Fabrication and Characterization of a Conformal Skin-like Electronic System for Quantitative, Cutaneous Wound Management

This article presents methods to fabricate and characterize a conformal, skin-like electronic system and protocols for the use in clinical applications, particularly on cutaneous wound management.

The overall goal of this procedure is to fabricate a skin like electronic device for quantitative cutaneous wound management. This is accomplished by first developing an electronic device on a carrier substrate. The second step is to prepare an elastomeric membrane to embed the electronics.

Next, the fabricated electronics are retrieved from their fabrication compound and transferred onto the membrane. The final steps are to enclose the electronics with a silicone coating and connect a flexible cable for data acquisition. The completed device is laminated near the wound tissue of a patient to monitor time, varying temperature and tissue thermal conductivity as wound management.

The main advantage of this device over existing tools based on microscopic visual inspections is that it can provide multifunctional quantitative monitoring of temperature and thermal conductivity near the wound tissue. The soft biocompatible and skin wearable device provides the functions and characteristics for the use in the clinical settings, and it has a great potential to address important issues in chronic wound management. This spin coating protocol can be performed on silicon wafers of any size.

To begin degrease the wafers with acetone and isopropyl alcohol. Then rinse them in deionized water and dry them under nitrogen. Dehydrate the wafer for three minutes on a hot plate set to 110 degrees Celsius.

Now spin coat five grams of PDMS onto the wafers at 3000 RPM For a minute, complete the process by curing the wafers on a hot plate set to 150 degrees Celsius for half an hour. After UV treating the PDMS coated wafers for three minutes, spin coat polyamide onto the wafers. Apply two milliliters with a pipette and spin at 4, 000 RPM for a minute for a 1.2 micron layer.

Then pre-bake the wafers on a hot plate at 150 degrees Celsius for five minutes and bake them in an oven at 250 degrees Celsius for two hours. Next, using an electron beam evaporation deposit, a 20 nanometer thick layer of chromium for adhesion, followed by a three micron thick layer of copper for conductivity. Next, spinco two milliliters of photoresist onto the wafers in three steps for a sub 10 micron layer.

Now cure the wafers on a 75 degree Celsius hot plate for 30 minutes. Next, use a UV aligner at 10 milliwatts per second to center the copper electronic pattern to the wafer. Use an exposure time of 25 seconds.

A fractal pattern is used to make sensors and an open mesh is made for the interconnects. Develop the photo resist in 33%developer solution for one minute. Then rinse the chips with deionized water and dry them under a nitrogen stream.

Before proceeding, check the patterns under a microscope for defects. Next, etch the copper on the wafer using a six minute bath in chemical etching. Following the etching, rinse and dry the wafers.

As before, check the etched patterns under a microscope greater than 20%over etching of the copper leads to mechanical failures. To etch the chromium layer, use five minutes of reactive ion etching. After checking the etching, remove the remaining photo resists with 10 milliliters of acetone, followed by 10 milliliters of isopropanol.

And thirdly, 20 milliliters of deionized water. Follow the baths by drying the wafers under a nitrogen stream. For this protocol, make 10 grams of an encapsulating silicone mixture for the target material onto which the electronics will be transferred onto a 10 centimeter wide Petri dish.

Spin coat, eight grams of mix at 150 RPM for a minute. For a half millimeter silicone coating. Let this cure overnight at room temperature.

Next, attach sized pieces of water soluble tape onto the pattern chips to act as a laminate surface for the electronic patterns. After attaching the tape, heat the wafers for three minutes at 130 degrees Celsius. Then rapidly peel the tape off to detach and retrieve the electronics onto the retrieve patterns.

Use e-beam evaporation to deposit 20 nanometers of chromium for extra adhesion. Then deposit 50 nanometers of silicon dioxide onto the chromium layer. Now retrieve the silicone layer and treat it with 365 nanometer UV light for two minutes at 8.9 milliwatts per square centimeter.

This activates the surface with the silicone activated. Press the retrieved electronics into the silicone, spread them out evenly on the target material. Then treat the tape with water to dissolve it, and after five minutes, peel away the tape.

Then rinse the silicone with deionized water and dry on a 90 degree Celsius hot plate for a minute. To create the device, start by covering the contact pads with a rectangular piece of PDMS using Vander Wall's bonding force. Now onto the transfer electronics spinco five grams of encapsulated silicone mixture at 4, 000 RPM for a minute.

This makes a five micron thick layer, which is cured overnight at room temperature. The next day, clean the sensor pads with half a milliliter of liquid steel flux for three seconds. Then bond a flexible ribbon cable to the contact points with pressure and heat in excess of 60 degrees Celsius.

Check the connections with a multimeter. The resistance between the sensor pad and the film cable one centimeter apart should be less than an ohm bond the other end of the ribbon cable to a customized circuit board in the same way. Finally, connect the device to data acquisition hardware by soldering conventional wires to the PCB.

Using the described protocol, the electronic sensor device was fabricated. The flexibility of the device was examined. Mechanical fracture under tensile strain did not occur and the device was functional.

The resistance at six sensor locations was measured using a high precision hot plate. There was a linear relationship showing that the sensors were well calibrated to detecting subtle changes in temperature. Furthermore, an adapted three omega signal was used to evaluate the thermal conductivity with the device in a clinical setting.

The device was used to monitor wound healing post-surgery for up to a month. The temperature of the wound healing process was thus measured. There was intense inflammation at the wound site at day three, which is reflected by the temperature spike measured by the device.

After watching this video, you should have a good understanding of how to fabricate a conformal schemic electric device for quantitative wound management on patients.