Thomas, T. L., Kalpathy Venkiteswaran, V., Ananthasuresh, G. K., & Misra, S. (2021). Surgical applications of compliant mechanisms: a review. Journal of mechanisms and robotics, 13(2).
1 Introduction
Compliant mechanisms (CMs) are designed to achieve transfer or transformation of motion, force, or energy through elastic deformation of flexible elements.
CMs have gained significant attention in the last few decades as they offer many advantages over traditional rigid-body mechanisms. A CM has monolithic structure, which reduces the number of assembly steps, thus simplifying the fabrication process and requiring reduced maintenance. High precision is attained, and the need for lubrication (润滑) is eliminated due to the absence of contact among members that causes wear, friction, backlash, and noise.
The structural compliance integrated in the main body of a device is exploited to perform object manipulation tasks such as grasping (抓取), cutting(切割), retracting(拉回), and suturing(缝合) for surgical procedures in the form of ablation(消融), laparoscopy(腹腔镜检查), endoscopy(内窥镜检查), and biopsy (活检), to mention a few. CMs also serve a secondary function in the device to transmit force/motion. Force sensing using CMs to monitor tool–tissue interaction has also been demonstrated, which serves as a feedback for safe operation of the device inside the human body.
This article aims to provide an overview of this process, which involves five major aspects involves five major aspects: (i) CM conceptual design and synthesis, (ii) analysis, (iii) material selection, (iv) fabrication methods, and (v) actuation methods. Furthermore, this article also reviews the existing literature on surgical devices that use CMs by classification into five major groups: (i) grasping and cutting, (ii) reachability and steerability, (iii) transmission, (iv) sensing, and (v) implants and deployable devices (植入物与可展开装置).
2 Design Aspects
Four techniques used in the synthesis of CMs: freedom and constraint topologies (FACTs), building blocks, topology optimization, and rigid-body replacement.
However, many compliant surgical devices are designed without explicit use of these conventional synthesis methods. This may be because the synthesis methods developed for CMs mostly apply to input-output transmission characteristics rather than guiding and maneuvering. The scope of the expected functions of surgical devices, described later in the article, offers a huge opportunity for designers.
| Synthesis method | Description | Applications and limitations |
|---|---|---|
| Freedom and constraint topologies (FACT) | Provides topological solution for known freedom space and constraint space based on screw theory (旋量理论), in which twists and wrenches are used to represent constraints and degrees-of-freedom of compliant elements. | 1. Synthesizing CMs with small to intermediate deflections; 2. Research on large deformation analysis and representation of elastomechanics, dynamics characteristics, and parasitic errors is limited. |
| Building blocks | Two main approaches based on: (i) instant centers and compliance ellipsoids (柔度椭球), and (ii) flexible building blocks and optimization. | 1. Synthesizing CMs with intermediate to large deflections; 2. Infeasible geometry may result depending on the chosen basic building block. |
| Topology optimization | Uses optimization algorithms to search for best CM topology to realize the design objective, subject to desired requirements and constraints generally through finite element methods. | 1. Most widely used CM synthesizing method within surgical
devices, with its ability to generate solutions from a wide
design space. 2. Difficult to account for localized stresses and buckling (屈曲). Resulting topologies are sometimes difficult to manufacture, warranting 3D printing or postprocessing for manufacturing. |
| Rigid-body replacement | Utilizes the pseudo-rigid-body model to replace compliant members and joints with equivalent rigid links and movable joints, with springs for capturing elastic deformation energy. | 1. Reduced-order method that relies on established rigid-body
kinematics methods, providing more intuitive analysis. 2. Accuracy of analysis suffers with increase in the complexity of CM. |
| Selection maps | Uses a catalog of CMs whose inherent stiffness and inertia characteristics are captured in two-port spring-lever and spring-mass-lever models for matching the user specifications for the purpose of selection. | 1. Can incorporate practical considerations of material selection,
manufacturability, strength, and scaling. 2. Limited to single-input-single-output CMs at present. |
From a clinical standpoint, other criteria that need to be considered are the biocompatibility (生物相容性), chemical resistance (耐化学性), elasticity (弹性), transparency (透明性), strength (强度), temperature resistance (温度耐性), and most importantly, sterilizability (灭菌性) of the chosen material.
