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

Summary

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.

Abstract

Recent advances in the development of electronic technologies and biomedical devices offer opportunities for non-invasive, quantitative assessment of cutaneous wound healing on the skin. Existing methods, however, still rely on visual inspections through various microscopic tools and devices that normally include high-cost, sophisticated systems and require well trained personnel for operation and data analysis. Here, we describe methods and protocols to fabricate a conformal, skin-like electronics system that enables conformal lamination to the skin surface near the wound tissues, which provides recording of high fidelity electrical signals such as skin temperature and thermal conductivity. The methods of device fabrication provide details of step-by-step preparation of the microelectronic system that is completely enclosed with elastomeric silicone materials to offer electrical isolation. The experimental study presents multifunctional, biocompatible, waterproof, reusable, and flexible/stretchable characteristics of the device for clinical applications. Protocols of clinical testing provide an overview and sequential process of cleaning, testing setup, system operation, and data acquisition with the skin-like electronics, gently mounted on hypersensitive, cutaneous wound and contralateral tissues on patients.

Introduction

In clinical study and biomedical research, monitoring of wound healing has focused on an invasive method that is based on the histological evaluation of tissue morphologic change in wounds^1,2. Recently, rapid advancements in electronic technologies enable the development of high-precision imaging and analysis tools that can visually inspect the wound healing process via digital imaging^3,4 or confocal scanning microscopy and spectroscopy^4,5. However, those imaging approaches require high cost, complicated optical tools and operations, and more importantly, patients need to be immobilized during testing. Therefore, there exists a need for new devices and systems that are quantitative, non-invasive, easy-to-use, inexpensive, and multifunctional to offer more accurate wound management.

Here, we introduce a skin-like electronic system that provides precise, real-time mapping of temperature and thermal conductivity and delivers a precise level of heating at wound sites via conformal lamination of the device non-invasively. This device presents a class of technology, skin-mounted epidermal electronic systems that are designed to match to mechanical and material properties (total thickness, bending stiffness, effective moduli, and mass density) of the epidermis^6-9.

The device is designed in a biocompatible, skin-friendly, water-proof, and reusable form that can be washed and disinfected for clinical applications on patients¹⁰. The conformal electronic device mounted near the wound tissues captures the inflammation phase (one of wound healing process), caused by increased blood flow and enzymatic reactions to the wound^11,12, through quantitative recording of temperature⁸ and thermal conductivity¹³, correlated to hydration. Experimental and computational studies determine an optimal mechanics design to accommodate natural motions and applied strains without mechanical fracture and capture the underlying physics of stretching mechanics of the skin-like electronics that laminates conformally on the skin surface, which offers acquisition of high fidelity signals.

The protocols described in this article present the methods of microfabrication for skin-like electronic systems, testing preparation including device cleaning, equipment setup in a clinical setting, and clinical applications for quantitative monitoring of temperature and thermal conductivity on cutaneous wounds.

Protocol

The experiments for device fabrication, skin lamination, and characterization shown in Figures 1, 2, and 4 involved two volunteers, all performed in the Bio-interfaced NanoEngineering Laboratory at Virginia Commonwealth University (VCU), Richmond, VA, USA. This study was approved by the VCU Institutional Review Board (protocol number: HM20001454) and followed the research guidelines from the VCU Human Research. The device and clinical data shown in Figures 3 and 5 were acquired from the published article¹⁰ where the experiments on patients were conducted under the protocol (number: STU69718) approved by the Institutional Review Board, Northwestern University, Chicago, IL, USA.

1. Device Fabrication  

NOTE: Figure 2 presents schematic illustrations for the overall fabrication process.

