Switching off smart adhesive while underwater

Materials World magazine
,
1 Apr 2020

A smart glue can be electrically deactivated in seconds while submerged in water. Shardell Joseph finds out how.

Using electrochemistry, an underwater adhesive can change from sticky to non-sticky in seven seconds with a zap of electricity. Funded by the Office of Naval Research, USA, the smart glue prototype has potential for underwater applications such as dismantling binding sensors on ships surfaces, but also for removable bandages and prosthetics. 

‘Most synthetic adhesives have trouble binding to surfaces underwater, i.e., a band-aid falling off in the shower,’ Michigan Technology University, USA, Biomedical Engineering Professor, Bruce Lee, told Materials World. ‘We incorporated catechol, an adhesive functional group found in mussel adhesive proteins, into designing synthetic underwater adhesive. 

‘This is a biomimetic technology that many others have also used to design underwater adhesives. However, the novelty of our work is that we found a way to turn off the adhesive with applied electricity, which makes it easier to detach.’ 

Sticky business 

Mussels secrete adhesive proteins with the amino acid 3,4-dihydroxyphenylalanine (DOPA), which contains the catechol. Because the reduced form of catechol exhibits strong interfacial binding strength to inorganic substrates, it can form strong bonds to metal, polymer and even soft tissues. As the adhesive strength of catechol is dependent on its oxidation state, the team used an electrochemical oxidation approach to control the pH near the adhesive joint. 

‘We demonstrated that it is feasible to directly deactivate catechol-containing adhesive using applied electricity, and studied the different conditions – level of voltage or current, pH or salt content of liquid in the interface – that could be used to control this process,’ said Lee.

A setup was developed that using a titanium (Ti) sphere and a platinum (Pt) wire electrode applied electrical stimulation to the adhesive, which was submerged in salty water. The Ti sphere was the electrode applying electricity to the adhesive, while the Pt wire was the counter electrode. This method makes it easy to control the voltage applied through the wire, glue and sphere, as well as how salty the water is around them. 

‘The titanium also served a second function,’ said Lee. ‘It is the surface that the adhesive binds to. We use Ti as the surface substrate for adhesion because the binding property between Ti and catechol is widely established. So, we can have good control over our experiment, knowing that it will stick well to Ti.’ 

Testing the glue

The team used the Johnson-Kendall-Roberts test, an accurate theory for strong adhesion energies of soft, slightly deformable material, to evaluate changes in the interfacial binding. The Ti sphere was deposited in an interfacial buffer solution – pH 7.5. 
The paper, In-situ deactivated of catechol-containing adhesive using electrochemistry, published in the Journal of the American Chemical Society in February 2020, reported the impact of the applied voltage and current levels, exposure time, and salt concentration of the interfacial buffer on the adhesive property of catechol. 

The team used different tools to determine the electrochemical oxidation of catechol, which included ultraviolet-visible diffuse reflectance spectroscopy and tracking the production of hydrogen peroxide (H₂O₂) – a byproduct generated during catechol oxidation. 
Lastly, the researchers performed contact mechanics testing and oscillatory rheometry to examine the effect of applied electricity on the mechanical properties throughout the adhesive. 

‘The interfacial binding properties of the adhesive decreased significantly with increased levels of applied voltage,’ the paper read. ‘The work and strength of adhesion decreased by 96% at the highest voltage tested, 9V, when compared to the virgin adhesive. ‘Adhesives oxidised in a pH9 solution also exhibited nearly zero adhesive properties confirming that the reduced form of catechol is responsible for strong wet adhesion.’ 

The team is currently exploring how to turn the adhesive back on, so that it can be repeatedly deactivated and reactivated.