NANOFIBERS + SOFT ROBOTIC MICRO-ACTUATORS

2017 - present

Video: Max Rousseau, Wyss Institute for Biologically Inspired Engineering, Harvard University

MOTIVATION //

Soft robotic actuators are promising tools for applications requiring delicate manipulation. Flexible silicones and other organic polymers replace traditionally metallic or hard plastic components, providing actuators that move smoothly and interact gently with organisms and target objects.

Two key challenges exist in fabricating thin (<1 mm) micro-scale soft actuators:

  1. Bending modes of elastomeric devices are currently controlled by changing the geometry of the entire actuator (e.g. angle of the inflating channel) 

  2. Device curvature and input pressure are limited by the thickness of the strain limiting layer

 

INNOVATION //

We use nanofibers as the strain limiting layer for soft micro-scale actuators, addressing these challenges as follows: 

  1. Modular nanofiber integration enables control of bending modes without changing actuator geometry

  2. We can vary actuator curvature without increasing the total device thickness due to the fibers' high specific strength

Based on these two advances, we can achieve pure bending and more complex modes (e.g. bend-twist) in a <1 mm thick actuator. We also developed an analytical model to predict actuator curvature as a function of applied pressure and device geometry. These actuators are the thinnest elastomeric actuators capable of achieving complex bending modes, and have the potential to provide unprecedented control of delicate structures. 

This project was published in the Journal of Micromechanics and Microengineering within a focus issue on "Soft Robotics and Smart System Technologies."

Single Actuator Inflation | PDMS/nanofiber actuators can be inflated pneumatically or hydraulically using a central channel that spans the length of each device. The pressurization range of our devices is among the highest of existing micro-scale soft actuators, and even matches that of some meso- and centimeter-scale robots. 

Controlled MotionThe following video shows two sample bending modes for soft nanofiber-reinforced micro-actuators: bending and bending-twisting. Due to the modular design of the actuators, additional orientations can be achieved by tuning the angle of nanofibers in the laminate structure. 

Actuator Design | We used soft lithography to stack layers of biocompatible silicone (PDMS, shown below in yellow) and a sheet of tough polymer nanofibers. A flat laminate consisting of a thick PDMS layer and the nanofabric is bonded to a concave PDMS layer; together, this forms the internal channel used to inflate the device.

Fiber Orientation Influences Bending Mode | We can generate pure bending motions by embedding monolithic nanofiber sheets in the soft laminate structure. Bending-twisting modes can be achieved by adjusting nanofiber orientation and aligning laser-cut fiber strips diagonally relative to the longitudinal actuator axis.

Just for FunTo highlight the scale of our devices, this image shows a soft actuator "petting" a tiny micro-knit rabbit, knitted with sewing needles. [No micro-bunnies were harmed in this photo shoot]

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