IEEE TRANSACTIONS ON INFORMATION TECHNOLOGY IN BIOMEDICINE, VOL. 14, NO. 2, MARCH 2010 371
Remote Wound Monitoring of Chronic UlcersSonja A. Weber, Niall Watermann, Jacques Jossinet, J. Anthony Byrne, Jonquille Chantrey, Shabana Alam,
Karen So, Jim Bush, Sharon O’Kane, and Eric T. McAdams
Abstract—Chronic wounds or ulcers are wounds that do not healin the usual manner. This type of wound is most common in theelderly and in paraplegic patients with an estimated 1% of thepopulation suffering from leg ulcers and the costs adding up to 4%of the annual National Health Service budget in the U.K. Thereis an identified need to develop a device capable of remote woundmonitoring that enables patients to take charge of their woundmanagement under clinical guidance. A new “wound mapping”device has been developed, which is based on electrical impedancespectroscopy and involves the multifrequency characterization ofthe electrical properties of wound tissue under an electrode array.A key feature of the prototype device is the anticipated incorpo-ration of the measuring array into standard commercial occlusivedressings, thereby protecting the wound from interference and con-tamination, and thus, promoting wound healing, while monitoringthe protected wound. Further development is planned includingwireless transmission, thus enabling telewound monitoring as de-scribed earlier.
Index Terms—Chronic ulcers, impedance spectroscopy, remotewound monitoring.
I. INTRODUCTION
CHRONIC wounds or chronic ulcers are wounds that donot heal in the usual manner. Such wounds stop healing
at an early stage of the wound healing process, and if they donot show any further signs of improvement after three months,are classified as “chronic” [1]. This type of wound is mostcommonly found in the elderly, above 60 years in age [1], andfrequently occurs in paraplegic patients. An estimated 1% ofthe population in industrial countries suffer from leg ulcers, andthe treatment and prevention of pressure ulcers in a typical largehospital costs up to £2 million a year, adding up to £1.4–£2.1billion annually in the U.K., which was equivalent to 4% ofthe total National Health Service (NHS) budget in 2004 [2].The cost of treatment of diabetic ulcers alone in the U.K., forexample, is estimated at £12.9 million per year [3].
Unfortunately, many ulcers do not respond to conventionaltreatments, and hence, research is required to develop cost-effective alternatives, which enhance wound healing, reduce as-
Manuscript received February 15, 2009; revised January 4, 2010. Currentversion published March 17, 2010.
S. A. Weber, N. Watermann, J. Jossinet, and J. A. Byrne are with theNanotechnology and Integrated BioEngineering Center, University of Ulster,Newtownabbey, BT37 0QB, UK (e-mail: [email protected]).
J. Chantrey, S. Alam, K. So, J. Bush, and S. O’Kane are with the RenovoLtd., Manchester M13 9XX, UK.
E. T. McAdams is with the Biomedical Microsensors Group, Institut Na-tional des Sciences Appliquees, Lyon 9621, France (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TITB.2010.2042605
sociated pain and improve the quality-of-life of patients. Givenconcerns of hospital-acquired infections and rising healthcarecosts, it is now important to discharge the patient as quickly aspossible. However, this does not remove the need for continuouswound monitoring over prolonged periods. This need is cur-rently addressed by tissue viability nurses or community nurseswho perform home visits to the patients, or by managing woundhealing clinically in community-based tissue-viability centers.There is, however, a growing need for patients to self-manage as-pects of their wound care, preferably under remote supervisionby a clinician using some form of telewound monitoring. Anobjective measurement is required that can be communicated tothe healthcare professional who can then advise the patient onnecessary dressing changes, changes in treatment, or if neces-sary, intervene more directly. This is particularly of interest forpatients, such as diabetics or the bed ridden who are being caredfor at home. Not only could the cost to the healthcare systembe reduced considerably but such a monitoring system could beused to improve the quality of care and give vital support andconfidence to the patients and their families.
II. CURRENT WOUND ASSESSMENT
In order to characterize the success of a given wound treat-ment, the most commonly measured wound-related parametersare [4]: wound volume, wound area, maximal wound depth,width of the wound margin (the layer between the wound andhealthy tissue), and type of tissue affected. It is critically im-portant to accurately and precisely determine and document thesize of a wound and the progress (or otherwise) of its healing inorder to chose/develop the most effective treatment [5].