1. Prepare a carrier substrate
   1. Cut a bare 3 in silicon (Si) wafer into the desired size of the electronics by using a diamond blade.
      NOTE: About half Si wafer gives an ideal size for the wound device.
   2. Degrease Si wafer with acetone and isopropyl alcohol (IPA). Rinse the wafer with deionized (DI) water and then dry with nitrogen and dehydrate on a hotplate at 110 °C for 3 min.
   3. Prepare 11 g of polydimethylsiloxane (PDMS) mixture with 10:1 volume ratio of base and curing agent and degas the mixture in a vacuum chamber for an hour.
      NOTE: PDMS is used for a dry pattern retrieval and transfer printing after the microfabrication, which is preferable to the wet chemical (acetone)-based approach from the previous study⁷.
   4. Spin coat 5 g of mixed PDMS solution on the wafer at 3,000 rpm for 1 min and fully cure on a hotplate at 150 °C for 30 min.
2. Deposit materials and pattern electronics
   1. Treat the PDMS-coated wafer with ultraviolet (UV)/ozone by using a UV lamp (8.9 mW/cm²) for 3 min to make the surface hydrophilic.
      NOTE: The hydrophilic surface offers uniform coating of additional layers on the PDMS.
   2. Spincoat polyimide (PI; 2 ml) on the PDMS coated wafer, by pipetting, at 4,000 rpm for 1 min to form a 1.2-µm-thick layer, pre-bake on a hotplate at 150 °C for 5 min, and post-bake at 250 °C for 2 hr.
   3. Deposit chromium (Cr) to form a 20 nm-thick layer and then deposit copper (Cu) to form a 3 µm-thick layer by using electron-beam (e-beam) evaporation (base pressure: ~1×10^-7 Torr, deposition pressure: ~1×10^-6 Torr, deposition rate: 1 – 5 Å/s). Monitor the film thickness by the deposition controller interface embedded in the evaporator.
      NOTE: The thick layer of Cu provides enough levels of electrical conductivity on the microscale resistors of the device and thin Cr layer is used to promote adhesion between PI and Cu.
   4. Spincoat a photoresist (2 ml) with three steps at 900 rpm for 10 sec, 1,100 rpm for 60 sec, and 4,000 rpm for 20 sec and then cure it on a hotplate at 75 °C for 30 min.
      NOTE: The sequential steps described above were used to deposit a thick (> 10 µm) photoresist.
   5. Align Cu electronic patterns (sensors; fractal ‘Peano’ design with 35 µm in width and interconnects; serpentine open mesh design with 50 µm in width) at the center of the Si wafer by using a UV aligner (power: 10 mW/sec) with exposure time 25 s.
      NOTE: The fractal structures are used to provide superior mechanical stretchability, compared to merely meandering features¹⁴.
   6. Develop the photoresist in a diluted base developer (1:2 ratio of developer and DI water) for a minute, rinse with DI water, and dry with nitrogen. Inspect the patterns (Cu fractals and interconnects) using a microscope to confirm the feature size and find any defects from particulates.
      NOTE: If there are any unwanted defects, then remove photoresist by rinsing with acetone/IPA/DI water. After drying with nitrogen, repeat the steps from 1.2.4 to 1.2.6.
   7. Etch the Cu layer on the Si wafer by immersing in a wet chemical etchant for ~6 min (10 ml; mixture of ammonium persulfate and water in ratio of 1:4; etch rate of 8 nm/sec in 40 °C), rinse with DI water, and dry with nitrogen. Inspect the patterns using a microscope for any over-etched patterns.
      NOTE: If the patterns are over-etched, it may cause unwanted sharp edges of the features, which could lead to mechanical fracture during device handling and washing process. The prior testing results showed that more than ~20% over-etching of the original patterns caused the aforementioned issues.
   8. Etch the Cr layer with reactive ion etching (RIE; pressure: 300 mTorr, power: 200 W, CF₄ gas: 5 sccm, O₂ gas: 10 sccm) for 5 min. Inspect the patterns.
      NOTE: For etching of the Cr layer, the RIE process is preferable to wet chemical etching that causes unfavorable reaction with the Cu layer.
   9. Remove the remained photoresist on the metal layers by immersing the wafer in acetone (10 ml), IPA (10 ml), and DI water (20 ml), respectively. Then, dry it with nitrogen.
   10. Spincoat PI (2 ml) on the metal deposited wafer, by pipetting, at 4000 rpm for 1 min to form a 1.2-µm-thick layer, pre-bake on a hotplate at 150 °C for 5 min, and post-bake at 250 °C for 2 hr.
   11. Spincoat a photoresist (2 ml) with three steps at 900 rpm for 10 sec, 1,100 rpm for 60 sec, and 4,000 rpm for 20 sec and then cure it on a hotplate at 75 °C for 30 min.
