Softening implantable bioelectronics: Material designs, applications, and future directions

Implantable bioelectronics, integrated directly within the body, represent a potent biomedical solution for monitoring and treating a range of medical conditions, including chronic diseases, neural disorders, and cardiac conditions, through personalized medical interventions. Nevertheless, contemporary implantable bioelectronics rely heavily on rigid materials (e.g., inorganic materials and metals), leading to inflammatory responses and tissue damage due to a mechanical mismatch with biological tissues. Recently, soft electronics with mechanical properties comparable to those of biological tissues have been introduced to alleviate fatal immune responses and improve tissue conformity. Despite their myriad advantages, substantial challenges persist in surgical handling and precise positioning due to their high compliance. To surmount these obstacles, softening implantable bioelectronics has garnered significant attention as it embraces the benefits of both rigid and soft bioelectronics. These devices are rigid for easy standalone implantation, transitioning to a soft state in vivo in response to environmental stimuli, which effectively overcomes functional/biological problems inherent in the static mechanical properties of conventional implants. This article reviews recent research and development in softening materials and designs for implantable bioelectronics. Examples featuring tissue-penetrating and conformal softening devices highlight the promising potential of these approaches in biomedical applications. A concluding section delves into current challenges and outlines future directions for softening implantable device technologies, underscoring their pivotal role in propelling the evolution of next-generation bioelectronics.

Magnetic steering continuum robot for transluminal procedures with programmable shape and functionalities

Millimeter-scale soft continuum robots offer safety and adaptability in transluminal procedures due to their passive compliance, but this feature necessitates interactions with surrounding lumina, leading to potential medical risks and restricted mobility. Here, we introduce a millimeter-scale continuum robot, enabling apical extension while maintaining structural stability. Utilizing phase transition components, the robot executes cycles of tip-based elongation, steered accurately through programmable magnetic fields. Each motion cycle features a solid-like backbone for stability, and a liquid-like component for advancement, thereby enabling autonomous shaping without reliance on environmental interactions. Together with clinical imaging technologies, we demonstrate the capability of navigating through tortuous and fragile lumina to transport microsurgical tools. Once it reaches larger anatomical spaces such as stomach, it can morph into functional 3D structures that serve as surgical tools or sensing units, overcoming the constraints of initially narrow pathways. By leveraging this design paradigm, we anticipate enhanced safety, multi-functionality, and cooperative capabilities among millimeter-scale continuum robots, opening new avenues for transluminal robotic surgery.

Fast-Response Variable-Stiffness Magnetic Catheters for Minimally Invasive Surgery

In minimally invasive surgery, such as cardiac ablation, magnetically steered catheters made of variable-stiffness materials can enable higher dexterity and higher force application to human tissue. However, the long transition time between soft and rigid states leads to a significant increase in procedure duration. Here, a fast-response, multisegmented catheter is described for minimally invasive surgery made of variable-stiffness thread (FRVST) that encapsulates a helical cooling channel. The rapid stiffness change in the FRVST, composed of a nontoxic shape memory polymer, is achieved by an active cooling system that pumps water through the helical channel. The FRVST displays a 66 times stiffness change and a 26 times transition enhancement compare with the noncooled version. The catheter allows for selective bending of each segment up to 127° in air and up to 76° in water under an 80 mT external magnetic field. The inner working channel can be used for cooling an ablation tip during a procedure and for information exchange via the deployment of wires or surgical tools.

Phase Transition Liquid Metal Enabled Emerging Biomedical Technologies and Applications

Phase change materials that can absorb or release large amounts of heat during phase transition, play a critical role in many important processes, including heat dissipation, thermal energy storage, and solar energy utilization. In general, phase change materials are usually encapsulated in passive modules to provide assurance for energy management. The shape and mechanical changes of these materials are greatly ignored. An emerging class of phase change materials, liquid metals (LMs) have attracted significant interest beyond thermal management, including in transformable robots, flexible electronics, soft actuators, and biomedicine. Interestingly, the melting point of LM is highly tunable around body temperature, allowing it to experience considerable stiffness change when interacting with human organisms during solid-liquid change, which brings about novel phenomena, applied technologies, and therapeutic methods, such as mechanical destruction of tumors, neural electrode implantation technique, and embolization therapy. This review focuses on the technology, regulation, and application of the phase change process along with diverse changes of LM to facilitate emerging biomedical applications based on the influences of mechanical stiffness change and versatile regulation strategies. Typical applications will also be categorized and summarized. Lastly, the advantages and challenges of using the unique and reversible process for biomedicine will be discussed.

