hydraulic hydrogel actuators and robots optically and sonically camouflaged in water

Marine animals such as hook-end insects develop tissues and organs composed of active transparent gels in water to achieve agile movement and natural camouflage. Hydrogel-
The actuator based on actuators cephali can imitate the function of letucephali, which will realize new applications in different fields.
However, the existing gel Drive, mainly penetration-
Driven, low in nature
Speed and/or lowforce;
Their ability to disguise has not yet been discovered.
Here, we show that the hydraulic drive of the gel with the design structure and performance can give high flexible actuators and robotsspeed, high-
Perform optical and sound camouflage in the water.
The actuator and robot of the gel can maintain its stability and function of the multi-cycle linkage, due to the-
Gel fatigue properties under medium stress.
We further demonstrate that agile and transparent gel actuators and robots have extraordinary features, including swimming, kicking rubber
The ball even catches a live fish in the water. Acrylamide (AAm; Sigma-Aldrich A8887)
Used as a monomer in a tough gel to cross-link the elastic network with a bonded bond.
In pam (PAAm)network, N,N-
Propylene amine (MBAA; Sigma-Aldrich 146072)
Used as a linker and 2-hydroxy-4′-(2-hydroxyethoxy)-2-
Acetone toluene (Irgacure 2959; Sigma-Aldrich 410896)
Used as a light trigger.
Sodium algae (Sigma-Aldrich A2033)
Cross-linking with calcium sulfate ions (Sigma-Alginate C3771)
Used for physical cross-linking dissipation networks in tough gels.
In order to alleviate oxygen inhibition during molding, glucose (Sigma-Aldrich G8270)
Glucose enzymes (Sigma-Aldrich G7141)
Added to pre-
Gel solution as oxygen scavenger.
Surface Treatment for elastic hydraulic connections, benzone (Sigma-Aldrich B9300)was used.
Silicone pipes of various sizes as hydraulic connections (McMaster Carr)
And metal needles (Nordson EFD)were used.
For better visual performance, red food dyes (McCormick)was used.
Ecoflex 30 (for the elastic body in the experiment)Smooth-On)
, Elostosil M 4601 (Wacker Chemical)
Sylgard 184 (Dow Corning)were used.
The hydraulic gel actuator is designed based on the soft actuator previously reported ().
The molds for each part are made using a 3D printer (Form2; Formlabs)
Laser cutting machine (
Finishing mini/spiral. Epilog Laser)
Computer-based
Auxiliary design drawings generated from commercial 3D modeling software (Solidworks;
Dassault System).
Each gel component is prepared by physical cross-linking gel using a mold.
The gel of physical cross-linking was prepared by pouring premix
Put the gel solution into the mold and cover the glass plate. The pre-
Gel solution of soft PAAm
Preparation of seaweed tough gel by mixing carefully degassed aqueous solution (22. 2 wt. % AAm, 1. 5 wt.
% Sodium algae, 0. 02 wt. % MBAA, 0. 2 wt.
% Irgacure 2959,2. 2 wt. % Glucose and 0. 01 wt.
Glucose Cox %)
Cross joint with ion (
Calcium carbonate slurry with final concentration of 20 × 10 µm). The pre-
Gel solution of stiff PAAm
Preparation of seaweed tough gel by mixing carefully degassed aqueous solution (22. 2 wt. % AAm, 3 wt.
% Sodium algae, 0. 02 wt. % MBAA, 0. 2 wt.
% Irgacure 2959,2. 2 wt. % Glucose and 0. 01 wt.
Glucose Cox %)
Cross joint with ion (
Calcium carbonate slurry in a cross-linked gel with a final concentration of 60 × 10 µm).
The physical cross-linked gel in the mold is kept in a damp chamber for 1 µh to ensure the formation of physical cross-linking.
After that, the gel part of the physical cross-linking is carefully separated from the mold and assembled together with the hydraulic connection (
Rubber tube, for example).
Treatment of elastic pipes with benzone solution (10 wt. % in ethanol)
For 2 min at room temperature (23u2009°C)
Form a solid interface with the gel.
The treated rubber tube is washed three times with methanol and completely dried with nitrogen before being assembled with a physically cross-linked gel part.
The assembled parts are further exposed to UV
Irradiation in the UV chamber (365u2009nm UV; UVP CL-1000)
For 1 hour, a polypropylene network with elastic bonded cross-linking is formed.
Please note that the UV chamber remains moist to avoid the potential drying of the gel during the second cross-linking process.
After polymerization in tough gels of a bonded cross-linked polypropylene network, assembled gel sections and hydraulic connections form a solid interface.
Before being used for experiments and measurements, the manufactured hydraulic gel actuators and robots were soaked for at least 48 hours in deionised water to achieve a balanced expansion state.
