Early detection and monitoring of chronic wounds

Author’s Accepted Manuscript Early detection and monitoring of chronic wounds using low-cost, omniphobic paper-based smart bandages Aniket Pal, Debkalpa Goswami, Hugo E. Cuellar, Beatriz Castro, Shihuan Kuang, Ramses V. Martinez www.elsevier.com/locate/bios

PII: DOI: Reference:

S0956-5663(18)30493-7 https://doi.org/10.1016/j.bios.2018.06.060 BIOS10580

To appear in: Biosensors and Bioelectronic Received date: 3 April 2018 Revised date: 6 June 2018 Accepted date: 27 June 2018 Cite this article as: Aniket Pal, Debkalpa Goswami, Hugo E. Cuellar, Beatriz Castro, Shihuan Kuang and Ramses V. Martinez, Early detection and monitoring of chronic wounds using low-cost, omniphobic paper-based smart bandages, Biosensors and Bioelectronic, https://doi.org/10.1016/j.bios.2018.06.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Early detection and monitoring of chronic wounds using low-cost, omniphobic paper-based smart bandages

Aniket Pala, Debkalpa Goswamia, Hugo E. Cuellara, Beatriz Castrob, Shihuan Kuangb, and Ramses V. Martineza,c*

a

School of Industrial Engineering, Purdue University, 315 N. Grant Street, West Lafayette, IN 47907,

USA b

Department of Animal Sciences, Purdue University, 270 S. Russell St, West Lafayette, IN 47907,

USA c

Weldon School of Biomedical Engineering, Purdue University, 206 S. Martin Jischke Drive, West

Lafayette, IN 47907, USA

*

Corresponding author. [email protected]

ABSTRACT The growing socio-economic burden of chronic skin wounds requires the development of new automated and non-invasive analytical systems capable of wirelessly monitoring wound status. This work describes the low-cost fabrication of single-use, omniphobic paper-based smart bandages (OPSBs) designed to monitor the status of open chronic wounds and to detect the formation of pressure ulcers. OPSBs are lightweight, flexible, breathable, easy to apply, and disposable by burning. A reusable wearable potentiostat was fabricated to interface with the OPSB simply by attaching it to the back of the bandage. The wearable potentiostat and the OPSB can be used to simultaneously quantify pH and uric acid levels at the wound site, and wirelessly report wound status to the user or medical personnel. Additionally, the wearable potentiostat and the OPSBs can be used to detect, in an

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in-vivo mouse model, the formation of pressure ulcers even before the pressure-induced tissue damage becomes visible, using impedance spectroscopy. Our results demonstrate the feasibility of using inexpensive single-use OPSBs and a reusable, wearable potentiostat that can be easily sterilized and attached to a new OPSB during the dressing change, to provide long term wound progression data to guide treatment decisions.

Keywords: smart bandages, chronic wound monitoring, pressure ulcers, wearable sensor, pH sensor, uric acid sensor.

1. Introduction Chronic wounds, where full regeneration of the damaged tissue does not complete in three months, are a worldwide health problem that causes a significant burden to healthcare systems; both in terms of the number of patients affected and the expenses derived from their prevention and treatment (Posnett and Franks, 2010; Sen et al., 2009). The need to reduce the burden of chronic wounds on patient’s quality of life and national health budgets has led to the development of advanced wound care technologies for automatic monitoring of wound status (Finlay, 2016). These “smart bandages” monitor wound biomarkers using sensors fabricated on flexible substrates in order to reduce the number of dressing changes and minimize the stress and pain suffered by the patient (Mehmood et al., 2014; Weber et al., 2010). Effective smart bandages should be mechanically flexible, breathable, easy to apply, and capable of reporting quantitative information about the wound status in real time to guide treatment decisions (Sen et al., 2009). Although a variety of smart bandages have been proposed to monitor physical and chemical parameters important in wound healing, most of these devices often require expensive and relatively cumbersome equipment, which limits the mobility of the patients and makes the dressings uncomfortable to wear (Jankowska et al., 2017; Phair et al., 2014; Schreml et al., 2014; Sharp and Davis, 2008). Moreover, the need of trained personnel to apply the smart dressings and to interpret the results limits the implementation of these devices outside clinical settings. Since it is recommended to change dressings frequently (at least once per day (Sood

