When Friction Meets Function

Abstract

In medical instrument design, friction is traditionally treated as a binary choice: either minimize it to reduce wear and trauma, or maximize it to anchor and stabilize. Yet many transluminal tools, such as catheters and endoscopes, need both: low friction to navigate safely and high friction to remain stable at the target site. This dissertation reframes friction from a static constraint into a controllable parameter that can be actively modulated to engineer systems that glide, grip, and propel.

Part I of this dissertation investigates the use of ultrasonic lubrication to modulate friction at the instrument interface between low friction (glide) and high friction (grip) on demand. We use high-frequency transverse vibrations to pressurize the squeeze film of fluid at the contact interface, increasing the interfacial separation and lubricating the contact on demand. We develop a theoretical understanding of how this fluid-film pressure buildup occurs in liquid environments relevant to catheter applications, such as blood vessels, and use it to design catheter-compatible friction-control modules, demonstrating a friction reduction of up to 42% in ex vivo tests on porcine aorta and confirming the viability of ultrasonic lubrication for this application.

Part II of this dissertation shifts from local-level friction modulation to system-level tuning of friction anisotropy for robotic propulsion. We show that sequencing the motion of multiple sliding contact elements creates the system-level friction anisotropy required for self-propulsion, inspired by how parasitic wasps sequence the motion of their ovipositor valves (sliders) to penetrate skin, steer, and transport eggs with minimal external forces. We develop a theoretical framework linking motion patterns to thrust and stability and validate it through a multi-slider self-propelling robotic capsule for colonoscopy that uses a mechanically encoded slider-motion sequence for control-free propulsion in the colon.

Part III combines the local friction modulation introduced in Part I with the system-level tuning of friction anisotropy developed in Part II to enable efficient robotic locomotion. We demonstrate this combination in two bio-inspired locomotion systems based on inchworm locomotion and ovipositor-inspired transport. Both systems achieve locomotion using only two modules under identical normal loads and isotropic surface conditions, unlike traditional systems, which typically achieve locomotion using a larger number of modules or dedicated mechanisms for normal-load modulation. These results underscore how coupling local and system-level friction tuning can improve locomotion efficiency, reduce design complexity, and unlock new possibilities for robotic propulsion.

By transforming friction from a passive constraint into an active control variable, this work shifts the design paradigm for medical instruments toward engineering with friction, not around it, enabling more optimal designs for physical interaction and unlocking a path toward a new class of friction-programmable instruments. Beyond medical applications, the methods developed in this dissertation provide a transferable toolkit for active friction control in soft robotics, human-robot interaction, and tribology, promising a future of mechanically intelligent, interaction-aware machines that adapt to the physical world with inherent safety and precision.