Please note the degree of swelling of soft and stiff PAAm-
The tough gel of seaweed stays at a similar level (
For example, the equilibrium volume expansion ratio is 2. 5)
To avoid geometric deformation after expansion.
Equilibrium water content of fully expanded PAAm-
The tough gel of seaweed is about 94. %.
To measure the interface toughness of the assembled gel, two types of samples-
Tough gel prepared as a whole and tough gel prepared by assembling two parts-were used.
The final size of the sample is fixed at a width of 15mm, a length of 100 and a thickness of 3mm.
For the tough gel prepared as a whole, the whole sample is using a soft PAAm-
Seaweed tough gel without assembly process.
For tough gel prepared by Assembly, two physical cross-linking soft PAAm-prepared separately-
The width is 15mm, the length is 100mm, 1.
According to the proposed method of giving the same final size, the thickness is 5mm assembled together.
By the method previously reported, one side of all samples was strongly adhered to the surface treated glass plate.
As a hard backing of the gel, the thin poly benzene diester (PETE)film (70 μm thickness)
It is introduced to the gel with acrylic adhesive.
Before measurement, immerse all samples in deionic water for 48 h to achieve balanced expansion-state.
In order to clearly measure the interface toughness between assembled gels, a 1 cm-long gap was introduced in the interface between assembled gels or in the same position as the prepared tough gels that were not assembled.
Then, with the standard 90-
Degree stripping test (ASTM D 2861)
With mechanical testing machine (20 kg/n load cell; Zwick/Roell Z2. 5)and 90-
Degree stripping fixture (G50; Test Resources)
In constant Cross
Head speed 50mm min ().
When separating bulk gel or assembled gel during testing, the measurement is up to a platform (
Slight oscillation)
When the separation process enters a stable statestate ().
The interface toughness is determined by removing the platform force from the width of the gel sample ().
It is worth noting that the measured interface toughness is also high (over 1,500u2009Ju2009m)
For both types of samples, it is shown that the interface strength of the assembled gel is as strong as that of the large tough gel.
Hydraulic gel actuator from multiple Programmable high-
Throughput syringe pump (
New era Pump System Co. , Ltd. ).
The pressurized water of the syringe pump is supplied by hydraulic connection (
For example, silicone tube and metal needle)
To the gel actuator.
The expansion and venting of the actuator is achieved by water injection and water intake through a syringe pump programmed by a custom code.
Control the drive speed by programming a syringe pump with an appropriate supply flow rate.
By using two sets of separate controlled syringe pumps, the synchronous drive on both sides of the gel fish is realized ().
Completely remove the bubbles in the hydraulic chamber of the gel actuator by squeezing or removing the gel actuator from the gas water.
Due to the diffusion transport of water through the gel, the gel is permeable to water. The steady-
Water diffusion transport through the state of the gel structure follows this relationship, which represents the flow rate of water infiltrated through the gel, represents the volume concentration of the moisture molecules in the gel, represents the diffusion coefficient of water molecules in the gel, represents the volume of each water molecule, typically representing a value of 10 µm, representing the surface area of the gel, representing the boerzman constant, representing the absolute temperature, representing the thickness of the gel wall of the Chamber, delta represents the pressure gradient applied to the gel hydraulic chamber.
Typical value of diffusion coefficient of fully swollen PAAm-
According to 3 × 10mm s, algae tough gel (ref. ).
In addition, the representative value of unit area and thickness-
The segmented gel actuators are 10 cm and 3mm, respectively.
Value at room temperature (23u2009°C)is 4 × 10J.
The water concentration is about 90% and the pressure gradient applied is 5kpa kPa ().
Therefore, for units, the typical value of the penetration water flow rate
The segmented gel promoter is 1.
1 × 10 mlmls s, several orders of magnitude smaller than the typical supply flow of the pump (
For example, the action frequency at 2 u2009 Hz 2 ml u2009 s).
Since the water penetration through the hydraulic gel chamber is much smaller than the typical supply flow rate, the effect of water penetration is negligible over time (
That\'s seconds)
Hydraulic drive.
In order to study the strain and stress distribution of hydraulic gel actuators as well as the driving time and force, we have established a 3D finite element model using commercial finite element software [6.
According to the method previously reported.
Import the geometry of the actuator into ansys CAE using STEP files generated from commercial 3D modeling software (Solidworks;
Dassault System).
The imported geometry is then mesh using a solid quadratic element (
C3D10M finite element software
For hydraulic Chambers and rigid layers modeled as gel with different mechanical properties (
That is, hard and soft gel)().
To capture the elastic response of the actuator, we modeled the hydraulic chamber and the rigid layer as hyper-
Using an elastic solid of a non-compressed Ogden model, its strain energy density is given by where, and represents the material parameters fitting from the experimental measurement data, and indicates the order parameters set here to 1.