   12. Align PI patterns to encapsulate the Cu electronics (sensors; fractal ‘Peano’ design with 35 µm in width and interconnects; serpentine open mesh design with 250 µm in width) with the pre-defined Cu fractals and interconnects by using a UV aligner (power: 10 mW/sec) with exposure time 25 sec.
   13. Develop the photoresist with a diluted developer (1:2 ratio of developer and DI water) for a minute, rinse with DI water, and dry with nitrogen. Inspect the patterns using a microscope to confirm the feature size and find any defects from particulates.
       NOTE: If there are any unwanted defects, then remove photoresist by rinsing with acetone/IPA/DI water. After drying with nitrogen, repeat the steps from 1.2.10 to 1.2.13.
   14. Etch the PI layer with RIE (pressure: 170 mTorr, power: 150 W, O₂ gas: 20 sccm) for 25 min. Inspect the patterns.
   15. Remove the remained photoresist by immersing the wafer in acetone (10 ml), IPA (10 ml), and DI water (20 ml), respectively. Then, dry it with nitrogen.
3. Prepare an elastomeric membrane
   1. Prepare a 10 g of encapsulating silicone mixture (1:1 volume ratio of base and curing agent) and add a black ink¹⁵ with one to one volume ratio, which is to facilitate control measurements of temperature variation on the skin using an infrared camera.
      NOTE: The utilized silicone (clear encapsulating rubber) provides unique characteristics of low viscosity, optical clarity, and electrical isolation/protection to the device¹⁶.
   2. Spincoat 8 g of the mixture in a petri dish at 150 rpm for 1 min to form a 500 µm thick elastomeric membrane and cure at RT for O/N.
      NOTE: The material needs to be placed on a flat surface for uniform thickness.
   3. Cut the membrane into the desired size of 70 mm x 30 mm by using a sharp razor blade and gently detach it from the petri dish.
4. Retrieve and transfer electronics
   1. Cut a water soluble tape (25 mm x 80 mm) and gently laminate onto the fabricated electronic patterns and place them on a hotplate at 130 °C for 3 min.
      NOTE: Temperature elevation expands the PDMS layer on the Si wafer to help dissociation of the electronic patterns from the surface.
   2. Detach the tape rapidly from the PDMS/Si wafer to retrieve the electronic patterns.
   3. Deposit a 20 nm thick Cr (for adhesion) followed by a 50 nm thick silicon dioxide (SiO₂) on retrieved patterns by e-beam evaporation.
   4. Treat UV/ozone by using UV lamp (365 nm, 8.9 mW/cm²) on the targeted silicone membrane for 2 min to activate the surface.
   5. Transfer the patterns to the silicone membrane by placing the retrieved patterns on the tape to the desired location and evenly adding pressure from top side of the patterns down to the substrate. Apply water to dissolve the tape for 5 min.
      NOTE: The described process of materials transfer is facilitated by covalent bonding (Si-O-Si) between the deposited silicon dioxide and the UV-activated silicone substrate¹⁷.
   6. Peel off the tape, rinse with DI water, and dry on a hotplate at 90 °C for 1 min.
5. Encapsulate the device using a silicone membrane
   1. Prepare an 10 g of encapsulating silicone mixture (1:1 volume ratio of base and curing agent).
   2. Cover the cable contact pads with a rectangular PDMS piece (22 × 6 × 1 mm³) by van der Waals bonding with the bottom silicone membrane, to avoid silicone coating the pads.
   3. Spincoat the 5 g of silicone mixture at 4,000 rpm for 1 min to form a 5 µm thick layer on the transferred electronics and then cure at RT for O/N.
6. Connect a flexible ribbon cable for data acquisition
   1. Apply liquid steel flux (0.5 ml), by pipetting, on the connector pads for 3 sec to make clean surface.
   2. Bond a thin, flexible ribbon cable on the contact points with pressure at high temperature (> 60 °C). A typical hair straightener offers easy handling and bonding.
      NOTE: The micro-film cable is preferable to the conventional hard-wire soldering to avoid any mechanical fracture of the transferred metal membranes on a silicone.
   3. Check the electrical connection using a digital multimeter. The resistance value is expected less than 1 Ohm between the sensor pad with one end and the other end of the film cable (distance: ~1 cm apart).
   4. Bond the other end of the ribbon cable to a customized printed circuit board with the same strategy described in the step 1.6.2.
   5. Check the electrical connection using a digital multimeter.
   6. Connect the device with the data acquisition hardware by soldering conventional wires on PCB.