The method of actuation is an important aspect to be considered in the design of a CM. Cable-driven actuation is the most widely used method among continuum manipulators and steerable instruments. SMAs and piezoelectric materials are seen more in high precision devices and for micro/nano manipulation. While fluidic actuation is used in a few flexible surgical instruments, there is a gradual increase toward the use of magnetic actuation in designing surgical devices for precise contactless control.
| Actuation method | Surgical applications | Integration with compliant mechanisms |
|---|---|---|
| Cable-driven actuation | Surgical robotic systems and flexible surgical instruments. + Uses lightweight and flexible cables for deformation of the structure. - Miniaturization is challenging due to the actuation unit away from the workspace of the device associated cables and moment arms |
+ Ability to transmit force/motion to remote joints and application
points enables convenient location of the actuation unit away from the
workspace of the device. - High pretension (预拉) in cable is necessary to reduce backlash and hysteresis |
| Shape memory alloys (SMAs) | Internal actuators for instruments like biopsy forceps (活检钳),
hingeless graspers (无铰链握把), and endoscopic and laparoscopic
(内镜和腹腔镜) instruments, among others. Also used in stents (支架),
stent grafts (支架移植物) and in orthopaedics (整形外科) as correction
rods and fracture fixators (矫正棒和骨折固定器). + Similar hysteresis behavior with bone and tendons (筋骨) and low sensitivity to MRI. +Shape memory effect provides a collapsible (可折叠) form during insertion and expands after deployment. - Limited by rise in temperature caused by heating. |
+ Reliable control on actuating CM by training the SMA to fine-tune
the performance. + Offers high power-to-weight ratio. + Easy to embed in complex structures. + Generally activated by Joule heating while deactivation takes place via convection heat transfer, which leads to a slow response time. |
| Piezoelectric materials | Actuators for micro/nano manipulation. + Delivers sub-nanometer positioning accuracy and is compact in size. - Expensive to fabricate. |
+ Offers high response speed. + Large force-to-weight ratio. -Limited by low strain range. -Transmission of forces to remote location is challenging. |
| Magnetic actuation | Endoscopic devices and surgical instruments with inherent
compliance. + Precise positioning and control. |
+ Enables contactless actuation of CM. - Adversely affected upon scaling to large surgical workspace. |
| Flexible fluidic actuators | Flexible surgical instruments. + Safe to operate under radiation and magnetic fields. + Ability of the inflatable membranes (充气膜) to lose and regain their shape facilitates the insertion of instrument inside a patient’s body. |
+ Causes no relative motion between parts, no wear and there is no
need for lubrication. - Risk of leakages, and controlling pressure is more complex when compared to electrical signals used in motors and other conventional actuators. |
3 Surgical Applications
3.1 Grasping and Cutting
CMs have been used to develop forceps (手术钳), scissors (剪刀), graspers, and needle holders for performing different surgical tasks such as grasping, cutting, suturing (缝合), and holding tissue.
Several forms of grasping tools have been investigated, which utilize the flexibility and stiffness that a CM can offer with different geometry, materials, and fabrication techniques.
The introduction of robot-assisted surgery has led to many designs of CM-based grasping end effectors, to deliver efficient manipulation with high dexterity.
The monolithic nature of CMs makes them easier to fabricate when compared to the pivoted jaw configurations of current grasping tools. Hence, CM was used in developing a disposable compliant forceps for HIV patients in which, the Q-joints methods was employed to replace a conventional pin-joint.
At micrometer scale, CM-based microgrippers and micromanipulators have been developed based on flexure hinges and cantilever beam structures. While the use of CMs contributes to the elimination of Coulomb friction and backlash, they have some inherent drawbacks. As noted in the design of a low cost flexure-based hand-held mechanism for micromanipulation, a drift in the major axis is caused by the imperfect rotation of most compliant joints. Flexure hinges have limited range of angular motion depending on the geometry and material properties of the hinge, and cantilever structures fail to produce perfect parallel motion. However, topology optimization aided by intuition has been used to design CM grippers with parallel-jaw motion.
3.2 Reachability and Steerability
This section describes applications of CMs to increase the range of motion and enhance steerability of the surgical instruments to reach difficult to reach surgical sites inside the body.
Continuum manipulators are devices that can be precisely steered inside the body to reach difficult-to-access surgical sites. CMs have been used to design flexible miniaturized continuum manipulators for robot-assisted surgery. A similar design was used in a flexible micro manipulator for neurosurgery.
Notched-tube compliant joint mechanisms are variants of aforementioned continuum manipulators, where different shapes, sizes, and patterns of notches made on tubes can enable different DoFs and range of motion.
