Femtosecond laser fabricated nitinol living hinges for millimeter-sized robots

TL;DR

This work demonstrates femtosecond laser micromachining of nitinol to fabricate living hinges with 2.5D cross sections for millimeter-scale robots, paired with analytical and Abaqus nonlinear FEM models and XRD validation to ensure preserved superelastic properties. The authors map processing parameters, validate hinge torque against models, and showcase a 7-layer nitinol wing mechanism actuated by a piezoelectric motor, illustrating rapid, monolithic prototyping potential. The results indicate negligible heat-affected zones, accurate torque-angle predictions, and significant design freedom for high-displacement, small-scale compliant mechanisms, with clear pathways for surface finishing and fatigue-life improvements. Overall, the approach enables robust, high-displacement nitinol hinges suitable for milli-robotics and medical devices requiring compact, durable joints.

Abstract

Nitinol is a smart material that can be used as an actuator, a sensor, or a structural element, and has the potential to significantly enhance the capabilities of microrobots. Femtosecond laser technology can be used to process nitinol while avoiding heat-affected zones (HAZ), thus retaining superelastic properties. In this work, we manufacture living hinges of arbitrary cross-sections from nitinol using a femtosecond laser micromachining process. We first determined the laser cutting parameters, 4.1 Jcm^-2 fluence with 5 passes for 5 um ablation, by varying laser power level and number of passes. Next, we modeled the hinges using an analytical model as well as creating an Abaqus finite element method, and showed the accuracy of the models by comparing them to the torque produced by eight different hinges, four with a rectangular cross-section and four with an arc cross-section. Finally, we manufactured three prototype miniature devices to illustrate the usefulness of these nitinol hinges: a sample spherical 5-bar mechanism, a sarrus linkage, and a piezoelectric actuated robotic wing mechanism.

Figures (4)

• Figure 1: (a): One of four rectangular cross section hinges used to characterize hinge torque and validate hinge models. (b): Living hinge manufactured with cutouts on both sides of bulk material, as used in the wing mechanism. (c): 5 bar mechanism for a millirobot manufactured from nitinol (left) and from the traditional carbon fiber and Kapton (right) kabutz2023design. (d): Wing mechanism manufactured from carbon fiber, nitinol, and mylar.
• Figure 2: Workflow for establishing laser cutting parameters. (a): Femtosecond laser micromachine. (b): Array of squares used for characterizing the nitinol sheet cut by our laser. The horizontal axis of the array includes the laser power levels 150-250 kHz in increments of 25 kHz. The vertical axis runs from 1-14 passes. Each square has a side length of 200 $\mu m$ and is filled with a grid of cuts spaced 5 $\mu m$ apart. (c): Confocal microscope. (d): Etch depth as a function of the number of passes for each of the five power levels tested along with lines of best fit. (e): Roughness comparison between the manufactured hinge and unmachined surface. (f): Plot comparing the 2 mm wide model hinge cross section with a manufactured one to demonstrate its accuracy and potential for realizing miniature features. (g) Graph of Intensity as a function of 2$\theta$ from XRD test for four different samples with major austenite and martensite peaks labelled. (110) and (211) peaks correspond to austenite and (111) corresponds to martensite.
• Figure 3: (a) and (b): Diagram of the notch and rectangular hinges. Half cross sections are depicted for ease of visualization. (c): Meshed hinge model with boundary conditions and applied torque shown in red. (d): Result of an Abaqus analysis showing the displaced hinge and von Mises stress. (e): Series of 4 images showing the progression of our test for measuring hinge torque. Inset shows a close-up of the hinge from the torque test. We rotated the servo at a speed of 0.05 rad/s to 40 degrees for 8 different hinges. (f): Stress vs. strain data from the nitinol used in this paper. Note the bilinear behavior. (g): Torque (N-$\mu m$) as a function of hinge angle (deg) for notch (top) and rectangular (bottom) hinges with varying hinge thickness. We provide the mean and standard deviation for each of the 8 hinges. (h): Table showing the experimentally determined material properties and dimensions of the hinges.
• Figure 4: (a): Manufacturing stack-up for the wing mechanism showing the 7 layers. (b) Manufactured wing mechanism on fingertip. (c): Overlay of 4 moments during the wing mechanism test, with t = 0s on the left and t = 2.5ms on the right. The wing had an angle range of 50°, and the yellow points show the location on the wing that we tracked in the graph below. (d): Graph of wing tip deflection in the X direction as a function of time for 200V actuator input at 10Hz, 100Hz, and 200Hz.