EPIDERMAL ELECTRONIC SYSTEM [EES]
A demonstrative platform of a
typical EES is shown in above Fig integrating a collection of multifunctional
sensors (such as temperature, strain, and electrophysiological), micro scale
light-emitting diodes (LEDs), active/passive circuit elements (such as
transistors, diodes, and resistors), wireless power coils, and devices for
radio frequency (RF) communications (such as high-frequency inductors,
capacitors, oscillators, and antennae), all integrated on the surface of a thin
(~30 μm), gas-permeable elastomeric sheet based on a modified polyester (BASF,
Ludwigshafen, Germany) with low Young’s modulus (~60 kPa).
In this paper i present a new technology called EES in which a
number of conventional medical diagnosis systems are fabricated onto a single
polymer and this polymer is attached to skin of a patient who needs to be
diagnosed regularly. This solves the problems of regular diagnosis of patients
faced by majority of health facilities.
we report
classes of electronic systems called EES that achieve thicknesses, effective
elastic moduli, bending stiffnesses, and areal mass densities matched to the
epidermis the capability which a Traditional wafer-based technologies do not
have .Therefore EES devices are being
developed which can be laminated onto the skin which leads to conformal contact
and adequate adhesion in a manner that is mechanically invisible to the
user.EES systems incorporate electrophysiological, temperature, and strain
sensors, as well as transistors, light-emitting diodes, photo detectors, radio
frequency inductors, capacitors, oscillators, and rectifying diodes for
electrophysiological measurements like heartbeat etc…
Introduction:
Physiological measurement and
stimulation techniques that exploit interfaces to the skin have been of
interest for more than 80 years, beginning in 1929 with electroencephalography
from the scalp. In these techniques small numbers of bulk electrodes are
mounted on the skin via adhesive tapes, mechanical clamps or straps, or
penetrating needles using conductive gels these are in turn connected to boxes
that house collections of rigid circuit boards, power supplies, and
communication components. These systems have many important capabilities, but
they are poorly suited for practical application outside of research labs or
clinical settings because of difficulties in establishing long-lived, robust
electrical contacts that do not irritate the skin and in achieving integrated
systems with overall sizes, weights, and shapes that do not cause discomfort
during prolonged use.
We introduce a different approach called
EES in which the electrodes, electronic devices, sensors, power supply, and
communication components are configured together into ultrathin, low-modulus,
lightweight, stretchable “skin-like” membranes that can be laminated onto the surface of the skin by
soft contact, in a manner that is mechanically invisible to the user, much like
a temporary transfer tattoo.
What does an EES consists of?
Material related design strategies:-
The devices and interconnects exploit
ultrathin layouts (<7 μm), neutral mechanical plane configurations, and
optimized geometrical designs.
The
active elements use established electronic materials, such as silicon and
gallium arsenide, in the form of filamentary serpentine nanoribbons and micro-
and nanomembranes.
To result in a high-performance system that
offers these reversible, elastic responses to large strain deformations with
effective moduli (<150 kPa), bending stiffnesses (<1 nN m), and areal
mass densities (<3.8 mg/cm2) the
orders of these parameters should
be smaller .Unfortunately this can’t be achieved using conventional electronics or even with recently
explored flexible/stretchable device technologies .Therefore a gas-permeable
elastomeric sheet based on a modified polyester developed by BASF,
Ludwigshafen, Germany with low Young’s modulus (~60 kPa).
Water-soluble polymer sheets [polyvinyl
alcohol (PVA) (Aicello, Toyohashi, Japan); Young’s modulus, ~1.9 GPa;
thickness, ~50 μm serve as temporary supports for manual mounting of these
systems on the skin in an overall construct that is directly analogous to that
of a temporary transfer tattoo.
Image
of a demonstration platform for multifunctional electronics with physical
properties matched to the epidermis. Mounting this device on a sacrificial,
water-soluble film of PVA, placing the entire structure against the skin, with
electronics facing down, and then dissolving the PVA leaves the device
conformally attached to the skin through van der Waals forces alone, in a
format that imposes negligible mass or mechanical loading effects on the skin.
