Self-Powering Electric-Field Skin Patch Speeds Wound Healing

What you’ll learn:

How an applied electric field for medical electrotherapy speeds wound healing.
Creation of the self-powered, non-electronic electrotherapy patch.
Results of the electrical and medical performance assessment of this patch, tested on mice.

Chronic wounds are open wounds that heal slowly, if they heal at all, and existing treatment options are relatively expensive and restrictive for the patient. Research has shown that electrotherapy—use of a modest, localized, external electric field—is an effective and complication-free technique for speeding healing wound closure. However, that technique can be awkward and costly, due to its reliance on bulky equipment that limits its clinical use. (There are other healing-accelerants in addition to electrotherapy, each with different practical and medical attributes.)

Under a DARPA grant, a multi-institute team lead by researchers at Columbia University and North Carolina State University, among others, has developed a water-powered, electronics-free dressing (WPED). It uses a flexible, biocompatible magnesium-silver/silver-chloride battery and a pair of stimulation electrodes. Upon the addition of water, the battery creates a radial electric field. (Note that this is not a skin- or sweat-powered energy-harvesting arrangement.)

Their approach is in sharp contact with other battery-free electrotherapy schemes which are powered by wireless RF transmission. Those systems require that patients be in the vicinity of a transmitting antenna (which obviously restricts patient mobility), require carefully designed transmission and receiving antennas, and rely on relatively bulky RF amplifiers, function generators, and advanced impedance-matching units.

Studies show that electrotherapy leads to increased cell migration, venous blood flow, and cell proliferation, all of which accelerate healing. However, achieving the best patient outcomes using this method requires daily treatment for several hours, typically for weeks at a time. Thus, the treatment arrangement tends to be very stressful for patients and often goes uncompleted.

How Does the Wound-Healing Patch Operate?

The wound-healing patch developed by this team includes a flexible, water-powered battery and a pair of thin-film stimulation electrodes seamlessly integrated with a commercial dressing. The battery produces an electrical field across the stimulation electrodes when activated with water (Fig. 1).

Materials, design, and working principle of the water-powered, electronics-free dressing — 1. Materials, design, and working principle of the WPED: (A) Photographs of the WPED illustrating its conformability. (B) Zoomed-in image of the WPED showing the water-powered battery. (C) Exploded view schematic illustration of the WPED showing the system’s main components. (D) Key steps involved in using the WPED demonstrated with a dummy wound on the ball of a foot of a human participant. (E) Cross-sectional view illustrations showing the activation and working mechanism of the WPED.

These water-powered dressings offer several hours of continuous stimulation with no restriction on patient mobility, robust performance (even under extreme temperatures, ambient humidity, and external pressures), and long shelf-life due to their materials, design architecture, and working principle.

Components of the Water-Powered Electronics-Free Dressing

The WPED involves a biocompatible magnesium-silver/silver-chloride (Mg-Ag/AgCl) battery (area: 0.64 cm2; weight: 47 mg; capacity: 0.4 mAh) affixed to the non-adhesive side of a commercial bandage (Tegaderm) and a pair of carbon-based stimulation electrodes on the adhesive side (facing the wound). The complete system (battery + electrodes + Tegaderm) is light (at 290 mg, only ~20% heavier than Tegaderm alone) and highly flexible for conformal attachment on curved body parts such as toes.

The battery, packaged within a polyethylene terephthalate film, is in an open-circuit, inactive state when the separator is dry. The separator includes an inlet pad where the user introduces water to activate the battery and a “check” pad to visually indicate complete hydration of the separator and successful activation of the battery.

When activated, the battery produces a voltage of about 1.5 V across the pair of stimulation electrodes, which include a central disk electrode connected to the anode (Mg; negative electrode) and an outer ring electrode attached to the cathode (Ag/AgCl; positive electrode). This configuration ensures a radial electrical field pointing to the wound center in line with the endogenous (meaning inside an organism or cell) electrical field, critical for promoting healing.

Testing the WPED

What about test and evaluation? Before testing “in vivo,” the biocompatibility of WPED was evaluated using NIH/3T3 mouse fibroblasts cultured with standard media, offering promising results in line with expectations. This was followed by in vivo tests using mice, where they evaluated the electrical and medical performance (Figs. 2 and 3).

2. In vitro electrical characterization of the WPED: (A) Battery capacity as a function of the volume of water added to the separator; discharge current: 1 mA/cm². (B) Effect of type of fluid used to activate the battery on capacity; discharge current: 1 mA/cm². DIW, deionized water; PBS, phosphate-buffered saline, pH 7; AWF, artificial wound fluid. (C) Duration of stimulation for different loads simulating different stages of wound healing. Effect of (D) temperature, (E) ambient humidity, (F) applied pressure, and (G) bending stress on duration of stimulation. (H) Duration of stimulation offered by the WPED of fixed battery capacity (0.4 mAh) for treating wounds of different diameters. (I) Effect of battery capacity on duration of stimulation for treating wounds of fixed size. n = 4. (D) to (G) WPED applied to a load resistance (25 kΩ) simulating average wound resistance. (H) and (I) phantom wounds were created using chicken breasts with a diameter of 2.5 cm and a thickness of 2 cm.

3. In vivo evaluation of the WPED in db/db (diabetic) mice: (A) Representative image of a db/db mouse wound before (left) and after (right) applying the WPED. (B) Infrared images of a mouse wearing the WPED before (left) and 5 min after (right) activation. (C) Representative macroscopic views of wounds on days 0, 3, 6, and 11 for the Control, Sham, and Stim groups. Scale bar, 10 mm. (D) Relative size of the wounds over time for the three groups. n = 8 for Sham and Stim. n = 7 for Control. (E) Wound closure on day 11. **P < 0.01 for both comparisons. ns, not significant. (F) Representative movement trajectories of individual Control, Sham, and Stim mice over 10 min. Scale bar, 5 mm.

Obviously, the metrics used for medical assessment are very different than those for electrical performance and involve statistical analysis and control groups. In brief, the WPED promoted healing by a meaningful and significant factor of about 30% by increasing epidermal thickness, modulating inflammation, and promoting angiogenesis (the process of new blood vessels forming from existing blood vessels in the body—good for wound healing, bad when related to cancer tumors).

The details of their work, including principles, materials, fabrication, dummy and in vivo tests, evaluation of data, and much more are in their lengthy yet highly readable paper “Water-powered, electronics-free dressings that electrically stimulate wounds for rapid wound closure” published in Science Advances.