actuation

Electrically-responsive materials undergo mechanical transformation in response to applied external electrical fields or current. These materials include dielectric elastomer actuators (DEAs) and liquid crystalline elastomers (LCEs).

In DEAs, a soft dielectric elastomer layer is sandwiched between compliant electrodes. Upon voltage application, electrostatic attraction between the electrodes generates Maxwell stress, leading to thickness compression and in-plane expansion of the elastomer membrane. Such electrostatically driven large-strain deformation has been widely explored in soft robotic systems, offering high compliance and mechanical adaptability suitable for minimally invasive deployment and biointegrated applications [29].

Beyond direct membrane deformation, DEAs have also been integrated with enclosed fluidic chambers to enable electrically controlled pneumatic or hydraulic pressure generation. In such configurations, voltage-induced membrane displacement modulates chamber volume, thereby producing controllable fluidic pressure output for soft robotic manipulation and implantable actuation. Recent studies have demonstrated that coupling electrostatic elastomer actuation with fluid-mediated force transmission can enhance output force while preserving structural softness, including frameless fluid-electrode architectures that enable translucent and mechanically compliant soft robotic platforms [30].

LCEs combine the ordered characteristics of liquid crystal materials with elastic polymer networks. Under electrical stimulation, these materials undergo reversible shape change due to electrically induced structural reconfiguration within the polymer matrix. This mechanism enables programmable deformation and tunable mechanical behavior in soft structures. Recent advances have demonstrated electrically addressable LCE architectures capable of spatially controlled and reversible three-dimensional morphing, highlighting their potential for soft robotic systems that require compact, lightweight, and minimally invasive actuation strategies. Such material-level programmability offers opportunities for integrated implantable devices and sensor platforms where adaptive mechanical conformity and electronic control are essential [31].

3.1 Long-term biocompatibility

The primary source of biocompatibility challenges in bioelectronic systems lies in the host immune system’s tendency to recognize and reject implanted artificial structures, a response known as the foreign-body reaction. Numerous strategies have been explored to mitigate this immune response and promote long-term integration. In this context, prior work can be broadly categorized into two complementary approaches: materials development, aimed at reducing immunogenicity at the device–tissue interface, and structural optimization, designed to enhance mechanical and geometric compatibility with surrounding biological tissues.

3.1.1 Materials development

Hydrogel is regarded as one of the most promising materials for long-term implantation. It features high-water content, tunable mechanical properties, and biochemical versatility. Their high hydration level enables mechanical compliance closely matching that of ultra-soft tissues such as brain and spinal cord, thereby reducing strain mismatch at the device–tissue interface and mitigating micromotion-induced injury [33]. The capability of hydrogel replicates key features of the extracellular matrix(ECM), which promote cell adhesion and neural integration [34]. These attributes, combined with their compatibility with microfabrication and 3D printing, make hydrogels a versatile platform for implantable systems that maintain stable electrical performance and biocompatibility over extended periods.

Zwitterionic materials present a further advantage by resisting nonspecific protein adsorption and inhibiting the adhesion of immune cells like macrophages (Fig. 2(a)). Yang et al. demonstrated that zwitterionic hydrogels exhibited strong resistance to foreign-body reaction in a mouse model [35]. Combined with robotics, Cabanach et al. developed zwitterionic 3D-printed non-immunogenic stealth microrobots that maintained stealth functionality to macrophages in vivo without triggering immune recognition [36]. These microrobots highlight the potential of zwitterionic coatings not only for static implants but also for mobile or shape-morphing biomedical devices, underscoring their versatility in chronic implantation scenarios. Collectively, these materials combine mechanical softness with surface chemistry that actively suppresses fibrotic encapsulation, extending the functional lifetime of implants.

Fig. 2.

Key functionalities of soft robotic actuators in implantable systems; (a) Immunogenic vs. stealth microrobots. Adapted from Ref. [36]. (b) shear-lag design visualization. Adapted from Ref. [37]. (c) Fully bioresorbable electrode array. Adapted from Ref. [43]. (d) Double-sided dry tape (DST) adhesion. Adapted from Ref. [45].

