(K-419) Laser-Induced Graphene as a Catalyst in Localized Oxygen Production for Wound Healing.

Introduction::

It has been shown that controlled oxygen levels in wound tissue can enhance wound-healing, induce vascularization, and reduce the formation of scar tissue [1]. Two major challenges arise for targeted oxygenation: mechanisms for producing solubilized oxygen in low overpotentials (or higher efficiency) at tissue pH , and mechanical matching of the oxygen producing agent to the flexible nature of soft tissue. A polymeric oxygen generating film has been used for this application with limited success, but an alternative approach uses electrochemistry for water-splitting to provide oxygen for the cells. Currently, the one of the standard materials that optimizes the production of oxygen via electrocatalysis is platinum (Pt). However, due to the scarcity and high cost of platinum, other materials must be investigated. Laser-Induced Graphene (LIG) shows promise as a comparable material due to its catalytic properties for oxygen production and biocompatibility. As LIG is also fabricated on a flexible substrate, polyimide, which makes it an ideal candidate for oxygenating soft tissue. Thus, it is important to quantify oxygen production from LIG and optimize it using functionalizing techniques, such as metal-doping and oxygen plasma treatment. Using a 3-electrode setup, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed on LIG fabricated on the flexible substrate polyimide. This work demonstrates that LIG has comparable overpotential and kinetics to platinum and is a potential material candidate for oxygen production applications in tissue models. 

Materials and Methods::

The first group of pristine LIG samples were scribed using a CO2 UV laser engraver (Sculpfun S30 5W laser) on Kapton polyimide film (McMaster-Carr, Cat. No. 2271K3, thickness 0.005”) at room temperature and air. The films were cleaned using acetone and isopropyl alcohol, and the laser used a 3000 mm/s scribing speed at 40% power to form a circular active electrode 3mm in diameter. The next group of LIG samples were also scribed using a CO2 IR laser engraver (RL-80-1290) at 50 mm/s and 8W power. Half the pristine IR and UV LIG samples were treated via reactive ion etching (Plasma-Therm 790 RIE) for 5 minutes under pure oxygen.

As a reference, platinum electrodes were fabricated on 3” Si/SiO2 wafers using a 2 mil PET mask in the same geometry as the LIG electrodes. A 250nm thick titanium layer was sputtered on the Si/SiO2 wafer before a 450nm platinum layer was added using a 5-target sputtering system to promote adhesion of the platinum to the wafer. Individual electrodes were diced before electrochemical analysis.

To obtain cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy data, a 3-electrode setup in PBS (pH 7.4) was used with a potentiostat (PalmSens 3). A platinum wire counter electrode was used for the LIG samples, while the platinum and titanium electrodes used a stainless steel counter electrode. Both types of samples used an Ag/AgCl reference electrode.

Results, Conclusions, and Discussions::

After obtaining raw current data from LSV scans, the current was normalized by the exposed area in the electrochemical cell. Onset potential was determined by finding the intersection between the linear Faradhaic and non-Faradhaic regions of the scans.Given that the theoretical potential of the oxygen evolution reaction (OER) is 1.23 V, the lowest overpotential was found in LIG fabricated using an IR laser without any oxygen treatment (pristine IR LIG) at 314 mV. The other fabrication conditions were not far behind this value, with the pristine UV LIG, UV LIG-O, and IR LIG-O displaying overpotentials of 341 mV, 349 mV, and 334 mV, respectively. This overpotential from the pristine IR LIG was less than 20 mV greater than the value obtained for the platinum electrodes manufactured in the same geometry, which had a 329 mV overpotential at the conditions in this experiment. The low overpotentials for OER for LIG demonstrates that it is a competitive material for this application.

Similar experiments have yielded lower overpotentials at this stage, but at more alkaline (pH 13 or greater) and while using measures to remove the diffusion limitation with a rotating disk electrode [2]. This would not be relevant for a wound-healing application, which although makes conditions for OER more difficult, the results from this experiment show that these materials can still function at these conditions. 

Despite the small electrode size, a capacitive current was observed in the LSV, possibly due to the porous structure. However, from the CV scans, it is evident that very little, if any, side redox reactions are occurring for the LIG. This indicates that the LIG does not have any redox competing reactions to the oxygen evolution reaction in the region scanned. 

 Although the results in tissue-level pH are promising, future work should focus on using these electrodes for in vitro study by testing viability on cells. This will give greater insight to the translational potential of LIG for wound healing. 

Acknowledgements (Optional): : I would like to thank Inkyu Lee and Professor Tzahi Cohen-Karni for their invaluable insight and guidance throughout the course of this project.

References (Optional): :

[1] B. S. Harrison, D. Eberli, S. J. Lee, A. Atala, and J. J. Yoo, “Oxygen producing biomaterials for tissue regeneration,” Biomaterials, vol. 28, no. 31, pp. 4628–4634, Nov. 2007, doi: 10.1016/j.biomaterials.2007.07.003.

[2] Zhang, J., Ren, M., Wang, L., Li, Y., Yakobson, B. I., Tour, J. M., Adv. Mater. 2018, 30, 1707319. https://doi.org/10.1002/adma.201707319