On-demand anchoring of wireless soft miniature robots on soft surfaces

Abstract

Untethered soft miniature robots capable of accessing hard-to-reach regions can enable new, disruptive, and minimally invasive medical procedures. However, once the control input is removed, these robots easily move from their target location because of the dynamic motion of body tissues or fluids, thereby restricting their use in many long-term medical applications. To overcome this, we propose a wireless spring-preloaded barbed needle release mechanism, which can provide up to 1.6 N of force to drive a barbed needle into soft tissues to allow robust on-demand anchoring on three-dimensional (3D) surfaces. The mechanism is wirelessly triggered using radio-frequency remote heating and can be easily integrated into existing untethered soft robotic platforms without sacrificing their mobility. Design guidelines aimed at maximizing anchoring over the range of the most biological tissues (kPa range) and extending the operating depth of the device inside the body (up to 75%) are also presented. Enabled by these advances, we achieve robust anchoring on a variety of ex vivo tissues and demonstrate the usage of such a device when integrated with existing soft robotic platforms and medical imaging. Moreover, by simply changing the needle, we demonstrate additional functionalities such as controlled detachment and subsurface drug delivery into 3D cancer spheroids. Given these capabilities, our proposed mechanism could enable the development of a new class of biomedical-related functionalities, such as local drug delivery, disease monitoring, and hyperthermia for future untethered soft medical robots.

Keywords: Wireless medical robots; medical devices; miniature robots; soft robots; surface anchoring.

Fig. 1.

Proposed RF-triggered spring-loaded surface-anchoring mechanism. (A) Conceptual illustration of the anchoring mechanism integrated with existing soft magnetic robots to achieve anchoring in open and enclosed environments. (B) Fabricated prototype. (C) Fabrication procedure. (C, I) The components (barb, needle, copper plate, spring, and casing) are brought together. (C, II) Cyanoacrylate adhesive is applied to the sides of the copper plate, and the barb is mounted on the needle and secured with adhesive. (C, III) The copper plate is then pushed into the casing after the adhesive has cured to create an interference fit. (D) Actuation sequence of the mechanism. (Insets) The temperature changes inside the copper plate as the mechanism is triggered. Red and blue represent higher and lower temperatures, respectively. (D, I) The mechanism is positioned. (D, II) Exposure to RF field heats up the copper plate. (D, III) Device is triggered (E) High-speed video image snapshots of the firing process in air. The brown line indicates the boundary between the substrate and the mechanism. The images are blurred primarily because some parts are moving out of plane, and the firing process takes place too quickly for the focus to be adjusted automatically or manually. The arrows indicate the direction of movement of the various parts as the mechanism is fired. (F) Infrared camera images of the firing process in air. In the first infrared image, the area bordered in orange indicates the boundaries of the substrate, and the area bordered in red indicates the approximate location of the mechanism. Cross from t = 1 s to t = 4 s indicates the location of the hottest region, which corresponds to where the copper plate is located.

Fig. 2.

Mechanism characterization. (A) Schematic of experimental setup. (B) Schematic of barb design. (C) Penetration depth with respect to the different substrates used in this work (n = 5). The dotted line denotes the position of the barb. Error bars represent the SD. (D) Pull-out force of different barb designs with respect to the different substrates (n = 5). Error bars represent the SD. (E) Pull-out force with respect to the force supplied by the spring (n = 5). Error bars represent the SD. (F) Pull-out force with respect to the extension length of the spring (n = 5). Error bars represent the SD. (G) Velocity at 1 mm with respect to substrate stiffness and spring constant at a mass ratio of 1. (H) Displacement–time graph of the mass $m_1$ at different $\frac{m_1}{m_2}$ mass ratios traveling into the substrate during phase I. The colors represent the different mass ratios. (I) Maximum displacement with respect to substrate stiffness and spring constant at a mass ratio of 1.

Fig. 3.

Mechanism characterization and modeling. (A) Pull-out force with respect to the distance of needle from the substrate (n = 5). Error bars represent the SD. (B) Pull-out force with respect to the angle of approach to the substrate (n = 5). Error bars represent the SD. (C) Schematic of experimental setup for heating experiments. (D) Predicted temperature of the copper plate after 25 s with respect to plates of varying thicknesses and distances at a constant angle ϕ = 0°. The red dotted line indicates that the temperature has exceeded 60 °C. (E) Predicted temperature of the copper plate after 25 s with respect to plates of varying thicknesses and angles at a constant distance of d = 0.02 m. The red dotted line indicates that the temperature has exceeded 60 °C. (F) Temperature rise of copper plate of varying thicknesses (t = 0.2, 0.5, and 1.0 mm) predicted from finite element simulation plotted against experimental values at d = 0.03 m, ϕ = 0° (n = 3). Error bars represent the SD. (G) Difference in temperature due to a change in angle at d = 0.02 m (n = 3). Error bars represent the SD.

Fig. 4.

Demonstration of the proposed anchoring mechanism. (A) Micro-CT image showing anchoring of the device on an ex vivo bladder tissue. The yellow and red lines represent the boundary of the bladder tissue and the orientation of the copper plate, respectively. (B) Robustness of the mechanism in various simulated body fluids (n = 5). Error bars represent the SD. (C) Pull-out force of the needle on biological tissues (n = 5). Error bars represent the SD. (D) Experiment to simulate dilation and relaxation of the bladder. (E) Mechanism integrated with a jellyfish robot for anchoring in fluid-filled three-dimensional spaces. (F) Ultrasound images of the mechanism integrated with a sheet-shaped soft robot for anchoring in confined spaces. Orange, yellow, and light green outlines represent the casing, needle mounted on copper plate, and spring, respectively.

Fig. 5.

Multifunctional needle toward medical applications. (A) Conceptual illustration of controllable detachment and passive removal of the device after anchoring. (B) Time to detachment of copper plate from the substrate using biodegradable needles of different diameters (n = 2). Error bars represent the standard deviation. (C) The 300-µm-diameter biodegradable needle before and after degradation. (C, I) Image taken on day 0 before it was anchored on a 10:1 PDMS substrate. The yellow line indicates the degradation site. (C, II) Close-up photo of the needle. The needle was removed from the PDMS substrate for imaging. (C, III) Detached copper plate. Images in C, II and C, III were taken on day 1, after the structure had been submerged in DI water in an incubator at 37$°\text{C}$. (D) DOX delivery to HT-29 spheroids. The needle was punched into the HT-29 spheroids, and time lapse images were recorded. The fluorescence signal of DOX distributed in the spheroid in time, demonstrating the drug release capability of the needles.