ADVERTISEMENT

Shape Deformable Hydrogels

Imagine what functions can be performed with a soft, flexible but tough transformer that can change its shape upon external stimuli? Hydrogels are three-dimensionally cross-linked polymeric networks that can absorb a large amount of water while maintaining their solid-like shapes. They are typically soft and wet materials. Some smart hydrogels can change their geometric shapes to external stimuli, such as temperature, pH value, solvents, etc. Shape deformable hydrogels have wide potential applications in manufacturing soft machines, soft robotics, and artificial muscles.

Shape deformation of hydrogels is generally driven by the nonuniform internal stresses caused by the uneven swelling/ deswelling of the different parts of a gel sample. Therefore, macroscopic intrinsic structural inhomogeneity or asymmetry is a prerequisite for the deformation of the gels. Recently, professor Huiliang Wang and his Ph. D. candidate Miss Xin Peng from Beijing Normal University, China reported a series of works on the simple preparation of shape deformable hydrogels, precise programming of their shape deformations and their applications.

ADVERTISEMENT

The first type of shape deformable hydrogels we reported are Janus hydrogels with different distributive components/cross-linking densities across the thickness prepared by using molds with different hydrophilicity/hydrophobicity. When the hydrogels are formed in the molds made of Teflon and glass plates, the extremely hydrophilic monomer sodium acrylate (NaAAc) mostly aggregates to the hydrophilic glass plate and forms a more hydrophilic phase, while a less hydrophilic phase is formed close to the hydrophobic Teflon side due to the aggregation of interpolymer complexes.

These Janus hydrogels undergo bending-unbending deformations upon swelling in water and deswelling in ethanol, due to the different swelling/deswelling rates of the two sides. Therefore, they can be used to prepare soft machines, such as a propeller and a manipulator (Figure 1a and b, Movies 1,2 ) (Adv. Funct. Mater. 2016, 26, 4491-4500).

Janus hydrogels are also prepared by using the polymer poly(N-isopropylacrylamide) (PNIPAM) and graphene oxide (GO) nanosheets as the main functional components. Hydrogels made from PNIPAM are thermo-sensitive since they have a sharp volume change about 33 °C. GO sheets absorb and transform near-infrared (NIR) irradiation into thermal energy. Therefore, the obtained hydrogels can undergo bending deformations upon heating by immersing in hot water or under NIR irradiation and unbending deformations upon swelling in cool water. The Janus PNIPAM/GO hydrogels are also used to prepare soft manipulators, which can be actuated by hot water or remote NIR irradiation (Figure 1c and d) (Chin. J. Polym. Sci. 2017, 35, 1268-1275).

The second type of shape deformable hydrogels are hydrogels with locally different swelling behaviors prepared by local introducing of metal cations into hydrogels containing a negatively charged polyelectrolyte. The complexation between metal cations and the negatively charged polyelectrolyte increases the cross-linking density and hence change the swelling/deswelling behaviors of the hydrogels. Inspired by fabric dip-dyeing and transfer-printing techniques, we developed ion-dip-dyeing (IDD) and ion-transfer-printing (ITP) methods to adjust the deformation rate and extent of the Janus hydrogels (Figure 2a).

ADVERTISEMENT

Complex patterns can be printed on the hydrogels through ITP on the assistance of filter papers (Figure 2b). By proper ion-transfer-printing on 1D hydrogel strips, 2D hydrogel sheets, and 3D hydrogel blocks, the hydrogel samples can undergo interesting shape deformations from 1D to 2D, 2D to 3D and simple 3D to complex 3D shapes (Figure 2c) (Adv. Funct. Mater. 2016, 26, 4491-4500).