2. Clinical Testing  

1. Clean the device using a disinfectant solution
   1. Prepare 205 g of a diluted disinfectant solution (40:1 volume ratio of water and solution).
   2. Spray the solution (10 g) on the device and soak it for 10 min.
      NOTE: The diluted disinfectant cleaner can be stored at RT.
   3. Rinse with water three times and dry it with clean tissues.
2. Set up a series of equipment for device testing
   1. Prepare and connect a lock-in amplifier with a current source, a multiplexer, and the custom software installed on a laptop computer for data recording.
   2. Place an infrared camera on a tripod and focus on a target object for thermography as a reference.
   3. Set up system parameters of a lock-in amplifier to measure thermal conductivity (frequency: 1 & 3 Hz; time constant: 3 and 1 sec; sensitivity: 1 mV; dynamic reserve: high reserve) and temperature (frequency: 997 Hz; time constant: 300 msec; sensitivity: 2 mV; dynamic reserve: low noise) with the applied constant current (2 mA).
   4. Connect two wound devices, prepared by microfabrication and transfer printing and mounted on wound and contralateral sites, to the multiplexer right before recording data from a patient.
3. Record temperature and thermal conductivity
   NOTE: The data acquisition software is custom-made, which can remote control the lock-in amplifier for real-time data monitoring and saving. In the temperature measurement, each data point is measured every 300 msec for 20 sec. The set of data for the first 10 sec and next 10 sec are used to calculate the average temperature value and standard deviation, respectively. The recorded data are then saved as a comma-separated value file, which is used to plot a graph for comparison with data from infrared thermography. In the measurement of thermal conductivity, the 3Ω signals are directly read from the hardware screen (amplifier), which is then utilized to calculate the thermal conductivity analytically.
   1. Gently rub the device application sites on the skin using antiseptic alcohol wipes¹⁰.
   2. 2.3.2) Laminate two devices on the desired skin locations by gently pressing the device to the skin with fingers to facilitate the soft bonding: one on the surgical wound site and the other on the contralateral location as a reference.
   3. Measure the electrical voltage (3Ω), related to thermal conductivity, of the device by starting the data acquisition.
   4. Evaluate the obtained data to verify the conformal contact of the device to the skin; abnormal value (< 0.1 W/mK) shows bad contact of the device.
   5. Measure the electrical resistance to determine temperature distribution and record data through the custom software.
   6. Take optical and IR images of two devices on the skin.
   7. Compare temperature values from the IR images with the data recorded from the wound device (2.3.5). Add both values to separate columns in a customized spreadsheet.
4. Analyze the recorded data
   1. Export the recorded data to the customized template to automatically calculate temperature and thermal conductivity from an array of sensors in the device.
   2. Plot the data (temperature and thermal conductivity according to the sensor location on a different time scale) for comparison over the course of one month (four sets of data on day 1, 3, 15, and 30).
   3. Analyze the data by comparing a series of temperature and thermal conductivity data according to time; Values with sudden elevation or drop tell the change of wound healing phase and/or unexpected abnormality on wound sites.

Results

Figure 1 presents an overview of the characteristics of the conformal, skin-like electronic system, designed for quantitative, cutaneous wound management on patients. The multifunctional electronic device consists of microscale fractal structures^3,14 and filamentary serpentine traces^9,17 on a thin elastomeric membrane that offers exceptional mechanical stretchability and bendability. The compliant device that is completely enclosed by silicone layers enables gentle, reversible lamin...

Discussion

This article highlights the methods and protocols to fabricate a conformal, skin-like electronics system that enables conformal lamination near the wound tissues, which offers quantitative measurement of skin temperature and thermal conductivity mapping on the skin.

The key features include the utilization of novel techniques of materials transfer printing and hard-soft materials integration to design and develop the flexible/stretchable, soft electronic device. The use of biocompatible, elec...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
3" Silicon wafer	University Wafer, USA		Use as carrier to fabricate the device
Acetone	Fisher Scientific, USA	A18-1	Use to clean a wafer and to remove photoresist
Isopropanol (IPA)	Fisher Scientific, USA	A459-1	Use to clean a wafer
AZ4620 photoresist	AZ Electrionic Materials, USA		Use to make patterns on metals and polymers
AZ400K developer	AZ Electrionic Materials, USA		Use to develop AZ4620 photoresist
Chromium etchant	Transene, USA	1020AC	Use to etch Cr layer of device
Copper etchant	Transene, USA	ASP-100	Use to etch Cu layer of device
Sylgard 184 Silicone Elastomer Kit (PDMS)	Dow Corning, USA	39100000	Use as a substrate for 'dry' retrieval
PI2545 polyimide	HD MicroSystem, USA		Use to encapsulate metal layer
Solaris	Smooth-On, USA		Use as substrate and to encapsulate device
Petridish	Carolina, USA	741255	Use as mold to make substrate
Water-Soluble Wave Solder Tape 5414	3M, USA	AM000000217	Use to retrive device from PDMS layer
High Activity Liquid Stainless Steel Flux	Worthington, USA	331929	Use to remove oxidation layer on Cu
Flexible, micro-film cable	Elform, USA		Use to make the electrical connection between the electronic device and the data acquisition system
pH Neutral Cleaner	Australian Gold, USA		Use as disinfectant solution to clean device in clinical testing
Solder	Kester, USA	24-6337-9703	Use as material to solder hard wires
Ultraviolet lamp	Cole-Parmer, USA	97600-00	Use to activate PDMS layer as hydrophilic surface
Multiplexer	FixYourBoard, USA	U802	Use to acquire measurements from six sensing components
DC/AC current source	Keithley, USA	6221	Use to supply current
SMD Digital Hot Air Rework Station	Aoyue, China	968A+	Use to solder hard wires, to electrically connect between the device and external instruments
Infrared camera	FLIR, USA	435-0001-01-00	Use to take infrared images in experiment
Digital multimeter	Fluke, USA	117	Use to check electrical connection
Lock-in amplifier	Stanford Research System, USA	SR830	Use to perform four-point-probe-measurement
Electron beam evaporator	9 scale Vacuum Products, USA		Use to deposit thin films (Cu and SiO2)