The image in Fig 1B, top, is of a device
similar to the one in Fig 1A, after mounting it onto the skin by washing away
the PVA and then partially peeling the device back with a pair of tweezers.
When completely removed, the system collapses on itself because of its extreme
deformability and skin-like physical properties, as shown in Fig 1B, bottom
(movie S1).
Fig 1B
EES
partially (top) and fully (bottom) peeled away from the skin. (Inset) A
representative cross-sectional illustration of the structure, with the neutral
mechanical plane (NMP) defined by a red dashed line.
Fig 1C
Multifunctional EES on skin:
undeformed (left), compressed (middle), and stretched (right).
Fig 1D
A
commercial temporary transfer tattoo provides an alternative to polyester/PVA
for the substrate; in this case, the system includes an adhesive to improve
bonding to the skin. Images are of the backside of a tattoo (far left),
electronics integrated onto this surface (middle left), and attached to skin
with electronics facing down in undeformed (middle right) and compressed (far
right) states.
Mechanics:
Understanding the mechanics of
this kind of device, the mechanophysiology of the skin, and the behavior of the
coupled abiotic-biotic system are all important. For present purposes, the skin
can be approximated as a bilayer, consisting of the epidermis (modulus, 140 to
600 kPa; thickness, 0.05 to 1.5 mm) and the dermis (modulus, 2 to 80 kPa;
thickness, 0.3 to 3 mm). This bilayer exhibits linear elastic response to
tensile strains ≲15%, which transitions to
nonlinear behavior at higher strains, with adverse, irreversible effects beyond
30%. The surface of the skin has wrinkles, creases, and pits with amplitudes
and feature sizes of 15 to 100 μm and 40 to 1000 μm,respectively. The devices
described here have moduli, thicknesses, and other physical properties that are
well matched to the epidermis, with the ability to conform to the relief on its
surface. We therefore refer to this class of technology as an “epidermal
electronic system” (EES).
Macroscopic view:
Macroscopically,
an EES on skin can be treated as a thin film on an epidermis–dermis bilayer
substrate. Microscopically, the sizes of the individual electronic components
and interconnects are comparable with those of relief features on the skin and
therefore must be considered explicitly. We began by considering aspects of
adhesion, in the macroscopic limit. Globally, detachment can occur in either
tension or compression because of interfacial cracks that initiate at the edges
or the central regions of the EES, respectively. Low effective moduli and small
thicknesses minimize the deformation-induced stored elastic energy that drives
both of these failure modes. Analytical calculation shows that compared with
silicon chips (thickness of ~1 mm) and sheets of polyimide (thickness of ~75
μm), the driving forces for delamination of the EES/skin interface are reduced
by more than seven and four orders of magnitude, respectively. The critical
delamination strain is plotted in Fig. 2A as a function of PDMS thickness for
two different formulations: one with a modulus of 19 kPa (50:1) and the other
145 kPa (30:1) (fig. S1C). The results, both theory and experiment, confirm
that reducing the modulus and thickness lowers the driving forces for interface
delamination for a given applied strain without lower bound.
Fig 2(A)
Plots of critical tensile (left) and
compressive (right) strains for delamination of a test structure consisting of
films of PDMS on substrates of polyester, designed to model the EES/skin
system. Data for formulations of PDMS with two different moduli are shown (red,
19 kPa; blue, 145 kPa). The critical strains increase as the PDMS thickness and
modulus decrease, which is consistent with modeling results.
Fig 2(B)
Optical micrograph of an
EES with FS design (left). The plot (right) shows the stress-strain data from
uniaxial tensile measurements for two orthogonal directions. Data collected from
a sample of pig skin are also presented. The dotted lines correspond to
calculations performed with finite element modeling.
