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Multifunctional Skin-Like Electronics for
Quantitative, Clinical Monitoring of Cutaneous
Wound Healing
Yoshiaki Haori
University of Illinois at Urbana-Champaign
Leo Falgout
University of Illinois at Urbana-Champaign
Woosik Lee
University of Illinois at Urbana-Champaign
See next page for additional authors
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1
Title: Multifunctional Skin-like Electronics for Quantitative, Clinical Monitoring of
Cutaneous Wound Healing
Woon-Hong Yeo
Prof. W. -H. Yeo
Department of Mechanical and Nuclear Engineering and Center for Rehabilitation Science
and Engineering
Virginia Commonwealth University
Richmond, VA 23284, USA
E-mail: whyeo@vcu.edu
Keywords: wound monitoring, clinical study, skin-like, multifunctional, epidermal electronics
Non-invasive, biomedical devices have the potential to provide important, quantitative data
for the assessment of skin diseases and wound healing. Traditional methods either rely on
qualitative visual and tactile judgments of a professional and/or data obtained using
instrumentation with forms that do not readily allow intimate integration with sensitive skin
near a wound site. Here we report a skin-like electronics platform that can softly and
reversibly laminate perilesionally at wounds to provide highly accurate, quantitative data of
relevance to the management of surgical wound healing. Clinical studies on patients using
thermal sensors and actuators in fractal layouts provide precise time-dependent mapping of
temperature and thermal conductivity of the skin near the wounds. Analytical and simulation
results establish the fundamentals of the sensing modalities, the mechanics of the system, and
strategies for optimized design. The use of this type of ‘epidermal’ electronics system in a
realistic, clinical setting with human subjects establishes a set of practical procedures in
disinfection, reuse, and protocols for quantitative measurement. The results have the potential
to address important unmet needs in chronic wound management.
2
1. Introduction
Monitoring of wound healing, a dynamic interactive biological process involving blood cells,
extracellular matrix, and parenchymal cells, is of great interest in biomedical research and
clinical practice. The most comprehensive means for assessment of wound healing is based
on histological evaluation of tissue morphologic change,
[1, 2]
but this process is invasive and
does not provide a means for continuous evaluation over time. Visual inspection by digital
photography
[1, 3, 4]
overcomes these limitations, however interpretation is inherently subjective
and the imaging often yields inconsistent information due to variations in lighting, focus and
angle. Quantitative imaging methods via confocal laser scanning microscopy or spectroscopy
can reveal microscopic level changes in the morphology of the epidermis and dermis.
[4, 5]
These methods, however, require patient immobilization during the testing. Also, the
required sophisticated optical systems are high in cost and require trained personnel for
evaluation. Recent work on simple, portable, point-of-care (POC) devices
[1, 6]
for optical
sectioning and topical determination of wound healing phases suggest promise, although their
use is ultimately limited by qualitative visual evaluation. In wound healing, calor is a primary
indicator of inflammation and possible infection.
[1, 7, 8]
Hydration is another factor that affects
wound healing.
[7]
Thus, monitoring of skin temperature and thermal conductivity (hydration),
along with numerous other potential markers such as bacterial load, cytokine release, DNA,
enzymes, hormones, pH, oxygen, and transepidermal water loss,
[1]
provides important clinical
information.
Practical biomedical devices, capable of non-invasive, quantitative and multifunctional
measurements of the healing process, are needed to complement optical and other techniques.
In this communication, we introduce a skin-like electronics system capable of precise and
real-time monitoring of cutaneous wound healing in a clinical setting. These devices
represent a type of epidermal electronics system (EES),
[8]
which adopts the soft mechanical
texture of the epidermis to allow conformal lamination and reversible bonding to the
3
epidermis via van der Waals interactions alone.
[9-11]
The result is a natural, non-irritating and
high quality interface to the skin that does not constrain natural motions or induce any
discomfort.
[10]
EES can be designed in biocompatible, waterproof forms that are easily
disinfected for clinical applications, enabling re-use. The sensors demonstrated here use
microscale, metal traces in fractal layouts on soft, elastomeric membranes, capable of
measurement and mapping of skin temperature with an accuracy comparable to that of a high-
end infrared (IR) camera, with the additional capabilities of recording thermal conductivity
and delivering precise levels of heating.
In wound healing monitoring, an EES records time-dynamic temperature and thermal
conductivity of the skin tissue. Mapping of skin temperature is important because it
presumably is capturing the ‘inflammation’ phase of the healing process, related to increased
blood flow to the wound.
[12, 13]
Thermal conductivity correlates strongly to hydration state,
which is another important aspect of wound care, and can serve as an early sign of the
emergence of local edema.
[14-16]
From a practical standpoint, thermal conductivity can also
serve as a sensitive indicator of quality of contact between the device and the wound,
allowing the healthcare professional to assess proper mounting on the skin. Three
dimensional mechanical and thermal simulations of this type of EES on human skin, through
the finite element method (FEM) and the finite volume method (FVM), respectively, capture
the underlying physics of this contact, as well as the mechanisms for physiological sensing
mechanism. The results establish critical design criteria for clinical applications.
2. Results
2.1 Device design and mechanics modeling
Figure 1a presents a schematic view of the layouts of a multifunctional EES of the type used
in the studies reported here. The device uses a multilayer construct that consists of metal
traces with fractal geometries (Peano curve motif) in an interconnected collection of ultrathin
