C.Santulli, G.JeronimidisUniversity of Reading, Centre of Biomimetics, Department of Construction Management and Engineering

S.I.Patel, F.J.Davis University of Reading, Department of Chemistry

G.R.MitchellUniversity of Reading, Department of Physics

DEVELOPMENT OF SMART ACTUATORSUSING N-ISOPROPYLACRYLAMIDE GELS

Website: www.reading.ac.uk/Biomimetics

Introductory research Principle of gel-driven actuatorsFE simulation of gel isotropic swellingExperimental set-up for force generation testsForce generation tests with commercial gel powder

NIPA gels synthesisSynthetic aims and materialsPolymerisation conditionsGel design for applicationFurther work

NIPA gels applicationMechanical characterisation Force generation testsDevelopments and future work

SUMMARY

PRINCIPLE OF GEL-DRIVEN ACTUATORS

BIOMIMETIC PRINCIPLE (worm’s locomotory mechanism)By contracting the muscles in the body wall and increasing its internal pressure the worm is able to change shape, with the fibres in the skin allowing the worm to go from short and fat to long and thin.

ENGINEERING APPLICATIONUsing cylinders of various fibre angles and by replacing the worm's muscles with a polymer gel which can absorb water, swelling and contracting of the gel can be designed to do useful work.

ACTIVE POLYMER GEL + FIBRE BRAID =TUNABLE ACTUATOR

Gel swelling forces the braided structure to pass from the minimum to the maximum angle, hence generating useful work

Gel swelling forces the braided structure to pass from the minimGel swelling forces the braided structure to pass from the minimum to um to the maximum angle, hence generating useful work the maximum angle, hence generating useful work

PROBLEM AREAS

Low force generation (low elastic modulus)

Limited response time (diffusion controlled swelling)

Isotropic swellingReversibility of effect

FE SIMULATION OF THE ISOTROPIC SWELLINGOF A POLYACRYLAMIDE GEL

INITIAL DIMENSIONS:6 MM DIAMETER30 MM LONG

VFINAL / VINITIAL = 5/1Isometric Swelling (Max. Force) Axial-Twisting Coupling

Axial-Bending Coupling Isotonic Swelling (Max. Displacement)

FORCE GENERATION (by isometric gel swelling)

FORCE GENERATION (commercial gel powder: repeated tests for different natural drying times)

0.00.20.40.60.81.01.21.41.61.82.0

0 10 20 30 40 50Time (minutes)

Forc

e (N

/g. d

ry g

el)

First test4 hours1 day3 days15 days

Completely dry gel (15 days) is slower in generating a forceCompletely dry gel (15 days) is slower in generating a forceCompletely dry gel (15 days) is slower in generating a force

BRAID CONTRACTION (isotonic swelling)

After the maximum force is generated, the gel is allowed to swell freely, and the braided structure contracts (~20% in length)

After the maximum force is generated, the gel is allowed to swelAfter the maximum force is generated, the gel is allowed to swell freely, l freely, and the braided structure contracts (~20% in length) and the braided structure contracts (~20% in length)

OBSERVATIONS ON FORCE GENERATION

Use of gel powder allows force generation (up to 2 N/g. dry gel)

Cycling can be assured (with fast gel drying) Diffusion of water in gel is effective

However: A membrane needs to be used to contain the gel

in the braid without spillage Mechanical characterisation of gel powder, to

serve for actuator modelling, presents problems

SYNTHETIC AIMS

• Gel.

• Shape.

• Response.

MATERIALS

N-Is opropyl acrylamide

N, N’-Methylene bis acrylamide

N,N,N,N-Tetramethyle thylene

diamine

Ammonium pers ulphate

O

NH CH3

CH3

F.W. 113.16 C6H11NO

O

NH NH

O

F.W. 154.17 C7H10N2O2

NN

CH3

CH3

CH3H3C

F.W. 116.21 C6H16N2

(NH4)2(SO4)2

F.W. 228.20 N2H8S 2O8

Monomer Cross -linking agent Genera tes free radica l with NH4PS

Initia tor

POLYMERISATION

(CH2)2(CH3)2N N(CH3)2(NH4)2(SO4)2

CH2CH(CH3)2N N(CH3)2

(CH3)2CHNH

C

CH

O

H2C

CH2CH(CH3)2N N(CH3)2

CH2

CHCO(NH)CH(CH3)2

Sci. Am., 244, 110 (1981)

TABLE OF CONDITIONS

Inhomogeneous30*-8Salt-ice bath

Inhomogeneous30*0Ice bath

Inhomogeneous20rtInert (Ar)

Inhomogeneous20rtUltrasound

Inhomogeneous20rtnone

GelTime (min)

T (°C)Conditions

* No reaction after this time, although rapid polymerisation on warming to room temperature.