Wound surface area is commonly determined using a range ofdiffering techniques [6], which involve removing the dressingand either:
1) tracing the wound on transparency film with a fine-tippedpen, and analyzing the traced area manually by “countingsquares,” or digitally by means of a planimeter or digitizeror;
2) taking a scaled photograph of the wound.Both these categories of techniques are found to be relatively
reliable as long as they are performed and analyzed by the sameinvestigator, the trend is more important than the exact woundarea (Intraclass correlation coefficients of 0.99 [7], and hence,systematic errors are negligible). The techniques fail, however,if a multicenter trial is performed as it is not always possible touse the same investigator, and hence, ensure the same systematicerrors. Unfortunately, given the general awkwardness of theposition of chronic wounds, it is generally not possible for thepatient to perform the measurement him/herself, though it couldbe performed in some cases by a family member.
372 IEEE TRANSACTIONS ON INFORMATION TECHNOLOGY IN BIOMEDICINE, VOL. 14, NO. 2, MARCH 2010
Fig. 1. Visitrack from Smith and Nephew.
Although direct-wound tracing is an inexpensive and con-venient method, albeit time-consuming; it is invasive as thetransparencies have to make contact with the wounds [8]. Thishas the real potential to further disrupt the wound healing pro-cess (ironically, dressing removal in itself can interfere withhealing), and thus, lead to contamination, the pathogens in thewound fluid spreading to clinicians and/other patients [9].
The “ruler technique,” where wound width and length aremeasured and multiplied, fails to provide reliable results if thewound is not a regular shape, such as a circle or oval. It alsotends to overestimate the actual size of the wound and becomesincreasingly inaccurate with increasing wound size [5].
A further example of a wound tracing technique is Smithand Nephew’s Visitrack system (see Fig. 1) [10], a clipboard-like device used to trace the wound. Initially, a layered gridis applied to the wound. The clinician then traces the wound,removes the top layer of the grid and attaches it to the electricalclipboard. He then has to trace the wound again to enable thedigital analysis. Any error in tracing will be multiplied by therepetition of the process.
The noncontact photographic technique (planimetry) elimi-nates the increased risk of contamination or wound interferenceassociated with direct-contact methods (dressing removal is stillrequired), however, the ease of use is offset by costly, bulkyequipment, and intensive training requirements. The digital im-age must be of a high resolution, however, even with this, it isstill very difficult and ultimately very subjective to determinethe wound perimeter [11]. In addition, a change in camera anglecan change the calculated area by around 10% due to the curvednature of most of the body’s surfaces [8].
Maximal wound depth is measured by the insertion of a cottonswab into the deepest recess of the wound. This “optimal” posi-tion is not always obvious and the process can be very painful,especially if the measurement has to be repeated at differentpositions.
Wound volume is usually calculated by multiplying the max-imal wound depth by the area. Errors associated with the tech-niques used to find these individual values will therefore havean accumulative effect. Alternatively, the wound can be filledwith saline solution and the volume thus deduced, obviouslynot possible for wounds located in awkward positions, and themeasurement will include errors due to the presence of wound
Fig. 2. PDA powered laser digitizer [13].
exudate. At present, there is no widely accepted standard tech-nique to determine wound volume [12].
To overcome the limitations of the photographic (and other)methods, the latest approach is to use stereophotogrammetry [5],a stereo camera combined with a computer system generates a3-D characterization of the wound. Once again expensive, bulkyequipment, and intensive training are required, making it a usefultool for clinical trials, but not practicable for routine care [5], [8].
A recently launched technique is the use of a personal digitalassistant (PDA)-powered laser digitizer, as shown in Fig. 2.Images can be analyzed and documented, and then, transferredto an electronic patient file [13]. No independent review of thetechnique appears to have been published to date, but it can beassumed that similar limitations exist to those for photographictechniques, however, ease-of-use should be greatly increased.
To date no technique is available that can be readily performedby the patient or his family that would enable them to take amore active role in wound management. The professional clin-ician is required to identify the progress of wound healing andto decide on any further treatment based on the wound charac-terization. No technique is available that could perform/enablethis characterization without the removal of the wound dressing,which can lead to disturbance of the wound healing processesand to increased risk of contamination.
III. DEVELOPMENT OF A NEW IMPEDANCE-BASED
WOUND-MAPPING SYSTEM
In order to measure ulcers and other wounds convenientlyand effectively, a nondisruptive 3-D technique is required,preferably operated by means of a small handheld device,which can be used with little training, or more optimally, re-motely/automatically operated, and viewed by a clinician. Onetechnique which could potentially provide nondisruptive 3-Dmeasurements is electrical impedance spectroscopy (EIS). It isbased on the measurement of the tissue impedance and could en-able the investigator to not only measure wound area and depthbut also to precisely assess the wound brink, and after somecalibration measurements, establish the type of tissue present,all without removing the dressing, and thus, avoiding interferingwith wound healing.