A variable stiffness manipulator with multifunctional channels for endoscopic submucosal dissection

PURPOSE

Endoscopic submucosal dissection (ESD) has become the main treatment for early esophageal and gastric cancers, but the insufficient stiffness and large diameter of current devices increase the difficulty in operation. To address the above problems, this study proposes a variable stiffness manipulator with multifunctional channels for ESD.

METHODS

The proposed manipulator has a diameter of just 10 mm and highly integrates a CCD camera, two optical fibers, two channels for instruments, and one channel for water and gas. Additionally, a compact wire-driven variable stiffness mechanism is also integrated. The drive system of the manipulator is designed, and the kinematics and workspace are analyzed. The variable stiffness and practical application performance of the robotic system are tested.

RESULTS

The motion tests verify that the manipulator has sufficient workspace and motion accuracy. The variable stiffness tests show that the manipulator achieves 3.55 times of stiffness variation instantly. Further insertion tests and operation test demonstrates that the robotic system is safe and can satisfy the needs in motion, stiffness, channels, image, illumination, and injection.

CONCLUSION

The manipulator proposed in this study highly integrates six functional channels and a variable stiffness mechanism in a 10 mm diameter. After kinematic analysis and testing, the performance and application prospect of the manipulator are verified. The proposed manipulator can promote the stability and accuracy of ESD operation.

A New Nonlinear Spatial Compliance Model Method for Flexure Leaf Springs with Large Width-to-Length Ratio under Large Deformation

Flexure leaf spring (FLS) with large deformation is the basic unit of compliant mechanisms with large stroke. The stiffness along the non-working directions of FLSs with large width-to-length ratio (w/L) is high. The motion stability of the compliant mechanism based on this type of FLS is high. When this type of FLS is loaded along the width direction, the shear deformation needs to be characterized. Nevertheless, currently available compliance modeling methods for FLS are established based on Euler-Bernoulli beam model and cannot be used to characterize shear models. Therefore, these methods are not applicable in this case. In this paper, a new six-DOF compliance model for FLSs with large w/L is established under large deformation. The shear deformation along the width direction model is characterized based on the Timoshenko beam theory. The new constraint model and differential equations are established to obtain a high-precision compliance model expression for this type of FLS. The effects of structural parameters on the compliance of the FLS are analyzed. Finally, the accuracy of the model is verified both experimentally and by finite element simulation. The relative error between theoretical result and experiment result is less than 5%.

Sigmoidal Auxiliary Tendon-Driven Mechanism Reinforcing Structural Stiffness of Hyper-Redundant Manipulator for Endoscopic Surgery

The overtube of an endoscopic surgery robot is fixed when performing tasks, unlike those of commercial endoscopes, and this overtube should have high structural stiffness after reaching the target lesion so that sufficient tension can be applied to the lesion tissue with the surgical tool and there are fewer changes in the field of view of the endoscopic camera from this reaction force. Various methods have been proposed to reinforce the structural stiffnesses of hyper-redundant manipulators. However, the safety, rapid response, space efficiency, and cost-effectiveness of these methods should be considered for use in actual clinical environments, such as the gastrointestinal tract. This study proposed a method to minimize the positional changes of the overtube end tip due to external forces using only auxiliary tendons in the optimized path without additional mechanical structures. Overall, the proposed method involved moving the overtube to the target lesion through the main driving tendon and applying tension to the auxiliary tendons to reinforce the structural stiffness. The complete system was analyzed in terms of energy, and the sigmoidal auxiliary tendons were verified to effectively reinforce the structural stiffness of the overtube consisting of rolling joints. In addition, the design guidelines of the overtube for actual endoscopic surgery were proposed considering hollowness, retroflexion, and high structural stiffness. The positional changes due to external forces were confirmed to be reduced by 60% over the entire workspace.

Thermoset Shape Memory Polymer Variable Stiffness 4D Robotic Catheters

Variable stiffness catheters are typically composed of an encapsulated core. The core is usually composed of a low melting point alloy (LMPA) or a thermoplastic polymer (TP). In both cases, there is a need to encapsulate the core with an elastic material. This imposes a limit to the volume of variable stiffness (VS) material and limits miniaturization. This paper proposes a new approach that relies on the use of thermosetting materials. The variable stiffness catheter (VSC) proposed in this work eliminates the necessity for an encapsulation layer and is made of a unique biocompatible thermoset polymer with an embedded heating system. This significantly reduces the final diameter, improves manufacturability, and increases safety in the event of complications. The device can be scaled to sub-millimeter dimensions, while maintaining a high stiffness change. In addition, integration into a magnetic actuation system allows for precise actuation of one or multiple tools.

