Science - USA (2022-02-25)

The ability of a membrane to make a con-
formal interface with the surface topography
can be determined by bending stiffness ( 22 ).
The effective bending stiffness (EI)foramulti-
layer membrane can be described as

EI¼

XN

i¼ 1

Ei

1 v^2 i

hi

(

1

3

h^2 iþ

"

Xi

j¼ 1

hj

!

hneutral

# 2

hi

Xi

j¼ 1

hj

!

hneutral

" #)

ð 3 Þ

hneutral¼

XN

i¼ 1

Ei

1 v^2 i

hi

Xi

j¼ 1 hj



1

2

hi



XN

i¼ 1

Ei

1 v^2 i

hi

ð 4 Þ

wherehneutraldenotes the neutral mechanical
plane;irepresents theith layer of the film;
hi,Ei, andvirepresent the thickness, the elas-
tic modulus, and Poisson’s ratio, respectively;
andNis the number of layers ( 40 ). Notably,
with its ultrasmall thickness and low elas-
tic modulus, the freestanding 10-nm-thick
VDWTF exhibits a bending stiffness of 4.2 ×
10 −^9 GPa·mm^3 , which is about eight orders
of magnitude smaller than that of the 1.6-mm-
thick VDWTF/PI film (0.97 GPa·mm^3 ).

The VDWTFs transferred onto human skin

show excellent natural adaptability to changing

skin textures and retain conformal contact

without apparent fracturing or flaking through-

out stretching, squeezing, and relaxing cycles

(Fig. 4H), highlighting the highly adaptable

nature of the VDWTFs to dynamically evolving

biological substrates. In contrast, the CVDTFs

transferred onto human skin easily fracture

and flake off when the skin is subjected to

similar deformation. Figure 4I shows the re-

maining area of both films on the skin replica

versus the number of squeezing-and-stretching

cycles. Because the freestanding CVDTFs are

not strong enough for processing and transfer-

ring, they are transferred onto the skin replica

with methyl methacrylate (MMA) support.

After the transfer process, the CVDTFs quickly

flakeoffoncetheMMAisdissolvedawaywith

acetone vapor. The remaining area instantly

decreases to ~50% of the original area and,

after 100 stretching cycles, further decreases

to 40% of the original area, with mostly frac-

tured domains. The fracturing and flaking are

attributed to the unstable membrane–skin in-

terface, which is associated with their limited

stretchability, conformability, and poor wetta-

bility. In contrast, the VDWTFs show superior

stretchability and conformability to the dynam-

ically changing skin replica with no apparent

fracturing or flaking, retaining essentially

100% surface coverage after the repeated

squeezing-and-stretching cycles.

The output and transfer curves of the skin-

gate VDWTF transistor demonstrate expected

transistor functions (Fig. 4, J and K) with

a low operating voltage suitable for biological

systems. Furthermore, the skin-gate VDWTF

transistor can maintain stable operation while

undergoing various mechanical deformations

(Fig. 4L), establishing a foundation for appli-

cations in probing and amplifying electro-

physiological signals.

Monitoring electrophysiological signals with

skin-gate VDWTF transistors

Given that many biopotential signals show

transient responses, we have evaluated the fre-

quency response of the skin-gate transistors.

The response times,t, of the skin-gate tran-

sistors are probed by measuring the current

response under a 20-ms pulse of 100-mV gate

voltage (Fig. 5A). A response time of 7ms is

achieved by fitting experimental data with an

exponential function (Fig. 5B). Furthermore, the

skin-gate transistors show a cut-off frequency

(at which the transconductance drops by 3 dB

from its plateau value) of ~100 kHz (Fig. 5C),

which is sufficient for monitoring most elec-

trophysiological signals from the human body.

We explored the skin-gate VDWTF transis-

tors for monitoring electrocardiography (ECG).

856 25 FEBRUARY 2022•VOL 375 ISSUE 6583 science.orgSCIENCE

Fig. 3. Leaf-gate VDWTF

transistors.(A) Diagram of a

Senecio mandraliscaeleaf.

(B) Cross-sectional view of the

leaf-gate transistor with Au source

and drain electrodes (“S”and

“D,”respectively) and an inserted

tungsten gate electrode (“G”).

(C) Schematic illustration

(top; leaf, light green; VDWTF,

dark green; Au electrodes, yellow;

tungsten probe, black dot) and

photograph of the leaf-gate

transistor (bottom). (D) Optical

image of a VDWTF with serpentine

Au electrodes transferred onto

the plant leaf. (E) Colorized SEM

image of the VDWTF on the leaf.

(F) Output characteristics of

a leaf-gate transistor.Vds, drain-

source voltage;Ids, drain-source

current. (GandH) Transfer curves

with (G) linear and (H) logarithmic

axis.Vg, gate voltage.

A B

CD E

FGH

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