Improving the Actuation Speed and Multi-Cyclic Actuation Characteristics of Silicone/Ethanol Soft Actuators

At a Glance

Metadata	Details
Publication Date	2020-07-28
Journal	Actuators
Authors	Boxi Xia, Aslan Miriyev, Cesar Trujillo, Neil Chen, Mark Cartolano
Institutions	Columbia University
Citations	31
Analysis	Full AI Review Included

Executive Summary

This research focuses on enhancing the performance of silicone/ethanol soft composite actuators, which rely on ethanol phase change for high actuation stress and strain.

Core Problem Addressed: Low thermal conductivity (0.190 W/mK) of the silicone matrix limits actuation speed, causes non-uniform heating, and leads to thermal degradation and ethanol loss, resulting in poor durability.
Solution Implemented: Incorporation of diamond nanoparticle-based thermally conductive filler (MasterGel Maker Nano) into the silicone/ethanol composite.
Optimal Performance (“Sweet Spot”): An 8 wt.% filler concentration provided the best balance, increasing thermal conductivity to 0.212 W/mK while minimally affecting mechanical properties.
Actuation Speed Improvement: The average actuation/de-actuation cycle duration was drastically reduced from 402 ± 55 s (0 wt.%) to 197 ± 18 s (8 wt.%).
Durability Enhancement: The total number of reliable operation cycles increased from 40 ± 7 cycles (0 wt.%) to 65 ± 2 cycles (8 wt.%).
Performance Prediction: A Long Short-Term Memory (LSTM) recurrent neural network was successfully implemented to predict the actuation force output in multi-cyclic tests, achieving a test RMSE of 1.8 N.

Technical Specifications

Parameter	Value	Unit	Context
Baseline Thermal Conductivity	0.190 ± 0.003	W/mK	0 wt.% filler composite
Optimal Thermal Conductivity	0.212	W/mK	8 wt.% diamond filler
Maximum Thermal Conductivity	0.248 ± 0.003	W/mK	20 wt.% diamond filler
Baseline Cycle Duration (Avg.)	402 ± 55	s	0 wt.% filler (First 30 cycles)
Enhanced Cycle Duration (Avg.)	197 ± 18	s	8 wt.% filler (First 30 cycles)
Maximum Total Cycles Achieved	65 ± 2	cycles	8 wt.% filler composite
Actuator Dimensions	25.4 x 76.2	mm	Diameter x Height (Cylindrical)
Actuation Target Force (F_max)	40	N	Upper bound for cyclic testing
De-actuation Target Force (F_min)	10	N	Lower bound for cyclic testing
Heater Material/Resistance	Ni-Cr alloy wire / 30	Ω	30 AWG, double-coiled
Silicone Matrix Components	Ecoflex 00-35 Part A/B	40 vol.% each	Platinum-catalyzed silicone rubber
Active Material Concentration	20	vol.%	Ethanol (≥99.5%)
LSTM Prediction Accuracy	1.8	N	Test Root-Mean-Squared Error (RMSE)
Tensile Strength (0 wt.%)	0.362 ± 0.031	MPa	Stress at failure
Tensile Strength (20 wt.%)	0.267 ± 0.036	MPa	Stress at failure (shows mechanical degradation at high filler load)

Key Methodologies

Composite Formulation: The base mixture consisted of 40 vol.% Ecoflex 00-35 Part A, 40 vol.% Part B, and 20 vol.% ethanol. The diamond nanoparticle filler was mixed first with Part A, then Part B, achieving concentrations from 0 wt.% to 20 wt.%.
Actuator Fabrication: Cylindrical specimens (25.4 mm diameter, 76.2 mm height) were cast in 3D-printed PLA molds. A U-shaped 30 Ω Ni-Cr wire was centrally inserted as the Joule heater.
Sensor Integration: A Negative Temperature Coefficient (NTC) thermistor was inserted into the material to monitor internal temperature (T_muscle), avoiding contact with the Ni-Cr wire.
Mechanical Characterization: Tensile tests were performed on Die A dumbbell specimens (ASTM D-412) using an Instron 5569A system at a strain rate of 500 mm/min.
Thermal Characterization: Thermal conductivity was measured using a custom-built guarded hot plate device, with specimens sealed in polyethylene bags to prevent ethanol evaporation effects.
Multi-Cyclic Actuation Protocol: Actuators were placed in a braided mesh sleeving (lubricated with WD-40) and tested on an automated unit. The cycle involved heating until force exceeded F_max (40 N), followed by passive ambient cooling until force dropped below F_min (10 N).
Failure Criteria: Cyclic testing was terminated if the actuator temperature exceeded 145 °C or if the heating time doubled the initial cycle duration (indicating significant ethanol loss/matrix damage).
Machine Learning Implementation: A Long Short-Term Memory (LSTM) recurrent neural network was trained on the time-series data (Force, T_muscle, T_env, PWM) of the 8 wt.% specimen to predict the force output 20 s into the future.

Commercial Applications

The improvements in speed, durability, and predictability of these soft actuators pave the way for broader implementation in several high-demand robotic fields:

Advanced Soft Robotics: Manufacturing of compliant robots and artificial muscles that require high force output combined with fast, repeatable actuation cycles (e.g., soft grippers, locomotion systems).
Haptic Feedback Devices: Creation of high-fidelity, responsive haptic interfaces and wearable devices where rapid thermal cycling is essential for realistic tactile feedback.
Biomimetic Systems: Use in prosthetics and orthotics where the combination of high strain, high stress, and increased operational life provides reliable, nature-like movement.
Predictive Maintenance and Control: The successful application of LSTM modeling allows for the development of self-aware soft robots capable of predicting their own performance degradation (e.g., ethanol loss) and adjusting control parameters accordingly.
Thermal Management in Flexible Electronics: The methodology for incorporating diamond nanoparticles to enhance thermal conductivity in elastomers is directly applicable to flexible electronic substrates requiring efficient heat dissipation.

View Original Abstract

The actuation of silicone/ethanol soft composite material-actuators is based on the phase change of ethanol upon heating, followed by the expansion of the whole composite, exhibiting high actuation stress and strain. However, the low thermal conductivity of silicone rubber hinders uniform heating throughout the material, creating overheated damaged areas in the silicone matrix and accelerating ethanol evaporation. This limits the actuation speed and the total number of operation cycles of these thermally-driven soft actuators. In this paper, we showed that adding 8 wt.% of diamond nanoparticle-based thermally conductive filler increases the thermal conductivity (from 0.190 W/mK to 0.212 W/mK), actuation speed and amount of operation cycles of silicone/ethanol actuators, while not affecting the mechanical properties. We performed multi-cyclic actuation tests and showed that the faster and longer operation of 8 wt.% filler material-actuators allows collecting enough reliable data for computational methods to model further actuation behavior. We successfully implemented a long short-term memory (LSTM) neural network model to predict the actuation force exerted in a uniform multi-cyclic actuation experiment. This work paves the way for a broader implementation of soft thermally-driven actuators in various robotic applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/act9030062

References

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