Electrical (bio-) impedance spectroscopy involves the char-acterization and analysis of electrical properties of tissue over
WEBER et al.: REMOTE WOUND MONITORING OF CHRONIC ULCERS 373
Fig. 3. Typical impedance locus taken over from [15]. As skin impedance doesnot show any inductive behavior, it is widely accepted to inverse the imaginaryordinate to facilitate presentation [16].
a range of frequencies and has been used by the authors tocharacterize and map wounds.
A. Impedance Basics
Skin impedance is measured over a range of frequencies, anda suitably large frequency range should be used to encompass theimportant features, typically 10 Hz to 1 MHz [14]. In the initialmeasurements, in order to speed up the many measurementsrequired to produce a map of the wound (see also Fig. 6), amore limited range between 11 Hz and 1 kHz was used. Themeasured impedance data can be displayed in two major ways:by plotting the imaginary impedance versus the real resistancein the form of a complex impedance plot (see Fig. 3) or in theform of a Bode diagram, which usually displays the logarithmof the magnitude of the impedance as a function of the logarithmof the frequency and the same for the phase angle. Both plotsenable the investigator to determine equivalent model variables,and therefore, to identify/characterize the tissue type present orthe thickness of the tissue under study.
B. Impedance Model
The complex impedance plot for measured skin impedance(and that of many tissues) generally has the form of a “de-pressed” semicircular arc (see Fig. 3). Cole derived the mathe-matical description, the so-called Cole equation, for such electri-cal behavior observed for a wide range of biological tissues [17]
Z = R +R0 − R∞
1 + (jω/j0)α (1)
where R0 = resistance at 0 Hz and R∞ = resistance at infinitelyhigh frequency. The exponent α is a descriptor of the level ofdepression of the observed impedance arc (see Fig. 3)
ω0 =1T0
(2)
and
T0 = time constant.
Fig. 4. Equivalent circuit of wound impedance.
In order to analyze the measured impedance, a simple equiv-alent circuit model is often used, which was derived from theoriginal Lapicque [18] model, and adapted by Cole [17] andFricke [19].
The outer, dry epidermis shows capacitive behavior at highfrequencies. At low frequencies, however, only resistive proper-ties can be measured for this layer. This has lead to the modelingof the epidermis by a resistor Rp in parallel with a capacitorC [18]. Underneath the epidermis lie the moister dermal layers.Currents can flow through these layers relatively unimpededand they are therefore represented by a small resistance Rs inseries with the aforementioned parallel circuit. It has been ob-served, however, that the phase angle of the capacitive elementof the circuit, although constant over a considerable range offrequencies, is not the expected 90 of an ideal capacitor. Thishas lead to the use of an empirical “constant-phase element,” asshown in Fig. 4, which has a phase angle typically between 45
and 90 [20]. The constant phase element can be described asfollows [21], [22]:
ZCPA = K(jω)−α (3)
and can be related to Cole’s equation [see (1)], such that
K =R0 − R∞
Tα0
=Rp
Tα[Ωs−α ]. (4)
If α = 1, φ = 90, the capacitive element is a true capaci-tance and the arc has its center on the x-axis. If α is less thanunity, as is generally the case, φ < 90, the capacitive elementis not an ideal capacitance and the center of impedance arcis depressed below the real axis. A typical value for skin isα ∼= 0.8 [22] and has been related to the hydration level [23].K has a typical value of up to 20 MΩ·cm2 ·s−α if measuredwith wet gel [24] or up to 50 MΩ·cm2 ·s−α measured with hy-drogel and is relatively constant and independent of time [25].RP varies considerably between different individuals and underdifferent circumstances. It is basically the current bypassing thecapacitive epidermal layers, largely traversing through skin ap-pendages (i.e., hairs, sweat glands, and sebaceous glands) [26].This parallel resistance varies over time, with sweating and skinpreparation.
A more in-depth review of these models can be found in [27].The model described earlier, using a constant phase element,is a compromise giving a good fit to the data and yet has anarrangement of components, which has physical significance[27].
374 IEEE TRANSACTIONS ON INFORMATION TECHNOLOGY IN BIOMEDICINE, VOL. 14, NO. 2, MARCH 2010
Fig. 5. (Left) Prototype ImpediMap device. (Right) Early electrode array.
Fig. 6. (Left) Artist’s impression of presentation of wound related data. (Right)Envisaged use of a PDA device.