Flexible Manipulator with Low-Melting-Point Alloy Actuation and Variable Stiffness

Flexible manipulators offer significant advantages over traditional rigid manipulators in minimally invasive surgery, because they can flexibly navigate around obstacles and pass cramped or tortuous paths. However, due to the inherent low stiffness, the ability to control/obtain higher stiffness when required remains to be further explored. In this article, we propose a flexible manipulator that exploits the phase transformation property of low-melting-point alloy to hydraulically drive and change the stiffness by heating and cooling. A prototype was fabricated, and experiments were conducted to evaluate the motion characteristics, stiffness performance, and rigid-flexible transition efficiency. The experimental results demonstrate that the proposed manipulator can freely adjust heading direction in the three-dimensional space. The experimental results also indicate that it took 9.2-10.3 s for the manipulator to transform from a rigid state to a flexible state and 15.4 s to transform from a flexible state to a rigid state. The lateral stiffness and flexural stiffness of the manipulator were 95.54 and 372.1 Ncm² in the rigid state and 7.26 and 0.78 Ncm² in the flexible state. The gain of the lateral stiffness and flexural stiffness was 13.15 and 477.05, respectively. In the rigid state, the ultimate force without shape deformation was more than 0.98 N in the straight condition (0°) and 1.36 N in the bending condition (90°). By assembling flexible surgical tools, the manipulator can enrich the diagnosis or treatment functions, which demonstrated the potential clinical value of the proposed manipulator.

Recent Advances in Design and Actuation of Continuum Robots for Medical Applications

Traditional rigid robot application in the medical field is limited due to the limited degrees of freedom caused by their material and structure. Inspired by trunk, tentacles, and snakes, continuum robot (CR) could traverse confined space, manipulate objects in complex environment, and conform to curvilinear paths in space. The continuum robot has broad prospect in surgery due to its high dexterity, which can reach circuitous areas of the body and perform precision surgery. Recently, many efforts have been done by researchers to improve the design and actuation methods of continuum robots. Several continuum robots have been applied in clinic surgical interventions and demonstrated superiorities to conventional rigid-link robots. In this paper, we provide an overview of the current development of continuum robots, including the design principles, actuation methods, application prospect, limitations, and challenge. And we also provide perspective for the future development. We hope that with the development of material science, Engineering ethics, and manufacture technology, new methods can be applied to manufacture continuum robots for specific surgical procedures.

Variable Rigidity Module with a Flexible Thermoelectric Device for Bidirectional Temperature Control

Dynamic stiffness tuning is a promising approach for shape reconfigurable systems that must adapt their flexibility in response to changing operational requirements. Among stiffness tuning technologies, phase change materials are particularly promising because they are size scalable and can be powered using portable electronics. However, the long transition time required for phase change is a great limitation for most applications. In this study, we address this by introducing a rapidly responsive variable rigidity module with a low melting point material and flexible thermoelectric device (f-TED). The f-TED can conduct bidirectional temperature control; thereby, both heating and cooling were accomplished in a single device. By performing local cooling, the phase transition time from liquid to solid is reduced by 77%. The module in its rigid state shows 14.7 × higher bending stiffness than in the soft state. The results can contribute to greatly widening the application of phase transition materials for variable rigidity.

Roboticizing fabric by integrating functional fibers

Fabrics are ubiquitous materials that have conventionally been passive assemblies of interlacing, inactive fibers. However, the recent emergence of active fibers with actuation, sensing, and structural capabilities provides the opportunity to impart robotic function into fabric substrates. Here we present an implementation of robotic fabrics by integrating functional fibers into conventional fabrics using typical textile manufacturing techniques. We introduce a set of actuating and variable-stiffness fibers, as well as printable in-fabric sensors, which allows for robotic closed-loop control of everyday fabrics while remaining lightweight and maintaining breathability. Finally, we demonstrate the utility of robotic fabrics through their application to an active wearable tourniquet, a transforming and load-bearing deployable structure, and an untethered, self-stowing airfoil.

Development of a variable-stiffness and shape-detection manipulator based on low-melting-point-alloy for minimally invasive surgery

There is an increasingly popularity for the continuum robot in minimally invasive surgery(MIS), because of the compliance and dexterity. In the first place, a variable stiffness manipulator can resolve the two contradictions of the demands for predominant flexibility and strong payload capacity. In the second place, to control the continuum robot more precisely and avoid the collision between robot and human body, real-time tracking of the shape of the continuum robot is of great significance. A new type of flexible manipulator with variable stiffness is proposed which can track the bending shape timely. The low-melting-point-alloy (LMPA) is used to realize the variable stiffness and shape detection for the flexible manipulator. The concept design for a single module is put forward. Then the stiffness control method and finite element simulation, the method of shape detection are presented. Moreover, the presented method of shape detection is evaluated by experiments.