2

et al., 2014)) smart bandages need to be low cost and disposable for single-use applications. Therefore, a low cost strategy to fabricate sensitive and easy to use smart bandages, so that they can be used as single use devices by minimally trained individuals, would be desirable to improve chronic wound healing outcomes; particularly in resource limited and home environments. Here, we propose to incorporate low-cost electrochemical and impedance sensors, fabricated using omniphobic (hydrophobic and oleophobic) paper, into commercially available bandages to monitor the status of open chronic wounds and pressure ulcers. The simple interfacing between these single-use smart bandages with a reusable, wearable measuring system facilitates their use as a non-invasive wireless platform for real-time monitoring of wound status. Recent advances in electrochemical sensors using flexible electronic platforms have provoked the emergence of several advanced dressings that quantitatively report wound status by monitoring physiological indicators such as temperature (Hattori et al., 2014), moisture (McColl et al., 2007), partial pressure of oxygen (Li et al., 2014; Schreml et al., 2014), concentration of biomarkers (Fernandez et al., 2014; McLister et al., 2017; Schneider et al., 2007), and bacterial load (Farrow et al., 2012; Zhou et al., 2010). The use of flexible substrates allows them to conformably cover the wound without damaging the repairing tissue. Some of these technologies demonstrated the ability to transfer the sensor reading wirelessly to an external device (Farooqui and Shamim, 2016; Kassal et al., 2017). Despite the high sensitivity of these smart bandages, the integration of a power source and the electronic circuitry necessary to acquire and transmit measurement data complicates their design and fabrication, increasing their cost and hindering their implementation as disposable, single-use solutions (Liu et al., 2017). Among the different biomarkers present in wound exudate, uric acid (UA) and pH are recognized to play a pivotal role in the biochemical events associated with wound healing (McLister et al., 2016). The continuous monitoring of UA has demonstrated to serve as an accurate indicator of S. aureus or P. aeruginosa colonization in open wounds since bacteria decrease the concentration of UA within wound exudate by metabolization (Ochoa et al., 2014; Sharp and Davis, 2008). Several bandage-based electrochemical sensors have used voltammetry and amperometry for the

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non-enzymatic detection of UA in wounds (Kassal et al., 2015; Liu et al., 2013; Phair et al., 2014). Unfortunately, voltammetry-based approaches to measure UA often require the application of a relatively large potential on the working electrode, which causes interferences from other metabolites and easily oxidizing species in the wound exudate (Dargaville et al., 2013). The low voltages typically needed to monitor UA using amperometry significantly decrease interferences; however, electrode biofouling has proved to decrease the detection accuracy of UA in wound exudate or blood after multiple measurements (Sharp et al., 2008). Elevated pH values of the wound exudate can also be used to identify bacterial infections, since the alkaline byproducts of bacterial proliferation raise the pH out of the narrow acidic range (5.5−6.5) of non-infected open wounds (Ochoa et al., 2014; Rahimi et al., 2016). While commercial pH meters (glass probes) have demonstrated the adequate evaluation of uniform acute wounds, most recent pH sensing platforms for wound monitoring rely on flexible electrodes coated with a pH-sensitive layer (mainly metal oxides (Korostynska et al., 2008) or conductive polymers (Ferrer-Anglada et al., 2006; Guinovart et al., 2014; Rahimi et al., 2017)). Compliant sensors with metal oxide pH-sensitive layers (e.g., SnO2, RuO2) exhibit good thermal stability, response times of only a few seconds due to their rapid absorption of hydrogen ions from the wound exudate, and high sensitivities (≈ -80 mV/pH) (Huang et al., 2011). Unfortunately, metal-oxide-based pH sensors require frequent calibrations due to drift and have not been widely adopted due to the high cost of their fabrication materials (Kurzweil, 2009). The use of polymeric pH-sensitive layers (e.g., polypyrrole, polyaniline), which are capable of measuring pH using the protonation and deprotonation of the nitrogen atoms in their structures, enabled the low-cost fabrication of highly sensitive (≈ -1300 mV/pH), flexible, and stable pH sensors by electropolymerization or drop-casting (Rahimi et al., 2017, 2016). While several sensors for UA or pH monitoring have been demonstrated using wireless platforms, the complexity of their fabrication process, the cost of the materials, and their often uncomfortable design make them unsuitable for chronic wound monitoring (Farooqui and Shamim, 2016; Liu et al., 2017). Patients with limited mobility are prone to develop pressure ulcers when a part of their body holds sustained pressure for a prolonged period. Elderly (Gist et al., 2009), obese (Sen et al., 2009), and