Fig 2 (C)
Skin of the forehead before (top left) and
after the mounting of a representative FS-EES, at various magnifications and
states of deformation. The dashed blue boxes at right highlight the outer
boundary of the device. The red arrows indicate the direction of compressive
strains generated by a contraction of facial muscles. The red dashed box at the
top right corresponds to the field of view of the image in the bottom left.
Fig 2(D)
Confocal
microscope image (top view) at the vicinity of the contacting interface between
an FS-EES laminated on a sample of pig skin. The FS structure and the skin are
dyed with red and blue fluorophores, respectively.
Fig 2(E)
Cross-sectional confocal
images at locations corresponding to the numbered, white dashed lines shown in
the top-view frame above.
The
mechanical properties of the EES depend on the effective modulus and thickness
of both the circuits and sensors and the substrate.
Multifunctional operation:
A key capability of EES is in monitoring electrophysiological
(EP) processes related to activity of the brain [electroencephalograms (EEGs)],
the heart [electrocardiograms (ECGs)] and muscle tissue [electromyograms
(EMGs)]. Amplified sensor electrodes that incorporate silicon metal oxide
semiconductor field effect transistors (MOSFETs) in circuits in which all
components adopt FS designs provide devices for this purpose. Here, the gate of
a FS-MOSFET connects to an extended FS electrode for efficient coupling to the
body potential (Fig.
3A; the inset shows an analogous design based on a rectangular
device island and FS interconnects) via contact with the skin in a
common-source amplifier configuration (Fig.3B, left). The measured frequency
response at different input capacitances (CIN) is indicated in Fig.
3B, right, and is in quantitative agreement with circuit simulations (fig. S3,
A and B). The value of CIN is determined by a series combination of
capacitances of the gate electrode, the encapsulating PI, and junction between
the gate electrode and the body surface. The bandwidth matches requirements for
high-performance EP recording. A typical layout for this purpose includes four
amplified channels, each comprising a FS-MOSFET, a silicon-based FS resistor,
and an FS electrode. One channel provides a reference, whereas the others serve
as sites for measurement. Results of demonstration experiments appear
subsequently.
Fig 3 A,B,C
(A)
Optical micrographs of an active electrophysiological (EP) sensor with local
amplification, as part of an FS-EES. (Left) The source, drain, and gate of silicon
MOSFET and a silicon feedback resistor before connection to sensor electrodes,
all in FS layouts. (Inset) Similar device with island design. (Right) The final
device, after metallization for the interconnects and sensor electrodes, with
magnified view (inset). (B) Circuit diagram for the amplified
EP sensor shown above (left). (Right) Measured and simulated frequency response
for different input capacitance (CIN = ∞, 1μF, 220pF). (C)
Optical micrograph of a temperature sensor that uses a platinum resistor with
FS interconnects (left) and a strain gauge that uses electrically conductive
silicone (CPDMS; right).
(D)
Image of an array of microscale AlInGaP LEDs and photo detectors, in an
interconnected array integrated on skin, under compressive deformation (left)
and of a FS silicon solar cell (right). (E) Image of a FS
wireless coil connected to a microscale InGaN LED, powered by inductive
coupling to a separate transmission coil (not in the field of view). (F)
Optical micrograph of a silicon RF diode. (G) Optical
micrograph of an interconnected pair of FS inductors and capacitors designed
for RF operation (left).
Many other classes of semiconductor
devices and sensors are also possible in EES, including resistance-based
temperature sensors built with meander electrodes, LEDs and photo detectors
based on AlInGaP and silicon FS photovoltaic cells Such cells can generate a
few tens of microwatts ; increasing the areas or areal coverages can improve
the output, but not without compromises in size and mechanics. Wireless
powering via inductive effects represents an appealing alternative. An example
of an FS inductive coil connected to a microscale InGaN LED is shown in Fig. 3E,
with electromagnetic modeling of its RF response. Such coils provide power
directly in this example; they can also conceivably be configured to charge
future classes of EES-integrated storage capacitors or batteries.