IV. SKIN-MAPPING SYSTEM
A new device has been developed by the authors called Im-pediMap, based on impedance spectroscopy, addressing manyof the issues outlined earlier. It is targeted at the monitoring ofchronic wounds, but could also be utilized in helping to preventthe formation of ulcers as well as in the study/diagnosis of acutewounds or burns. The initial prototype can be seen in Fig. 5 withfurther improvements presently being developed.
An array of electrodes is embedded in a sterile carrier dress-ing and the tissue impedance underlying each of the individualelectrodes is measured by the associated system and the datapresented to the clinician in a range of formats to provide infor-mation concerning the wound and the healing progress in termsof, for example, wound size/volume and wound severity. Sim-plistically, intact skin has a high impedance, whereas an openwound has a very low impedance, the latter largely due to theresistance Rs , of the underlying dermis. Maps with pixels ofvarying resistance/impedance are produced to show the shapeand size of the wound, and to establish the degree of healing,all without the need of removing the dressing (see Fig. 6). Mul-tifrequency impedance measurement furnishes more detailedcharacterization of the wound tissues and the stages of healingacross the wound.
The device hardware is illustrated in Fig. 7 with a DSP con-trolling the measurement application and multiplexers switchingthe measurement signal across the electrode array.
The device is battery powered to ensure patient safety. Dis-creet frequencies between 11 and 935 Hz were initially chosenfor early trials. The choice of frequencies involves a tradeoff
between measurement time and amount of information. Themore and lower the frequencies used in the measurement, thelonger the total measurement time will be for the multielectrodearray. The current setup of ten different frequencies allows formeasurements in less than 30 s at 25 distinct positions over thewound, providing enough information to apply data interpreta-tion modeling as described earlier.
The measurement current is achieved by a voltage-controlledcurrent source, chosen to restrict the output current to a safelimit, i.e., 10 µA (rms) [28]. The output signal from the voltage-controlled current source is then switched to the electrode arrayvia a bank of multiplexers.
An instrumentation amplifier is used to measure individu-ally the voltage between each electrode under test (VTEST ) andthe reference electrode (VREF ). The output signal from the in-strumentation amplifier is fed into an amplifier/filter stage forfurther conditioning. The gain of the instrumentation amplifierneeds to be variable, since the skin impedance measurements arefrequency dependent, however, good signal strength is requiredfor the A/D conversion.
A current-to-voltage converter is used to convert the appliedcurrent through the electrode-skin impedance into a voltage tobe measured. Again, this voltage is later amplified by a sec-ond stage to ensure that good use is made of the A/D’s range(±5 V).
The amplifier/filter circuitry is used to condition the signals(voltage and current), to amplify the input signals so as to makegood use of the A/D’s range (±5 V) and also to reduce aliasingerrors from out-of-band noise and interference, i.e., antialiasingfilter.
Custom-designed software has been programmed to facilitatecommunication between the device and the computer. It enablesthe clinician to add patient data, take an impedance reading, anddisplay the information about wound size and wound healingstatistics.
In these trials, the device is only being used in the threeelectrode setup with a fixed reference electrode. This supplies“instant” information on wound size and severity. In order togain information about the wound volume a four electrode tech-nique will be utilized, where, after a quick scan against a definedreference electrode, the device can determine, which electrodesare on healthy and which on wounded tissue. The appropriateelectrode combinations can then be chosen automatically, pro-viding information on the depth of the wound as well as the typeof tissue present.
Clinicians involved in the treatment of chronic as well as acutewounds have shown an interest in this tool, not only for woundmonitoring but also as an objective outcome measure in clinicaltrials investigating wound healing products and techniques. Itcould be used in the objective assessment of new dressing typesor alternative techniques, such as electrical stimulation, laser, orvacuum therapy, which claim to enhance the healing process.
V. CLINICAL ASSESSMENT OF THE TECHNIQUE
In order to assess the impedimetric wound-assessment de-vice, a collaborative research project was set up with Renovo,
WEBER et al.: REMOTE WOUND MONITORING OF CHRONIC ULCERS 375
Fig. 7. Hardware overview of the wound-mapping system.
a Manchester based biopharmaceutical company. Renovo spe-cialize in the development of pharmaceutical products for theacceleration of wound healing and improvement of scarring.A phase 2 clinical trial to investigate the wound-healing ac-celerating efficacy of Juvidex (mannose-6-phosphate) on splitthickness skin graft donor sites in healthy volunteers was con-ducted in their clinical trials unit. The trial included five treat-ment groups, each exploring different dose levels and dosingregimens. The fifth group was used to compare placebo withstandard care (no additional treatment) controls. In the collabo-rative project, 34 patients in group five were also assessed withthe wound-mapping system.