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diabetic (Blakytny and Jude, 2006) patients are more likely to develop pressure ulcers, especially while recovering from surgical operations. Pressure ulcers pose the additional problem of being difficult to predict, since the amount of pressure that causes them varies widely between patients (Ayello and Lyder, 2008; Sen et al., 2009; Wong, 2011). Several techniques have been proposed for the early detection and monitoring of pressure ulcer development using ultrasounds (Aoi et al., 2009; Kim et al., 2008),

digital photography (Chang et al., 2017; Lima et al., 2018), impedance

spectroscopy (Swisher et al., 2015), or measurements of the partial pressure of carbon dioxide of the damaged tissue (Mirtaheri et al., 2015). To date, however, there are no wearable wireless devices capable of detecting and continuously monitoring the healing process of pressure ulcers. Our previous work in point-of-care diagnostics demonstrated the low-cost fabrication of microfluidic devices (Glavan et al., 2013) and self-powered electrochemical sensors using hydrophobic paper (Pal et al., 2017). Here, we present omniphobic paper-based smart bandages (OPSBs), a low cost platform capable of measuring multiple parameters to perform real-time monitoring of all kinds of chronic wounds (both open wounds and pressure ulcers). OPSBs, in combination with a wireless wearable potentiostat, can provide simultaneous quantitative measurements of pH and uric acid in open wounds, and assess tissue damage in closed chronic wounds like pressure ulcers. OPSBs also offer several advantages as follows: (i) They are flexible, lightweight, breathable, easy to apply and dispose, and suitable for single-use applications; (ii) their manufacturing is simple, inexpensive, and compatible with mass-scale production techniques, such as spray deposition or roll-to-roll printing; (iii) they can be used to monitor both open wounds and pressure ulcers; and (iv) their wireless measurement and communication module is wearable and reusable, enabling non-invasive remote monitoring of wound status.

2. Materials and Methods

2.1. Chemicals and instruments

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We purchased Whatman #1 paper from GE Healthcare Inc. (Pittsburgh, PA) and two conductive inks, Ag/AgCl (AGCL-675) and carbon (C-200), from Applied Ink Solutions (Westborough, MA). Potassium ferricyanide, potassium ferrocyanide, uric acid, uricase (from Candida sp., 4.1 U/mg), polyaniline emeraldine base (PANi-EB, Mw = 50 kDa), disodium phosphate (sodium hydrogen phosphate),

citric

acid

(2-hydroxypropane-1,2,3-tricarboxylic

acid),

and

RFSiCl3

(CF3(CF2)5(CH)2SiCl3, trichloro-(1H,1H,2H,2H-perfluorooctyl)silane) were purchased from Sigma Aldrich Inc. (St. Louis, MO). We used a commercial, benchtop potentiostat (Reference 3000; Gamry Instruments, Warminster, PA) to test the performance of the electrochemical sensor and the wearable potentiostat. We purchased BAND-AID® adhesive bandages from Johnson & Johnson Consumer Inc. (New Brunswick, NJ).

2.2. Fabrication of the wearable potentiostat We fabricated a rechargeable, wearable potentiostat using a low-power programmable front end for electrochemical sensing applications (LMP91000, Texas Inst. Inc.) and a high precision impedance analyzer (AD5933, Analog Devices Inc). The wearable potentiostat is powered by a rechargeable battery (LIR2032, Duracell Inc.), and controlled by an open-source microcontroller prototyping platform (Arduino Nano v3.0, Arduino Inc.) (code provided in Supplementary Information). An RF transceiver IC (nRF24L01, Nordic Semiconductors Inc.) inside the wearable potentiostat performs for wireless communication through the 2.4 GHz ISM band. We sterilized the wearable potentiostat by spraying 70% ethanol before attaching it to a new OPSB. After the ethanol dried, we used a laser cut ring of double-sided adhesive tape (410M, 3M Inc.) to attach the wearable potentiostat to the bandage and provide a stable electrical connection with the paper-based sensors embedded in the OPSB.

2.3. Fabrication of omniphobic paper-based uric acid sensors to monitor open chronic wounds We rendered Whatman #1 paper omniphobic by spraying it with a 2% solution of fluorinated alkyltrichlorosilane (RFSiCl3) in iso-propyl alcohol (IPA) and drying it in a desiccator at 36 Torr for 20 min (Glavan et al., 2013). We stencil printed three flexible electrodes on the omniphobic paper

