Micro-deformation response of a holographic
sensor in highly stretchable polymer hydrogel
DAN YU,
1,
*HONGPENG LIU,
2
RUI WANG,
2
LI LI,
2
SUHUA LUO,
3
JIANG LV,
1
AND WEIBO WANG
2
1
College of Science, Tianjin University of Technology, Tianjin 300384, China
2
Department of Physics, Civil Aviation University of China, Tianjin 300300, China
3
Department of Physics, Harbin Institute of Technology, Harbin 150001, China
*Corresponding author: yudanhit@126.com
Received 10 April 2018; revised 6 May 2018; accepted 28 May 2018; posted 30 May 2018 (Doc. ID 327991); published 18 June 2018
A volume grating-based holographic sensor in highly stretch-
able PVA/AA polymer for sensing its micro-deformation and,
hence, the displacement has been studied. One-dimensional
micro-displacement induced by tensile stress is analyzed
using a diffraction spectrum in two kinds of sensor construc-
tion methods, i.e., transmission and reflection. The depend-
ence of the peak wavelength on the displacement presents a
good linear relationship which provides a quantitative sensing
strategy for hydrogel micro-deformation. The available sensi-
tivity is better than 4 μm/0.5 nm (displacement/wavelength
shift) using a commercial spectrometer with a resolution of
0.5 nm. Finally, the reversible deformation response further
validates the practical applicability of a holographic sensor
constructed by photosensitive hydrogel. The optical measure-
ment of micro-displacement as a novel sensing strategy can
accelerate the development of the holographic optical
element.
© 2018 Optical Society of America
OCIS codes: (090.0090) Holography; (160.5335) Photosensitive
materials; (230.1950) Diffraction gratings.
https://doi.org/10.1364/OL.43.003037
A smart hydrogel with crosslinked polymer network can
reversibly swell or shrink in response to the change in environ-
mental factors [1,2]. So far, many methods have been devel-
oped to characterize the modification, including electrode
impedance pressure sensors [3], piezoresistive membranes
[4], and optical methods [5,6]. Synthetic hydrogel-based pho-
tonic devices currently face a great challenge. These hydrogel
devices are relatively brittle against the external deformation
[7]. Consequently, the practical application of photonic sen-
sor-based on hydrogel in sensing deformation is restricted.
Recently, the researcher attempted to improve the low mecha-
nism strength by hydrogel optical fibers [8]. However, the de-
sign of grating-free hydrogel optical fiber actually lost the
sensing advantage of optical diffraction spectrum. A holo-
graphic sensor as a novel optical sensing device can extend
the optical applicability [9,10]. It is a potential sensing platform
due to its narrow diffraction spectrum, high visualization, and
stability [11,12]. Photopolymer is a significant candidate for
the next holographic sensor because of its high diffraction effi-
ciency and self-process [13–16]. Compared to other sensing
strategies, especially the photonic sensing method, the holo-
graphic sensor provides a simple, quick, and intuitive identifi-
cation. The previous report provides a holographic sensor for
pressure observation based on the Denisyuk reflection grating
[17]. A change in the color of the reconstructed grating is vis-
ible from red color to blue with increasing pressure. However,
so far, sensing the polymer deformation induced by tensile
force and the quantitative result using holographic sensor
has been absent.
Here we report on the fabrication of a novel micro-
displacement holographic sensor using high flexible polymer.
A linear displacement sensing response with high precision
and a large range is implemented. The dependence of micro-
deformation on the spectrum response provides a quantitative
basis for sensing the displacement. The reve rsible response fur-
ther evidences its applicability. The novel sensing method is
expected to apply in sensing the feedback of a hydrogel-based
micro-actuator in microfluidics.
In our experiments, acrylamide (AA) photopolymer hydro-
gel was prepared to implement the micro-displacement mea-
surement. The polymer system consisted of AA monomer, a
triethanolamine ( TEA) initiator, an NN’-m ethylene bisacryla-
mide (MBAA) cross-linking agent, methylene blue dye for the
red laser, and a polyvinyl alcohol (PVA) binder (1788 type).
The degrees of polymerization and hydrolysis of the PVA
binder were 1700 and 88% (mol/mol), respectively. The cor-
responding average molecular weight of the PVA binder was
approximately 75,000.
The procedure of sample preparation was described as fol-
lows. First, PVA powder was dissolved into deionized water
to form a 10 w/v% mixed solution at temperature 70°C. Then
the solution was cooled to room temperature. Simultaneously,
other components, namely, AA, TEA, BAA, EB, were weighted
into another bottle according to the proportion. Then the
cooled PVA solution was mixed into the bottle and stirred until
the mixed liquid became transparent. Finally, the mixed solu-
tion was spread on a glass sheet using a micro-pipette and dried
for 36–48 h for holographic application. After the drying
Letter
Vol. 43, No. 13 / 1 July 2018 / Optics Letters 3037
0146-9592/18/133037-04 Journal © 2018 Optical Society of America