Matthew Sze; Daniel BroweApril 27, 2016

Optimization of Actuating Hydrogels: Dependence on,

Molecular Weight, Mass, Hydrogel Thickness, and

Concentration

2

Electroactive Hydrogels

• Movement of ions causes bending actuation• Repeatable, forceful movement

3

Poly(acrylic acid) and PEG• Poly(acrylic acid) (PAA) or acrylic acid monomer (AA)

– Contains the ionic side group, acrylic acid, which drives the movement

• Poly(ethylene glycol) di-acrylate (PEGDA)– Improves swelling of the hydrogel and allows for potential

future modification of the copolymer gel

Acrylic Acid

Polyethylene Glycol Diacryalate

4

Crosslinked Hydrogel

5

Goal/Objective• Determine what, molecular weight, mass of PEG-DA,

thickness of hydrogel, and concentration ratio (PEG-DA: Acrylic Acid) facilitates the movement of the electroactive hydrogel the most.

6

Actuation Method• Hydrogels placed into a PBS solution and placed under a 20V

electric field for 1 minute.• Electric field switched polarities three times• Degree of movement from each endpoint of the hydrogel was

recorded.• Movement speed was calculated by recording degree of

movement divided by the time the electric field was applied.

7

8000 Dalton Actuation Example

Before Electric Field Applied After 20V Electric Field Applied for 60 Seconds

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Method for Varying Molecular Weight (Equal Moles)

• Varied the weight of PEG-DA added to keep the amount of moles of PEG-DA among all samples constant

• Equal moles implies equal number of molecules, which normalizes the amount of potential crosslinks

• Samples mixed with photoinitator solution and crosslinked under UVA radiation for 1 minute.

SampleAmount of AA

Added(mL)Amount of PBS

(mL)Weight of PEG-DA Added (g)

1000 Dalton 1.2 2 0.02

4000 Dalton 1.2 2 0.08

8000 Dalton 1.2 2 0.16

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Amount of Movement Varied Molecular Weight

8000 4000 10000

20

40

60

80

100

120

140Amount of Movement

Forward ReversePEG-DA Molecular Weight(Daltons)

Deg

rees

10

Movement Speed Varied Molecular Weight

8000 4000 10000

0.20.40.60.8

11.21.41.61.8

2Movement Speed

Movement Speed Forward Movement Speed ReverseMolecular Weight (Daltons)

Deg

rees

/Sec

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Amount of Movement (Equal Mass)

12

Angular Speed (Equal Mass)

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Equal Moles vs. Equal Mass• “Equal moles” implies the same number of molecules of

each molecular weight– Normalizes the total number of potential crosslinks

• However, larger molecular weight molecules will then take-up more space– More swelling

• Polymers are sold by mass (grams), not amount (moles)• Therefore, the experiments were repeated with an equal

mass of PEGDA

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Method Varying Hydrogel Thickness

• The hydrogel thickness was varied for three samples (.29mm, .42mm, .55mm)

• Samples mixed with photoinitator solution and crosslinked under UVA radiation for 1 minute using different sized templates that change the thickness of the hydrogel.

• Hydrogel Solution kept constant among all samples

Amount of AA (mL) Amount of PBS (mL)

Weight of PEG-DA Added (g)

2.4 4.0 .32

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Varied Thickness Actuation Example

.55m After Electric Field Applied

.55mm Initial.29mm Initial

.29 m After Electric Field Applied

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Amount of Movement Varied Thickness

.66 mm .55 mm .42 mm .29 mm0

20

40

60

80

100

120

140Degree of Movement Varied Thickness

Forward Reverse

Deg

rees

17

Angular Speed Varied Thickness

.66 mm .55 mm .42 mm .29 mm0

0.20.40.60.8

11.21.41.61.8

2

Movement Speed Varied Thickness

Forward Reverse

Hydrogel Thickness

Deg

rees

/Sec

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Method for Changing Concentration

• Ratios of AA to PEG-DA varied to account for concentration change

• Changing ratio would for potential crosslinking sites1) introduce more potential charged groups on the side chain2) create more potential crosslinking sites

• Samples mixed with photoinitator solution and crosslinked under UVA radiation for 1 minute.

SampleAmount of AA

Added(mL)Amount of PBS

(mL)Weight of PEG-DA Added (g)

4:1 AA:PEG-DA .4 1.0 .1

6:1 AA:PEG-DA .6 1.0 .1

8:1 AA:PEG-DA .8 1.0 .1

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Methods of Analyzing Degree of Movement and Angular Speed

• Method 1- movement and measured until movement stopped (no fixed time interval)

• Method 2- movement recorded for fixed 60 second intervals (could reduce error bars because time is variable when accounting for angular speed)

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Amount of Movement Varied Concentration (Method 1 of Analysis)

(8:1 AA to PEG-DA) (6:1 AA to PEG-DA) (4:1 AA to PEG-DA)70

75

80

85

90

95

100Amount of Movement Varied Concentration

Amount of Movement Forward Amount of Movement Reverse

AA to PEG-DA Ratios

Deg

rees

21

Angular Speed Varied Concentration (Method 1 of Analysis)

(8:1 AA to PEG-DA) (6:1 AA to PEG-DA) (4:1 AA to PEG-DA)0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80Angular Speed Varied Concentration

Forward Reverse

Acrylic Acid (mL) to PEG-DA(g) Ratios

Deg

rees

/sec

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Degree of Movement for Varied Concentration (2nd Method of Analysis)

4:1 AA:PEG 6:1 AA:PEG 8:1 AA:PEG 10:1 AA:PEG0

10

20

30

40

50

60

70

80Amount of Movement Varied Concentration

Forward Reverse AA(mL) : PEG-DA(g) Ratios

Deg

rees

23

Angular Speed Varied Concentration (2nd Method of Analysis)

4:1 AA:PEG 6:1 AA:PEG 8:1 AA:PEG 10:1 AA:PEG0

0.2

0.4

0.6

0.8

1

1.2Angular Speed Varied Concentration

Forward Reverse AA(mL) : PEG-DA(g) Ratios

Deg

rees

/Sec

ond

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Discussion/Conclusions• Experiments with equal moles of PEGDA indicate larger

molecular weight will lead to more movement– Larger ether groups leads to more swelling

• Experiments with higher thickness will equal more movement– More mass = more swelling = more movement

• Experiments with varied acrylic acid amounts show change in actuation– Difference can be attributed to the amount of charged

groups on the side chain, (more charged groups = greater number of hydrated cations being pulled = more actuation)

– Optimal Concentration ratio of AA(mL): PEG-DA(g) is 8:1

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Future Directions• Future Directions

– Change ratios of concentration of Acrylic Acid to PEG-DA to a wider range to see if trend is consistent

– Increase the thickness until the amount of material eventually inhibits hydrogel movement to find optimal thickness