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using conductive inks: working (WE) and counter (CE) electrodes with carbon ink, reference electrode (RE) and contact pads with Ag/AgCl ink (Fig. S1). We dried the conductive inks in a desiccator at 36 Torr for 30 min, producing electrodes with highly reproducible conductivities (Fig. S2). Prior to mounting the paper-based UA sensors on the adhesive bandages, we drop cast 5 μL of a uricase solution on the electrochemical test zone and allowed it to dry at room temperature. We used uricase (from Candida sp.) as it has been shown to have excellent selectivity towards UA (Erden and Kiliç, 2013; Liu et al., 2013; Mostafalu et al., 2015; Zhang et al., 2004). The presence of potential interferants available in wound exudate—such as creatinine, glucose, lactate, or ascorbic acid—has also been demonstrated to have no significant effect on the selectivity of uricase (Kassal et al., 2015; Mostafalu et al., 2015; Piermarini et al., 2013). We prepared the uricase solution by mixing 3 μg/mL uricase with a 100 mM solution of potassium ferricyanide in 1:1 ratio. All solutions were made in phosphate buffered saline (PBS, 1x, pH 7.4). To calibrate the wearable potentiostat, we used different concentrations (0.2 to 1 mM in steps of 0.2 mM) of UA in PBS. We pipetted 5 µL of the uric acid solutions over the uricase modified electrodes and performed chronoamperometric assays using a 300 mV step potential (with respect to the RE) at a sampling rate of 10 Hz. The use of such a low working potential ensures minimal interference from other easily oxidized species in the wound exudate. Integrating the measured current with respect to time enabled the calculation of the net charge exchanged during the redox reaction by chronocoulometry (Eq. S2).

2.4. Fabrication of omniphobic paper-based pH sensors to monitor open chronic wounds We printed the electrodes of the pH sensors using Ag/AgCl ink. We prepared a pH-responsive polymeric composite by mixing 150 mg of PANi-EB with 250 mg of silver microflakes (particle size 2−5 μm, Inframat® Advanced Materials™ LLC) (Fig. S5a, b) in 5 mL of IPA. The mixture was sonicated for 1 h to create a uniform suspension. We pipetted 10 μL of the Ag/PANi-EB solution between the electrodes of the pH sensor and dried it at 60 °C for 30 mins to create a thin solid film of Ag/PANi-EB composite (blue colored). We exposed the Ag/PANi-EB composite to hydrochloric acid (HCl) vapors in a desiccator at 36 Torr for 30 min. The H+ ions from the HCl vapors transform the PANi-EB part of the composite into its emeraldine salt (ES) form (green colored), which exhibits a

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higher conductivity (Fig. S3, S5c, d). We rinsed the Ag/PANi-ES pH sensors with deionized water and dried them in a nitrogen stream before their embedding into a commercial bandage. We prepared pH buffer solutions (McIlvaine) across the clinical range (Ochoa et al., 2014; Rahimi et al., 2016) of open wound exudate (5.5−8.5) to calibrate the pH sensors. McIlvaine buffers were prepared by mixing 0.2 M solution of disodium phosphate and a 0.1 M solution of citric acid in different ratios (Pearse, 1968) (Table S1). We verified the pH of all the resulting solutions using a digital pH meter (Model IQ125, IQ Scientific Instruments, USA). We pipetted 10 μL of the pH buffers on the Ag/PANi-ES composite and performed impedance spectroscopy across the electrodes by applying sinusoidal signals with an amplitude of 100 mV and frequencies ranging 10 Hz−100 kHz to calibrate the measured impedance with pH.

2.5. Fabrication of omniphobic paper-based impedance sensor array to monitor pressure ulcers We printed seven equidistant electrodes in a hexagonal array using Ag/AgCl ink on omniphobic Whatman #1 paper to measure, in vivo, tissue impedance across pressure ulcers models induced on mice (Fig. S1). To improve the electrical contact between the electrodes and the skin of the mice, we selectively coated the electrodes with a conductive hydrogel (SPECTRA® 360, Parker Laboratories Inc.) using a stencil mask. The omniphobic paper substrate impeded the spreading of the conductive hydrogel over the paper, and avoided short circuits among the electrodes. After placing the electrode array on the shaved skin of the mouse and ensuring its uniform contact, impedance data was recorded from each pair of nearest neighbor electrodes. The wearable potentiostat enables the detection of tissue damage using a AD5933 impedance analyser chip and transmits the results to the user via an RF transceiver module. The AD5933 chip applied signals with an AC voltage of 0.97 V, DC bias of 0.76 V, and frequencies ranging 1 to 100 kHz to perform impedance spectroscopy. The wearable potentiostat reads the response signal, and calculates the magnitude and phase of the impedance of the underlying tissue.