Systems for electrophysiological
recording:
EES configured for
measuring ECG, EMG, and EEG in conformal, skin-mounted modes without conductive
gels or penetrating needles provide important, system-level demonstrations (fig.
S4A and movie S2). All materials that come into direct contact with the skin
(Au, PI, and polyester) are biocompatible Devices worn for up to 24 hours or
more on the arm, neck, forehead, cheek, and chin showed no degradation or
irritation to the skin (figs. S14 and S15). Devices mounted in challenging
areas such as the elbow fractured and/or debonded under full-range motion (fig.
S16). ECG recordings from the chest
revealed high-quality signals with information on all phases of the
heartbeat, including rapid depolarization of the cardiac wave, and the
associated QRS complex (Fig. 4A, right).
EMG
measured on the leg with muscle
contractions to simulate walking and resting are presented in Fig. 4B, left.
The measurements agree remarkably well with signals simultaneously collected
using commercial, bulk tin electrodes that require conductive gels, mounted
with tapes at the same location (Fig. 4B, right, and fig. S4B, right). An
alternative way to view the data (spectrogram) is shown in Fig. 4C, in which
the spectral content appears in a color contour plot with frequency and time
along the y and x axes, respectively.
To
demonstrate EMG recording in a mode in which conventional devices are
particularly ill suited, an EES mounted on the throat can monitor muscle
activity, noninvasively, during speech (fig. S5A) . Here, recordings collected
during vocalization of four words (“up,” “down,” “left,” and “right”), repeated
10 times each (fig. S6) exhibit distinctive patterns, as in Fig. 4D.
Measurements from another set of words (“go,” “stop,” and “great”) (figs. S5B
and S7) suggest sufficient structure in the signals for recognizing a
vocabulary of words. These capabilities create opportunities for EES-based
human/machine interfaces. As an example, dynamic time-warping
pattern-recognition algorithms applied to throat-based EMG data (Fig. 4D)
enable control of a computer strategy game , as illustrated in Fig. 4E. The
classifications occur in less than 3 s on a dual-core personal computer running
codes in MATLAB (Math Works, Natick, MA), with an accuracy of >90% (fig.
S8).
As a
human/machine interface, EEG data offer additional promise. EES mounted on a
region of the forehead that is first prepared by exfoliating the stratum
corneum with Scotch tape yields reproducible, high-quality results, as
demonstrated in alpha rhythms recorded from awake subjects with their eyes
closed (fig. S9A) . The expected feature at ~10 Hz appears clearly in the
Fourier-transformed data of Fig. 4F, left. The spectrogram of Fig. 4F, center,
shows similar signatures. This activity disappears when the eyes are open. The
signal-to-noise ratios are comparable with those obtained in otherwise
identical experiments that used conventional, rigid bulk electrodes with
conductive coupling gels. In further demonstrations, EEG measured with EES
reveals well-known cognitive phenomena such as the Stroop effect. In these
experiments, subjects randomly presented with congruent or incongruent (fig.
S9B) colored words whisper the color (not the word) as quickly as possible. The
data show that the motor responses pertaining to the whispering are manifested
by two peaks at ~650 ms (congruent case) and ~1000 ms (incongruent case), as
shown in Fig. 4F, right. The time delay implies that the congruent stimuli
require fewer cognitive resources and are quicker to process than are the
incongruent ones, which is consistent with the literature.
Conclusions:
The
materials and mechanics ideas presented here enable intimate, mechanically
“invisible,” tight and reliable attachment of high-performance electronic
functionality with the surface of the skin in ways that bypass limitations of
previous approaches. Many of the EES concepts are fully compatible with
small-scale integrated circuits that can be released from ultrathin-body
silicon-on-wafer substrates. For long-term use, materials and device strategies
to accommodate the continuous efflux of dead cells from the surface of the skin
and the processes of transpiration will be needed.
References:
www.wikipedia.in
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