Figure 2.Ion-dip-dyeing and ion-transfer-printingtechniques. (a) Schematic illustration of the IDD and ITP. (b) Patterns printed by ITP. (c) Shape deformations from 1D to 2D, 2D to 3D and 3D to complex 3D (Reproduced from Ref. 1, with permission from [Wiley]).
To print more complex patterns on hydrogels, a computer-assisted ion-inkjet-printing (IIP) technique has been developed. This computer-assisted design and printing process allows the direct printing of batched and very complicated patterns, especially those with different or gradient grayscale distributions, on only one or both surfaces of a large-sized hydrogel sample (Figure 3). A variety of complex 3D shapes, such as cylindrical shell, righthanded helix, forsythia flower, saddle, blooming flower, and lily leaf-like shapes, are obtained by printing appropriate patterns on the hydrogels (Figure 3c and Movie 3). (Adv. Funct. Mater. 2017, 27, 1701962).

Figure 3. Ion-inkjet-printingtechnique.(a) A flatbed inkjet printer and schematic illustration of the IIP. (b) Patterns printed by IIP. (c) Shape deformations from 2D to 3D (Reproduced from Ref. 3, with permission from [Wiley]).
We also used NIR laser to locally irradiate homogeneous PNIPAM/GO hydrogels. Under local NIR irradiation, the irradiated side of the PNIPAM/GO hydrogels shrinks more than the un-irradiated side due to the slow heat transfer rate of the hydrogel, which leads to the folding/bending of the hydrogels to the direction of NIR laser in the irradiated region.

By programming the local irradiation on different parts of 2D hydrogel sheets, the hydrogel samples can undergo complex shape deformations from 2D to 3D (Movie 4) (J. Mater. Chem. B 2017, 5, 7997-8003).

The study, Mechanically strong Janus poly(N-isopropylacrylamide)/graphene oxide hydrogels as thermo-responsive soft robots was recently published in the Chinese Journal of Polymer Science.

ADVERTISEMENT

Reference

  1. Peng, X.; Li, Y.; Zhang, Q.; Shang, C.; Bai, Q.-W.; Wang, H. Tough Hydrogels with Programmable and Complex Shape Deformations by Ion Dip-Dyeing and Transfer Printing. Funct. Mater.2016, 26, 4491-4500.
  2. Peng, X.; Liu, T.; Shang, C.; Jiao, C.; Wang, H. Mechanically Strong Janus Poly(N-isopropylacrylamide)/Graphene Oxide Hydrogels as Thermo-Responsive Soft Robots. J. Polym. Sci.2017, 35, 1268-1275.
  3. Peng, X.; Liu, T.; Zhang, Q.; Shang, C.; Bai, Q.-W.; Wang, H. Surface Patterning of Hydrogels for Programmable and Complex Shape Deformations by Ion-Inkjet-Printing. Funct. Mater.2017, 27, 1701962.

Comments

READ THIS NEXT

Can Babies Think Hard?

Our lab studies how well babies can remember objects. The test that we have developed is a baby-friendly version of […]

People From Different Countries, Age Groups, And With Different Formal Education Level Mainly Are Very Similar In Their Values And Attitudes

“We are far more united and have far more in common with each other than things that divide us.” — […]

The Dust That We Breathe Inside Our Homes

Published by Pat Rasmussen Exposure and Biomonitoring Division, Environmental Health Science and Research Bureau, HECSB, Health Canada, Ottawa, and the […]

How Platelet Regulation Varies In Humans With Cyclic Thrombocytopenia

Making all of the blood cells in your body is no small feat. Every day, the hematopoietic system produces over […]

A Novel Class Of Anti-Cancer Drugs

Most current anti-cancer drugs are not very specific for killing cancer cells. They preferentially kill cancer cells because they kill […]

Forest “Hotspots” Of High Conservation Interest – Using The Past As A Guide For Management

The past can leave significant and surprising legacies Preserving large connected areas of valuable vegetation types in their natural, or […]

Moth-Eye Mimicking By Electrochemical Oxidation Of Aluminum

Nature has long inspired scientists around the world to engineer the materials of particular functional properties, such as anti-icing, non-wetting […]

Science Trends is a popular source of science news and education around the world. We cover everything from solar power cell technology to climate change to cancer research. We help hundreds of thousands of people every month learn about the world we live in and the latest scientific breakthroughs. Want to know more?