2.6. Early in vivo detection of pressure ulcers

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We used ten laboratory mice (C57B6J, 8−15 weeks old, male) with mixed backgrounds to detect pressure-induced tissue damage in vivo. A ketamine-xylazine cocktail (0.1 g per kg of body weight). was used to anaesthetize the mice. We used two disc magnets (D601, www.kjmagnetics.com; NdFeB, 10 mm diameter, 2 mm thickness) to controllably create a pressure ulcer model on mice (Swisher et al., 2015). Prior to the application of the magnets, hair was removed from the back of the mice using depilatory cream (Nair) and then the area was washed with mild detergent (Dawn). The shaved skin of the mice was gently tented up and placed between the two disc magnets (Fig. S6), which applied a ~6.7 kPa pressure during the ischaemia cycle. The magnets did not interfere with the normal activity of the mice after they recovered from the anesthesia. The magnets were kept in place for 1 or 3 h to create different degrees of tissue damage. Each mouse received only one pressure-induced wound, and impedance measurements of the damaged tissue were collected for three days after the ischaemia cycle. During the measurements, the wearable potentiostat placed over the anesthetized mouse performed impedance spectroscopy across each pair of nearest neighbor electrodes in the array (Fig. S1). After sampling all the electrodes and transmitting the results wirelessly to a laptop, a map of the impedance of the tissue was constructed and used to assess the healing process of tissue damaged by 1 h- and 3 h-long ischaemic events. We drew registration marks onto the skin of the mice to locate the different features of the pressure ulcer with respect to the position of the electrodes. We used the same OPSB to collect impedance measurements, once per day, over three consecutive days. All procedures involving mice were performed in accordance with Purdue University’s Animal Care and Use Committee.

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3. Results and discussion

3.1. Design and assembly of OPSBs Fig. 1a depicts the fabrication steps followed to make OPSBs. We fabricated flexible electrodes by screen printing conductive inks on paper, which was previously rendered omniphobic by spraying a solution of RFSiCl3 in IPA. The silane, while chemically modifying the cellulose fibers, does not block the pores of the paper, preserving its breathability to ensure oxygenation of the wound (Fig. S3, Movie S1). These paper-based sensors are then embedded into commercial bandages to create the OPSBs (Fig. 1a−c), without compromising the flexibility of the bandage. We laser cut openings on the adhesive bandage and folded the contact pads of the sensors to allow them to interface with the wearable potentiostat (Fig. 1a). The absorbent pad of the bandage transfers the wound exudate to the surface of the paper-based sensors. We fabricated two sets of OPSBs: one with sensors capable of monitoring both UA and pH levels in the exudate of open wounds (Fig. 1b), another with an electrode array capable of detecting pressure ulcers (Fig. 1c). After the bandage is applied on top of the wound, a single-use, double-sided adhesive layer sticks the wearable potentiostat to the OPSB, making it easy to apply, and securing the electric contacts with the paper-based sensors. To change these smart dressings, we simply remove the OPSB (while still attached to the wearable potentiostat) from the skin of the user and then peel the wearable potentiostat from the adhesive layer.

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Fig. 1. OPSB fabrication and assembly process. (a) Schematic diagram of the fabrication of OPSBs: 1) Whatman #1 paper is rendered omniphobic by spraying a 2% solution of RFSiCl3 in IPA; 2) stencil printing is used to pattern flexible conductive electrodes using carbon and Ag/AgCl inks; 3) openings are laser cut on the adhesive layer of the bandage to interface the wearable potentiostat with the paper-based sensors in the OPSB. OPSBs are assembled by placing the paper-based sensors between the adhesive layer and the absorbent pad of the commercial bandages. (b) OPSBs used to monitor uric acid and pH levels in open wounds. (c) OPSBs used for the early detection of pressure ulcers. (d, e) Interfacing of the wearable potentiostat with OSPBs for monitoring open wounds and detecting

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pressure ulcers, respectively. (f) Packaging of the electronics in the rechargeable, wearable potentiostat.

3.2 Characterization of the wearable wireless potentiostat We developed a lightweight (~8 g) and low-cost (~$18, table S4) wearable potentiostat, capable of performing 3-electrode electrochemical measurements and impedance spectroscopy (Fig. 2). We designed the housing of the potentiostat as a “smiley face” to promote patient adoption of the device (Fig. 1d−f). The wireless communication module integrated in the wearable potentiostat transmits the measurements so that the results can be stored and displayed on the user’s phone (Pal et al., 2017) or laptop and then transferred from on-site to experts, over the web, to facilitate remote consultation. Additionally, the wireless card in the wearable potentiostat enables the wireless reconfiguration of the LMP91000 and the AD5933 chips through the microcontroller, making it possible to choose between impedance spectroscopy, chronoamperometry, and chronocoulometry, as well as to select the proper scanning parameters, for a fully automated wireless monitoring of wound status. A 3.6 V rechargeable battery powers the microcontroller and the low-power chemical sensors using low supply currents (