By fitting with the measured stress
Strain curve, the material parameters of the hydraulic chamber are determined to be = 6. 17895, kPa, =3.
78464, while the installation of the rigid layer as = 27. 92793, kPa, =2. 52527 ().
Note that the stiff and soft PAAm-
In this study, the seaweed tough gel deform to the maximum deformation in actual actuators and Robots (that is, 1.
2 is hard and tough gel and 2.
7 for soft and tough gel).
After that, the experimental boundary conditions were simulated by applying the flux of non-compressed water on the closed surface of the chamber and constraining one side of the actuator, and dynamic explicit simulations were performed.
In order to apply the condition that the gel material is not compressed, we set the Poisson\'s ratio to 0. 49.
Use of quality scale technology to maintain accuracy
The static process in the execution process.
Determine the drive speed by checking the time it reaches the full drive state (
For example, 20 degree bending of the unit
Sub-Implementation Agency)
According to the traffic specified on the surface of the cavity (and ).
To further predict the force generated by the unit-
Segment actuator, we specify the flux on the interior surface of the confined cavity on both sides (and ).
Since the gel hydraulic actuator operates in water, the density is almost the same as that of the water in a fully expanded state, so gravity is not considered in the finite element simulation.
The induced time is defined as the time required to reach the fully induced state (
For example, 20 degree bending of the unit
Segment gel actuator).
The driving time is real-
Time video of hydraulic start-up under different supply flow ().
Induced volume (
That is to say, produce the amount of water needed to drive completely)of the unit-
The water condensate actuator was found to be 1 ml ().
The flow rate of 120 ml min is used to generate the excitation frequency (
That is, the countdown of driving time)
2 hz in units-
Sub-Implementation Agency ().
The complete linkage of the bending gel actuator is defined as the contact of the fixed end through its free end, and it is found that its induced volume is 10 ml ().
The driving frequency used to produce 1 hz for the bending actuator ().
Measuring the force generated by the unit-
Segment gel actuator, the actuator is placed between the rigid bottom plate and the acrylic clamp connected to the force sensor (NeuLog;
Fisher Science)().
The water is then pumped slowly by the syringe at a supply flow rate of 1 mlmlmin into the actuator, while the pressure inside the actuator is recorded by the pressure sensor (PX 409;
Omega Engineering Limited)
Connect via T-junction.
The injection rate is low enough to maintain accuracy.
Static conditions.
The driving force is recorded by the force sensor and then plotted according to the recorded pressure ().
Measurement stops in units of internal pressure around 20kpa kpa-
The segment gel actuator experienced unstable expansion (
Drum kit for example)
For higher pressure levels ().
To analyze the mass transmission properties of hydraulic gel actuators, the pressure-
The volume lag curve is generated by Hydraulic Expansion and venting of the actuator in water, according to the previously reported method.
The syringe pump is connected to a gel actuator that is clamped in a vertical position and immersed in water, and the pressure sensor also passes through T-junction.
Water is injected into the gel actuator at a low enough supply flow rate (
For example, 5 ml min)
Until the volume of water supply reaches the driving volume (
For example, the unit is 1 ml-
Segmented actuator and 10 ml
Bending actuator)
And then deflate by taking out the water from the actuator.
Once the actuator has completed a complete cycle, the pressure is restored to zero, and the same cycle is repeated at least three times to ensure repeatability.
The recorded pressure is drawn into pressure according to volume-
Volume lag curve and energy analysis using area under pressure
Volume lag curve ().
In order to further study the reliability of hydraulic gel actuators under cyclic drive, we performed cyclic fatigue tests on gel actuators.
The bending actuator is driven by more than 1,000 programmed syringe pumps that are continuously inflated-
Deflating cycle with drive frequency of 0. 5u2009Hz (
Supply flow of 300 ml min).
In each 1st, 10, 100 and 1, 000 cycle, pressure-
The volume lag curve is obtained through the above procedure ().
After 1,000 cycles, neither a material failure nor an interface failure occurred with the gel actuator.
Characterize the cyclic fatigue failure of PAAm --
Samples of seaweed gel about 1mm thick were tested using DMA Q800 machine (TA Instruments).
The sample is loaded cyclically at a frequency of 1 hz in tension between 0 and various stresses.
The clamping device on the machine is closed by a chamber equipped with a humidifier to prevent dehydration of the gel.
In addition, spray water to the sample regularly to ensure full expansion.
We applied varying degrees of cyclic stress in the range of 50-950 kpa.
The maximum strain in the sample continues to increase until the final failure ().
The number of failure cycles for each applied stress was recorded, and the number of failure cycles and experimental data for applied stress were plotted ().
The experimental data in the log fits a line
It can be used to push the fatigue results outside the log scale of other stress levels.