INITIAL RESULTS

• Inhomogeneous network formation. (Polymer, 31, 1546 (1990)).

• Varying cross-link density.

• Rapid reaction time.

Regular, cross-linked gel:

– Greater volume of solvent.

– Improved mixing methods.

– Longer reaction times at reduced temperatures.

• 0 °C, 24 hours

• Transition occurs at 33.6 °C

• Lower temperature leads to clear gels

Science, 253, 1121 (1991)

GEL DESIGN FOR SPECIFIC APPLICATION

• Dimensions: 5mm diameter, 30-40 mm length

• Varying cross-link density

• Degree of swelling

SUMMARY OF GEL SAMPLES

Monomer : Water

Ratio

% Cros s -linked Gel Length(s ),

(cm)

1:5 *Concentra tion too high for ge la tion.

1:7 1 3

1:7 2 3, 4

1:10 1 3, 4

1:12 1 3, 4

1:12 2 3, 4

1:12 3 3, 4

FURTHER WORK

• Modification by hydrolysis:

C

O

R NHR'

OHC

O

R NHR'

OH

C

O

R OHNHR'+

Sci. Am., 244, 110 (1981).

FURTHER WORK

• Prepare new monomer:

J. Biomater. Sci. Polymer Edn., 11 (1), 101 (2000)

CHH2C

C O

NHCH

H3C CH2COOH

COMPRESSION TESTS on gels with different water/monomer ratios

0.00E+002.50E-045.00E-047.50E-041.00E-031.25E-031.50E-031.75E-032.00E-032.25E-032.50E-03

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10Strain

Stre

ss (M

Pa)

50:1 (12 mm.)12:1 (5 mm.)10:1 (5 mm.)7:1 (5 mm)

Influence of water/monomer ratio on gel stiffnessInfluence of water/monomer ratio on gel stiffnessInfluence of water/monomer ratio on gel stiffness

WATER CONTENT EFFECT ON STIFFNESS(natural drying of 12:1 gel, 1% crosslinker)

0.001

0.01

0.1

1

10

100

012345678910111213Water/gel ratio

E (M

Pa)

Constant stiffness for a water/gel ratio between 3 and 12Constant stiffness for a water/gel ratio between 3 and 12Constant stiffness for a water/gel ratio between 3 and 12

CROSSLINKER EFFECT ON STIFFNESS(12:1 gel)

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14Strain

Stre

ss (M

Pa)

1% crosslinker2% crosslinker3% crosslinker

Passing from 1 to 3% of crosslinker,Young’s modulus grows by a similar factor

Passing from 1 to 3% of crosslinker,Passing from 1 to 3% of crosslinker,Young’s modulus grows by a similar factor Young’s modulus grows by a similar factor

COMPRESSION TESTS RESULTS

Water/monomer ratio, crosslinker content and cylinder diameter all influence gel stiffness

Young’s modulus remains constant for water/gel ratios higher than ~ 3Therefore:

A compromise between a high swelling rate (higher force generation) and a good stiffness (easier actuator development ) has to be reached

The use of small gel cylinders (diameter ≤ 5 mm.) is preferable for improved diffusion

FORCE GENERATED IN THE HIGHER ∆V REGION

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 10 20 30 40 50Time (minutes)

Nor

mal

ised

forc

e (N

/g. d

ry g

el) A

B

A = 10:1 gel (5 mm cylinder)

B = 50:1 gel (12 mm cylinder)

The use of 5 mm. cylinders allowed the generation of a larger forceover the same time (50 minutes)

The use of 5 mm. cylinders allowed the generation of a larger foThe use of 5 mm. cylinders allowed the generation of a larger forcerceover the same time (50 minutes)over the same time (50 minutes)

A 40 cylinders (total gel = 1.5 g.) in 12 mm. braid (low diffusion)B 4 in-braid (5 mm.) cylinders (gel = 0.24 g.) in 12 mm. braid (empty volume)C 5 cylinders (gel = 0.3 g.) in 5 mm. braid

FORCE GENERATION (larger structures)

BA

C

DEVELOPMENTS

To maximise the force obtained, a number of fibre-like (~ 1 mm diameter) dry gel cylinders has to completely fill the braid volume

To minimise the response time, the actuator has to work in the interval in which the volume change in the gel is maximum

FUTURE WORK

Possible modification of the experimental environment (e.g., salted water)

Triggering force generation e.g., electricallyFind alternative ways of water diffusion (e.g., by

capillarity inside the braid) In-braid gelation

THIS WORK IS SUPPORTED BY EPSRC’S SMART MATERIALSAND STRUCTURES PROGRAMME, GRANT NO. GR/N27583/01