3.2 Bioresorbability

Bioresorbable electronics offer a transformative approach for temporary implantable devices by enabling complete in vivo degradation after fulfilling their functional purpose, thereby eliminating the need for surgical retrieval. Such devices are fabricated from bioresorbable materials—including transient metals [39], bioresorbable polymers [40], and naturally derived substrates [41]—that dissolve into biocompatible byproducts via hydrolysis, enzymatic degradation, or metabolic absorption. The degradation profile can be precisely tuned through copolymer composition, degree of crosslinking, and protective encapsulation layers, allowing device lifetime to be synchronized with clinical requirements [42].

Clinically relevant systems have been demonstrated, such as a fully bioresorbable, flexible hybrid opto-electronic neural implant capable of simultaneous electrophysiological recording and optogenetic stimulation in the brain (Fig. 2(c)) [43]. Another case is a wireless, dual-mode bioresorbable stimulator for peripheral nerve regeneration that operated reliably over therapeutic timeframes before complete resorption [44]. These examples underline the potential of bioresorbable platforms for short-term neuromodulation, post-operative monitoring, and pediatric applications, where avoiding repeat surgeries is of particular benefit.

As materials and their structural design strategies advance, future bioresorbable systems are expected to integrate multimodal sensing, targeted actuation, and closed-loop therapeutic control into single, transient platforms—maximizing clinical impact while minimizing long-term tissue burden.

3.3 Contact reliability

Reliable contact stability is one of the major challenges in soft bioelectronics due to the arbitrary 3D shapes of organ systems and various physiological motions from respiration, heartbeat, peristalsis, and muscle contraction. Such motions can lead to micro gaps at the device–tissue interface, causing signal drift, reduced stimulation accuracy, and premature device failure. Thus, adhesion strategies ensuring stable, conformal contact are of traditional interest for high-fidelity signal transduction over prolonged implantation.

Permanent wet-tissue adhesion has been achieved using double-sided dry tapes that rapidly bond hydrated tissues to device surfaces without external stimuli (Fig. 2(d)) [45]. This method provides robust mechanical coupling even under challenging surgical conditions. For applications requiring temporary adhesion, bioresorbable adhesives present an effective solution. Yang et al. developed a photocurable, bioresorbable adhesive that forms a mechanically stable, electrically conductive interface between flexible bioelectronic devices and soft tissues, which gradually degrades after the intended operational period [7]. Bioresorbable adhesive interfaces for biomedical applications further emphasize tunable degradation rates, strong wet adhesion, and compatibility with diverse device substrates [46].

Collectively, these advances provide a strong foundation for contact-reliable bioelectronic systems capable of maintaining signal integrity over extended timeframes, while allowing non-invasive removal or complete bio resorption—an essential requirement for next-generation multifunctional soft robotic implants.

4.1 Implications of actuator-induced mechanical deformation on neural tissues

The integration of soft actuators into implantable bioelectronic systems inevitably introduces mechanical deformation to surrounding tissues, raising fundamental questions regarding mechanobiological safety, particularly in the brain and spinal cord. While recent soft robotic implant studies have demonstrated promising in vivo feasibility, the majority of these works infer safety indirectly through acute implantation outcomes or post hoc histological analysis rather than through systematic mechanical characterization.

From a broader biomechanical perspective, prior neurotrauma studies clearly demonstrate that neural injury is governed not only by deformation magnitude but also by deformation rate and mode. For example, three-dimensional in vitro compression models have shown that neuronal injury strongly depends on both strain amplitude and strain rate, indicating that dynamic loading conditions critically influence neural damage thresholds [47]. Similarly, in vivo spinal cord injury models have revealed a strong correlation between maximum principal strain and tissue damage across contusion and dislocation scenarios [48]. These findings establish that neural tissues exhibit quantifiable strain-based injury criteria, even in the absence of implantable devices.