For each patient, two grafts were harvested from the lowerback, using an electric Dermatome (Nouvag AG 12 mm Der-matome) at a depth setting of 0.4 mm. Treatment of group fiveinvolved 100 µL of placebo being injected intradermally per1 cm2 of the 3 cm2 wound and a 0.5 cm border of unwoundedskin around the donor site, no more than 20 min before graftharvest (total dose 750 µL). A further 300 µL of placebo wereapplied topically within 30 min of graft harvest and 24 h af-ter graft harvest. Standard care (moist wound healing dressingsalone) was applied to the other 3 cm2 donor site.
It must be noted that in this study, the ability of EIS to char-acterize tissue and monitor healing was of more importancethan mapping of the very small wound with a relatively smallnumber of electrodes. The impedance results were comparedto clinically relevant parameters, such as reepithelialization andtransepidermal water loss (TEWL). [The reepithelialization ofsites in this trial was determined by the clinical trial investiga-tors, who at each follow up visit, decided whether the woundwas 0–25%, 26–50%, 51–75%, 76–99%, or 100% reepithelial-
ized.] Transepidermal water loss was determined with a DelfinVapometer (Delfin Technologies Ltd., Finland). Three measure-ments were taken from the center of the wound.
The impedance plots evidenced very small impedances, al-most purely resistive, for day 2 and day 3, mainly due, in theabsence of the removed skin layer, to the series resistance, andto a limited extent, the small electrode–gel interface. Larger,almost complete impedance arcs (see Fig. 3) were observed fordays 8 and 9 indicating a significant regrowth of the skin. The arcdiameter (reflecting RP , Fig. 4) increased dramatically from oneday to the next. It was therefore obvious from this data that thereis a major difference in impedance between an open wound andeven partially regrown skin. The concept of mapping of a woundbased purely on the magnitude of the electrode-skin impedance,especially at lower frequencies, was therefore validated.
During subsequent days, the impedances, particularly RP ,tended to continue to increase, indicating an increasing stra-tum corneum impedance. As a result of the increasing size ofRP , and its subsequent effect on the “characteristic frequency”of the impedance “arcs,” only “straight-line,” ZCPA -dominatedbehavior was observed in later days. Calculated values of RP
were therefore wildly inaccurate in this range. Suffices to say,RP increased greatly over the trial duration.
The averages of the K and alpha values [see (1)–(4)] forthree sites were calculated and compared to the clinical param-eters recorded for these sites over time. Fig. 8 similarly showsK in comparison with TEWL. Fig. 9 presents the comparisonof K, derived from the impedance data, with the reepithelial-ization rate as determined by the panel of clinicians by meansof color-scaled photographs and visual classification of woundclosure.
376 IEEE TRANSACTIONS ON INFORMATION TECHNOLOGY IN BIOMEDICINE, VOL. 14, NO. 2, MARCH 2010
Fig. 8. Average K and TEWL.
Fig. 9. Average K and reepithelialization.
Fig. 10. Average alpha and reepithelialization.
Figs. 10 and 11 display the comparison of alpha with thesesame clinically used parameters. For all four parameters, corre-lation levels of 95% were found. The best correlation, with r =0.85, was between K and the reepithelialization rate.
Fig. 11. Average alpha and TEWL.
VI. CONCLUSION
A new device has been developed that will enable clinicians tomonitor wound healing without disturbing, the wound-healingprocess.
Initial results indicate that as skin regrows, the skin’s par-allel resistance RP increases dramatically so much that it wasquickly difficult to measure accurately within the frequencyrange chosen. There was a major difference in the magnitude ofthe impedance for an open wound and that of even only partiallyregrown skin. Therefore, the concept of mapping of a woundbased purely on the magnitude of the electrode–skin impedancewas validated.
For the skin’s pseudocapacitive properties, K increases andα decreases progressively. There were good correlations be-tween both K and alpha with TEWL and reepithelializationdemonstrating the potential usefulness of impedimetric charac-terization of tissues.
The vision for the future is to greatly miniaturize the de-vice and the electrodes (increasing their number/density) andto incorporate a wireless transmitter into the electrode array/dressing, thus enabling the use of a PDA or similar device to dis-play and store the recorded data. The recorded data can then besent to the clinician if results outside the anticipated boundariesoccur. This will enable patients and their families to optimallymanage the ulcers themselves under the guidance of a clinician.
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Authors’ photographs and biographies not available at the time of publication.
