Gravity pulls on everything in the universe. How can hydrogels defy this overpowering force? Let’s look at how to go against gravity using adhesive hydrogels.

What’s Pulling On You

We all know that gravity pulls on us by now. It keeps us planted to the ground. So if we want to stay up, then we need another force to pull us upwards! Specifically, we want an attractive force that is pointing opposite from gravity.

What are the various types of attractive forces other than gravity? We have

• magnetic (north and south)

• electrostatic (plus and minus)

• van der Waals (more on this later)

• hydrogen bonding

We need to find ways to turn on one of these forces.

One of the ways is through the use of adhesive hydrogels. Adhesive hydrogels are a type of hydrogel that possess exceptional adhesive properties. According to recent studies, the synthesis of hydrogels with adhesive properties remains a challenge but could be utilized in various applications including wound healing, electronic devices, and underwater adhesion in the marine sector.

This newsletter edition will highlight key research in hydrogen bonding and van der Waals based hydrogels.

Hydrogen Bonding Hydrogels

One strategy to increase adhesion in hydrogels is to introduce hydrogen bonding. The bonding occurs between two groups. One of them needs to have a hydrogen atom bonded to an electronegative atom, such as oxygen or nitrogen. Thus, the hydrogen atom can form a weak bond with a neighboring electronegative atom in another functional group. Common functional groups used by scientists are -2N, -SO3H or -COOH.

Zhang and coworkers designed a multiple hydrogen bonding hydrogel system for wearable electronics. Made out of PEDOT:PSS, these hydrogels can stick onto human skin and transmit electric signals to a computer. Their best-performing hydrogel, γ-GM-P1.3, has an adhesive strength of 53 kPa with glass and can even maintain its strength after 20 peeling and sticking cycles.

Another work by Gao and coworkers engineered hydrogen bonding between three chemical groups for enhanced adhesion strength. Their hydrogel displayed over 40 kPa adhesive strength on steel. When applied onto polycarbonate surfaces, the initial adhesive strength was ~20 kPa. Amazingly, adhesion strength increased by 6x over 120 minutes. The authors suggest this time-strengthening effect is due to the diffusion of interfacial polymer chains into the porous substrate or more hydrogen bonds were formed between the gel and substrates. These results show the promising future for cathechol-based adhesive hydrogels.

Van der Waals Hydrogels

Van der Waals forces are another type of attractive force that could potentially enhance the adhesion strength of hydrogels. They occur in all objects due to the fluctuation of electrons, which induces transient dipoles that can interact with neighboring molecules.

Yi and coworkers developed a universally adhesive hydrogel made of PEGDMA or PDMS. They employed van der Waals forces to increase the hydrogel's adhesion strength. The hydrogels were able to adhere in dry and wet environments, although PDMS adhesion significantly deteriorates in water. On the other hand, PEGDMA adhesion mechanism changes from van der Waals to capillary adhesion in a wet environment, allowing it to maintain over 90% adhesive strength relative to dry environment.

In another work, Liu and Scherman demonstrated dynamic hydrogel adhesive that can work on both porous and non-porous substrates. Based on CB[8], this supramolecular hydrogel can form tough and strong adhesive interlayers between two substrates without curing reagents. The adhesive mechanism relies on the van der Waals forces between polymer backbone and the substrate. Its simplicity of bonding mechanism highlights its utility in hybrid systems for biomedical implants/devices and tissue/bone regeneration.

In this edition, we have thought about ways to oppose gravity with adhesive hydrogels. Among the many attractive interactions, we focused on four research articles covering hydrogen bonding and van der Waals mechanisms. This is an exciting and rapidly developing space worthy that I will continue to cover in future editions.

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