Concentric tube robots (CTRs, 同心管机器人) are a special type of continuum manipulators that are made of multiple precurved elastic tubes that are concentrically nested within one another. CTRs have been deployed for “follow-the-leader” insertion, and their steering is not affected by the tissue interaction forces. Thus, they have found several applications as steerable needles and miniaturized surgical manipulators.
3.3 Transmission
Transmission refers to the use of CMs in augmenting an actuator in the transfer of force, displacement, or energy. In some surgical devices, CMs made for force or displacement transmission serve as an input or feedback for the principal function of the device. Lan and Wang developed an adjustable constant-force forceps for robot-assisted surgical manipulation to aid in grasping soft tissues. It employs a compliant constant-torque mechanism made using flexible arms to transmit the required force to forceps tips. The motion of a flexure-based parallel manipulator for an active hand-held micro-surgical instrument was tracked to cancel the hand tremors (颤抖) using piezo-actuators.
A drawback of CMs is that energy efficiency is challenged due to energy storage in the flexible members of the mechanism.
3.4 Sensing
Sensing application refers to the use of CMs in detecting or measuring physical quantities. Several kinds of sensors rely on the change in deflection or stiffness of CMs in conjunction with other transducers like optical sensors and strain gauges to measure physical parameters. Alternatively, vision-based force sensing integrated with miniature grippers was reported by Reddy et al.. Subsequently, a compliant end-effector to passively limit the force in tele-operated tissue-cutting using the vision based force sensing for haptic feedback was demonstrated.
Force sensing forms an integral part of different surgical applications that involve tissue palpation (触诊), pulling, and pushing of tissue during biopsy (活检), to name a few.
Magnetic resonance imaging (MRI)-compatible force sensors, in particular, benefit from a CM-based design as the metallic and electric elements (金属和电器元件) can be placed outside MRI. The force sensing element typically consists of an elastic body which deforms under the influence of an applied force, which in turn is measured by a transducer like optical fiber.
Achallenge with multi-axial force sensors lies in the decoupling of forces along the axes. Linear decoupling methods proved to be inaccurate since local deformation of flexures affects the strains measured. A method to decouple pulling and grasping forces of a 2DOF compliant forceps was derived using the serial connections of two torsional springs, which was realized by optimizing the shape of two circular-type flexure hinges.
Other factors to be considered while designing force sensors include thermal sensitivity, hysteresis, plastic deformation and friction due to contact between internal components that can alter the elastic behavior of flexures.
3.5 Implants and Deployable Devices
Implants are medical devices embedded inside the body via surgery to replace or enhance damaged biological tissue. Within the context of this article, deployable devices refer to CMs designed to change in shape and size that facilitate insertion of the surgical device in a compact form to reduce invasiveness of the procedure. Origami works well with flexible nonmetallic materials, thus making them ideal for MRI-guided procedures, which is hazardous in the presence of magnetic materials.
4 Discussion
This study began with the aim of assessing the utility of CMs in designing surgical devices. There are some challenges that hinder the further development and implementation of these devices in clinical practice. A drawback concerning CMs is the adverse effect of stress concentrations and fatigue, especially in flexure-based designs under cyclic loading. This is a major challenge in the medical field where device failure is not acceptable. To tackle this issue, there is a growing interest towards developing multi-material CMs and functional grading of CMs to enhance structural integrity. Soft robotics is another emerging field of interest, which utilizes flexibility to function but is not classified under CMs. Inspired by the softness and body compliance of biological systems, continuum devices based on soft robotics systems are designed using compliant materials.
The behavior of CMs with geometric nonlinearity caused by large deflections is disregarded in many studies. There is scope for improvement by analyzing and understanding the deformation of flexible members of CMs under these complex conditions.
This review highlights the merits of CMs over conventional rigid body mechanisms due to elimination of joint friction, backlash, wear, and need for lubrication. This aspect is leveraged by integration of CMs with modern actuators such as magnets, SMAs, and piezoelectric materials.
However, a major challenge lies in analyzing an overall system of CM consisting of multiple flexible members. While the monolithic nature of most of the CMs simplifies the fabrication and assembly processes, the flip side is that the whole design may fail if even one part of the mechanism breaks.
From a clinical standpoint, the protection of instruments from contamination due to contact with fluids is important. As a potential solution, some researchers have suggested soft elastic coating of the instrument. However, further analysis of the implications of in-vivo operating conditions on the instrument’s performance, while maintaining sterilization, is necessary.