Dynamic Subsurface Deformation and Strain of Soft Hydrogel Interfaces Using an Embedded Speckle Pattern With 2D Digital Image Correlation

Abstract

Background

Subsurface mechanisms can greatly affect the mechanical behavior of biological materials, but observation of these mechanisms has remained elusive primarily due to unfavorable optical characteristics. Researchers attempt to overcome these limitations by performing experiments in biological mimics like hydrogels, but measurements are generally restricted due to the spatio-temporal limitations of current methods.

Objective

Utilization of contemporary 3D printing techniques into soft, transparent, aqueous yield-stress materials have opened new avenues of approach to overcoming these roadblocks. By incorporating digital image correlation with such 3D printing techniques, a method is shown here that can acquire full-field deformation of a hydrogel subsurface in real-time.

Methods

Briefly, the method replaces the solvent of a transparent and low polymer concentration yield-stress material with an aqueous hydrogel precursor solution, then a DIC speckle plane is 3D printed into it. This complex is then polymerized using photoinitiation thereby locking the speckle plane in place.

Results

Full-field deformation measurements are made in real-time as the embedded speckle plane (ESP) responds with the bulk to the applied load. Example results of deformation and strain fields associated with indentation, relaxation, and sliding contact experiments are shown.

Conclusions

This method has successfully observed the subsurface mechanical response in the bulk of a hydrogel and has the potential to answer fundamental questions regarding biological material mechanical behaviors.

Appendix

Materials and Supplies

Poly(ethylene glycol) methyl ether acrylate Mn 480 (PEG-A), Poly(ethylene glycol) diacrylate Mn 700 (PEG-DA), sodium hydroxide 10 N, and 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure) were supplied by Sigma Aldrich. The granular microgel which forms the LLS is Ashland™ 980 carbomer purchased from Chempoint. UV irradiation is preformed using a MelodySusie 36 W UV Nail Lamp Dryer purchased from Amazon (ASIN: B012MEZP2E).

mLLS Preparation

• 1. Create PEG stock solution.

  • (a) Create a PEG-A stock solution by dissolving 6.15 g of PEG-A into 3.85 g of ultra pure DI H2O

  • (b) Create a PEG-DA stock solution by dissolving 4 g of PEG-DA into 6 g of ultra pure DI H2O

  • (c) Create an Irgacure solution by dissolving 1.25 g 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone into 10 mL of ethanol

• 2. Create LLS stock solution

  • (a) Combine 0.25 g Ashland 980 to 100 g ultrapure DI H20 and mix vigorously until dissolved.

    • 1 Allow the mixture to rest overnight to fully hydrate

  • (b) Pass the carbomer solution through a 40 µm mesh filter to remove all undissolved clumps.

  • (c) Add 1 mL of 10 N NaOH and shake vigorously

    • 1 The resulting mixture should be a highly jammed solid with many air bubbles

• 3. Create the PEG stock + LLS mixture according to the following table (increasing DA% corresponds to increasing elastic modulus)

	0.05% DA	0.1% DA	0.15% DA
PEG-A (mL)	7.9	7.5	7.0
PEG-DA (mL)	0.415	0.83	1.2
Irgacure (mL)	2.6	2.6	2.6
LLS (mL)	25	25	25

• 4. Mix vigorously and wrap the container in foil to prevent the initiation of free radical polymerization

• 5. Place the mixture in a vacuum chamber reaching a gauge pressure of -85 kPa for 10 minutes, then centrifuge the mixture to remove all entrapped air.

• 6. Irradiate a small sample of the resulting mixture with UV light for 5–15 min at a low power to ensure sample polymerization

Prepare Polymeric Speckles

This procedure is adapted from Mcghee et al. [41]

• 1. Create a monomer mixture from the prepared mLLS according to the table above, but replace the LLS component with ultrapure DI H2O

• 2. Add a small amount of water soluble paint and mix thoroughly

• 3. Place the mixture into a fine misting spray bottle and spray the mixture onto a foil surface

• 4. Irradiate the sprayed surface with UV for 1 h on a low power setting

• 5. Wash the surface while collecting the effluent into a large container.

• 6. Allow the particles to settle to the bottom of the container and dump the excess water

• 7. Repeat this process until the water is clear of excess paint

• 8. Collect all of the particles into a centrifuge tube, spin the particles down into a puck, and drain the excess water.

• 9. Mix a small amount of mLLS with the particle puck and mix thoroughly to form the mLLS-speckle mixture for use as printing ink

  • (a) Try to obtain a high-density speckle printing mixture

3D Printing

• 1. Place a printing container onto the printing bed, and preform a dry run of the expected print to ensure correct positioning and alignment

• 2. Load the mLLS-speckle mixture into the syringe

• 3. Choose a needle gauge that is close to the same diameter as the largest particle in the syringe

• 4. Begin the print

  • (a) Follow guidelines associated with McGhee et al. [41]

Photocure

• 1. After the print is complete, place the UV irradiator on top of the sample and irradiate on a low power setting for 15–30 min

• 2. If the sample is somewhat translucent then it has cured correctly

• 3. Allow the sample to rest for 1 h

Swell the Sample in Ultrapure DI H2O

• 1. Remove the sample from the glass container by placing a small flat plastic spatula on the edge of the gel and gently pry the gel from the bottom

• 2. Place the gel block into a large container of ultrapure DI H20 and allow it to swell for 48–72 h until the polymer has become transparent

  • (a) Expect a significant volume change

Once the sample has swollen it is ready for testing.