Low-friction Medical Tool Links with Curved Guide Paths
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Solution Overview
Problem
Existing medical instruments face challenges in reducing size while maintaining functionality, particularly in minimally invasive surgery, where small diameter instruments with low friction and easy assembly are required.
Innovation Solution
The development of low-friction medical devices featuring a first link, a second link, and a tension member, where the second link is rotatable relative to the first link, and the tension member has specific portions to minimize friction and facilitate easy assembly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the size of medical instruments is reduced for minimally invasive surgery, then the operating footprint is reduced allowing smaller entry incisions, but the cable friction increases and component strength decreases
Solution Approach 1:
A pulley is introduced as an intermediary component between the cable and the wrist mechanism. The pulley has a groove that guides the cable and a surface that reduces friction through rolling contact or low-friction material coating. This mediator allows the cable to transmit force efficiently while minimizing direct friction against the wrist mechanism components.
Solution Approach 2:
The friction coefficient of the cable-wrist mechanism interface is reduced by changing material parameters. The pulley surface is coated with low-friction materials such as PTFE (Teflon) or diamond-like carbon, reducing the coefficient of friction from typical metal-to-metal contact (0.5-1.0) to below 0.1. This parameter change enables reliable operation with reduced cable diameter.
2Volume of moving object
If the size of medical instruments is reduced, then smaller entry incisions are enabled, but cable lengthening and creep increase
Solution Approach 1:
The cable is pre-treated with a coating or sheath during manufacturing that protects against creep and lengthening. This preliminary protective action creates a barrier layer that prevents moisture ingress and reduces stress concentration points, thereby maintaining cable dimensional stability throughout the instrument's service life.
Solution Approach 2:
The cable is constructed as a composite structure with a core material optimized for tensile strength and a protective outer layer. The composite design combines materials with complementary properties - for example, a stainless steel or tungsten core for strength surrounded by a polymer or ceramic coating for creep resistance and corrosion protection.
3Volume of moving object
If the size of wrist mechanism components is reduced, then instrument diameter is decreased, but assembly complexity increases
Solution Approach 1:
The wrist mechanism is divided into modular segments that can be assembled in a standardized sequence. Each segment contains specific functions (pulley mounting, cable routing, joint articulation) and interfaces with adjacent segments through standardized connection features. This segmentation allows for simplified assembly procedures even with reduced component sizes.
Solution Approach 2:
Components are designed with multi-functionality to reduce the total number of parts. For example, a single pulley assembly may serve as both a cable guide and a joint articulation point, while also providing structural support. This universal design reduces assembly steps and minimizes the complexity of small-component fabrication.
4Volume of moving object
If cable diameter is reduced for smaller instruments, then operating footprint is reduced, but localized forces increase causing excessive creep
Solution Approach 1:
The pulley acts as a force-distributing intermediary that spreads localized stresses over a larger contact area. The pulley groove geometry is designed to distribute the cable tension forces uniformly across the pulley surface, preventing stress concentration at any single point and thereby reducing creep in the cable and surrounding structures.
Solution Approach 2:
The contact pressure between the cable and pulley is reduced by changing the pulley surface parameters. A larger pulley diameter with optimized groove geometry distributes the cable force over a longer contact arc, reducing the peak contact pressure. This parameter change allows smaller cable diameters while maintaining acceptable stress levels.
Data Source
AI summary
A low-friction medical device includes a first link, a second link, and a tension member. The first link is coupled to an instrument shaft and a first guide path is defined. The second link is rotatable relative to the first link through an angular range. A distal end portion of the second link is rotatably coupled to a tool member. A curved guide path is defined within the second link between the tool member and the first guide path. A curved guide surface of the second link defines a portion of the second guide path. A first portion of the tension member is parallel to a centerline of the first guide path, and a second portion is coupled to the tool member. A third portion of the tension member between the first and second portions is in contact with the curved guide surface throughout a portion of the angular range.