Deployable, Variable Stiffness, Cable Driven Robot for Minimally Invasive Surgery

Minimally Invasive Surgery (MIS) imposes a trade-off between non-invasive access and surgical capability. Treatment of early gastric cancers over 20 mm in diameter can be achieved by performing Endoscopic Submucosal Dissection (ESD) with a flexible endoscope; however, this procedure is technically challenging, suffers from extended operation times and requires extensive training. To facilitate the ESD procedure, we have created a deployable cable driven robot that increases the surgical capabilities of the flexible endoscope while attempting to minimize the impact on the access that they offer. Using a low-profile inflatable support structure in the shape of a hollow hexagonal prism, our robot can fold around the flexible endoscope and, when the target site has been reached, achieve a 73.16% increase in volume and increase its radial stiffness. A sheath around the variable stiffness structure delivers a series of force transmission cables that connect to two independent tubular end-effectors through which standard flexible endoscopic instruments can pass and be anchored. Using a simple control scheme based on the length of each cable, the pose of the two instruments can be controlled by haptic controllers in each hand of the user. The forces exerted by a single instrument were measured, and a maximum magnitude of 8.29 N observed along a single axis. The working channels and tip control of the flexible endoscope remain in use in conjunction with our robot and were used during a procedure imitating the demands of ESD was successfully carried out by a novice user. Not only does this robot facilitate difficult surgical techniques, but it can be easily customized and rapidly produced at low cost due to a programmatic design approach.

A foldable manipulator with tunable stiffness based on braided structure

Minimally invasive surgery (MIS) has recently seen a surge in clinical applications due to its potential benefits over open surgery. In MIS, a long manipulator is placed through a tortuous human orifice to create a channel for surgical tools and provide support when they are operated. Currently the relative large profile and low stiffness of the manipulators limit the effectiveness and accuracy of MIS. Here we propose a new foldable manipulator with tunable stiffness. The manipulator takes a braided skeleton to enable radial folding, whereas membrane is used to seal the skeleton so as to adjust stiffness through creating negative pressure. We demonstrated experimentally, numerically, and analytically that, a flexible and a rigid state were obtained, and the ratio of bending stiffness in the rigid state to that in the flexible state reached 6.85. In addition, the manipulator achieved a radial folding ratio of 1.95. The proposed manipulator shows great potential in the design of surgical robots for MIS. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B, 2019.

E-BiInSn Enhanced Rigidity Alterable Artificial Bandage

surgery, orthopedic cast is often implemented to stabilize and fix anatomical structures like broken bones. Plaster could harden after mixed with water, thus it is commonly utilized with cotton bandage to form a solid structure to encase a limb or other body parts. As plaster is heavy and impervious, cast could easily result in itching, rashes, allergic contact dermatitis or other cutaneous complications. In this paper we present a novel implementation for surgical fixation with low melting point alloy (LMPA) stuffed in silicone tubes, which is dubbed "LMPA enhanced bandage The alloy is heated by an enameled copper wire to alter the stiffness. When the alloy is in solid state, the bandage could withstand high load without significant deformation, while if heated to its melting point, the entire bandage would soften. We present several conceptual experiments to evaluate the mechanical performance and body fixation of the proposed bandage. Phase change process and temperature variation were recorded by an infrared camera. Preliminary results showed that the present fixation bandage design owns sufficient mechanical strength and necessary thermal response performance to meet the requirement of clinical applications.

Water-jet outer sheath with braided shape memory polymer tubes for upper gastrointestinal tract screening

BACKGROUND

Flexible endoscopes have become an important tool for the diagnosis and treatment of gastric cancer. However, there are several limitations to the use of endoscopes in rural areas, including their high cost, poor portability, and unstable platform.

METHODS

This paper presents a novel low-cost outer sheath for stomach screening. The sheath uses braided shape memory polymer (SMP) tubes with a water-jet to control the stiffness and bending motion. The insertion part of the prototype is 250 mm long with a maximum outer diameter of 16 mm and incorporates an internal charge-coupled device camera.

RESULTS

We have tested the workspace and stiffness of the outer sheath. The prototype has also been validated with phantom and ex vivo porcine stomach experiments.

CONCLUSIONS

By controlling the water-jet and temperature of the braided SMP tubes, the outer sheath achieves a large workspace and a remarkable variability in stiffness, demonstrating the potential clinical value of the outer sheath system.