Refractive index of water and completely swollen PAAm-
Seaweed gel and Sylgard 184 were measured with a digital refraction instrument (Science).
Note that since both materials are optical opaque, the refractive index of Ecoflex and elostosil cannot be measured.
Light transmission spectrum of water, fully expanded PAAm-
Seaweed gel, Ecoflex, elostosil, and Sylgard 184 were measured using a spectrometer (BioMate;
Thermal Science)
Test Tube with quartz (
10mm optical travel length)
For the entire range of visible light (400750u2009nm).
Use a quartz tube with water as a reference to reduce the reflection caused by exponential mismatch.
The transmission ratio is defined as the intensity of the transmitted light, which is the intensity of the transmitted light ().
In order to measure the sound speed of the gel and the elastic body, the ultrasonic signal is transmitted at three different frequencies (
That is, 40 kHz, 200 kHz and 1 MHz)were used.
All measurements are carried out in the tank using samples with a width of 120mm, a length of 120mm, and a thickness of 50mm ().
Pulse of central frequency (
That is, 40 kHz, 200 kHz and 1 MHz)
Generated by a signal generator (Tektronix)
And send it to the sensor (CTG Model ITC-
40khz 1042,200khz and 1mhz Olympus Non-Destructive Detection ultrasonic sensor).
The transmitted signal collected by the sampled signal and by the water listener (CTG Model ITC-
1089D at 40 khz, 200khz and Olympus Non-Destructive Detection ultrasonic sensor at 1 mhz).
The measured signal is sent to the computer through the connected oscilloscope (Agilent)
The data is processed with MATLAB.
The data is drawn on the curve of measuring signal amplitude and travel time ().
= 0 corresponds to the time when the ultrasonic signal is sent from the transducer, and when the ultrasonic signal transmitted through the sample arrives, the water listener measures the signal amplitude.
The ultrasonic signal emitted at each frequency has the same amplitude, while the attenuation between the sample materials is different due to the different acoustic impedance and viscosity effects of each material.
At each frequency, the sound speed in pure water is measured as a control parameter.
The time difference of arrival after receiving the pulse through the sample and through the purified water ()
The sound speed in the sample is calculated to represent the thickness of the sample, the sound speed in the purified water, and the sound speed in the sample.
The acoustic impedance of the gel and the elastic body is calculated according to the definition of acoustic impedance aswhere represents known material density and represents the measured material sound speed.
The acoustic reflection coefficient of the interface between the water and the sample material can be calculated from the acoustic impedance of the material, here is the acoustic impedance of the water, is the acoustic impedance of the sample material, and it is the angle of the plane sound wave relative to the water and the sample interface.
Note that the incident angle of the sound wave is perpendicular to the water and sample interface (that is, ==90°)
In the measurement ().
The degree of acoustic reflection of projected sound waves on the interface of water and sample materials is determined by the acoustic reflection coefficient ().
Ultrasonic images of gel and elastic actuators were taken through commercial imaging probes (GE LOGIQ E9; GE Healthcare)
4mhz ultrasound pulse mode is used.
To take an ultrasonic image of the sample in the water, immerse the sample in a large tank filled with deionized water and place the probe in the tank above the sample.
First adjust the contrast and other settings of the ultrasonic image so that the elastic actuator clearly displays the boundaries of the sample, and then take the ultrasonic image of the gel actuator without further changing the settings ().
Note that all optical and acoustic measurements are performed at room temperature (23u2009°C).
In order to demonstrate the swimming of gel robot fish, gel robot fish with two hydraulic actuators on both sides are prepared ().
Connect the gel robot fish to two separate sets of programmable syringe pumps to drive both sides simultaneously in the water.
Floating block (
Polystyrene foam)
Used to stabilize the position of the actuator and allow it to swim ().
Both sides of the hydraulic gel actuator use a supply flow rate of 300 µmlmin min to produce a fast tail stroke (
That is, the driving frequency of 1 hz).
A very flexible soft silicone pipe is used as a hydraulic connection and fixed to the water tank to minimize the possible impact of the hydraulic connection during swimming.
Display ball-
Kicked by gel actuator, rubber-ball (7. 5u2009cm diameter)
A curved gel actuator is used (). The full-
The curved gel actuator is fixed in a vertical position in the water and then rubber-
The ball is placed at the free end of the gel actuator ().
In order to demonstrate the gel holder catching fish, a gel holder with six curved actuators was prepared ().
The gel holder is driven by a single hydraulic input connected to the syringe pump.
Put a live goldfish in the tank, and without hurting it, be caught, lifted and released by the gel holder ().
Please note that all procedures are in compliance with the guide to experimental animal care and use and are approved by the MIT committee of animal care institutions (CAC).
Upon request, the corresponding authors provided data supporting the results of this study.