Despite this foundational knowledge, such mechanobiological principles have not yet been systematically incorporated into the design or evaluation of soft robotic implants. Existing implant studies—including fluidically deployable spinal cord stimulation systems, deployable ECoG platforms, and shape-memory–based implants triggered by external stimuli—suggest that actuator-induced deformation can be tolerated under carefully constrained conditions [25,26]. However, these observations remain qualitative and application-specific, and do not yet translate into generalizable design guidelines.

Taken together, these considerations highlight a critical gap between established neural injury biomechanics and the emerging field of soft robotic implants. Bridging this gap will require future studies that explicitly link actuator deformation modes, strain levels, and actuation dynamics to tissue-level mechanical and biological responses. Such efforts are essential not only for improving safety but also for enabling rational design and reliable clinical translation of soft robotic implant technologies.

4.2 Sensing as a key enabler for soft robotic implants

The paradigm of soft robotic implants proposed in this review envisions soft robotic platforms that integrate actuation and sensing to enable minimally invasive implantation of bioflexible electronic devices. To realize this vision, proprioception, achieved through self-sensing and environmental sensing, is essential for reliable deployment, safe actuation, and adaptive interaction with biological tissues.

Despite its importance, sensing integration in soft robotic implants remains extremely limited. To date, explicit self-sensing has been demonstrated only in rare cases, such as the on-board strain sensor used to detect deployment status in a soft robotic ECoG system. In most other reported platforms, sensing is either restricted to environmental measurements, such as neural signal recording after deployment, or inferred indirectly through external imaging modalities, including fluoroscopy, MRI, or ultrasound. As a result, actuation and sensing are largely decoupled, and most systems operate in an open-loop manner.

These observations indicate that while sensing is already present at the system level, its integration as an intrinsic feedback mechanism remains largely unexplored. Rather than a limitation, this gap highlights a significant opportunity for future research. The development of robust self-sensing and multimodal environmental sensing will be critical for enabling closed-loop control, improving safety, and advancing soft robotic implants toward intelligent and clinically translatable systems.

The rapid advancement of implantable soft robotic systems underscores their transformative potential in next-generation bioelectronics. Across fluidic, thermal, optical, and magnetic actuation modalities, recent studies in functional integration of these soft robotic actuators with bioelectrical interfaces have demonstrated minimally invasive surgical implantations to diverse target organs enabled by programmable deployment. Each actuation strategy carries distinct strengths and limitations: fluidic actuation provides large force density and rapid deployment but is constrained by tethered fluidic hardware peripherals; magnetically responsive systems offer precise spatiotemporal control but require a bulky external magnetic field generator and sophisticated control system; shape memory polymers enable compact delivery with remote stimulation triggers, though their slow recovery speed and long-term durability remain challenges. A key insight from these diverse approaches is that no single mechanism is universally optimal, and hybrid or multi-modal actuation strategies may provide the best fit to a specific clinical practice.

Equally important are the materials and structural design considerations that dictate chronic stability. Biocompatibility challenges arise from the foreign-body response, necessitating surface modifications such as zwitterionic coatings to reduce immune rejection. Structural optimization, exemplified by shear-lag inspired composites, provides a powerful means to match mechanical compliance with tissue and maintain conformal electrode contact during dynamic motion. Bioresorbable systems further expand the design space by eliminating the need for retrieval surgeries, an essential milestone toward transient biomedical implants. Together, these strategies highlight the necessity of integrating both material science and mechanical engineering principles in soft implant design.

In conclusion, soft robotic implants represent a compelling frontier at the intersection of materials science, biomedical engineering, and robotics. By uniting advances in soft robotic actuation, biomaterials, and structural optimization, these systems hold promise for overcoming long-standing barriers in bioelectronics, including chronic immune responses and mechanical mismatch between devices and tissues. Continued interdisciplinary research will be essential to realize their potential in enabling minimally invasive, adaptive, and multifunctional implants for next-generation healthcare.