a b c - Nature Research€¦ · b, Photoinitiated polymerization of the monomer and subsequent...
Transcript of a b c - Nature Research€¦ · b, Photoinitiated polymerization of the monomer and subsequent...
Supplementary Figures
Supplementary Figure S1. Preparation of composite gel sheets and their
transformation in helical structures. a, Schematic of the photolithographic patterning
of the hydrogel sheet. A sheet of PNIPAm gel (a primary gel or PG) is swollen with a
monomer mixture, sandwiched between the two glass slides, and exposed through a
photomask to ultraviolet irradiation. The mask contains black stripes (1 mm wide, 1 mm
apart) passing at an angle to the long axis of the mask. b, Photoinitiated polymerization
of the monomer and subsequent removal of unreacted monomer and linear polymer yield
a PNIPAm/PAMPS binary gel (BG) in the light-exposed regions of PG. c, A helix
formed by the patterned gel sheet in a 1M NaCl solution or in water heated to 45 oC,
above the dehydration temperature of PNIPAm.The dark and light-blue colors correspond
to the stripes of PG and BG, respectively.
a b cstimulus
Photopolymerization
Supplementary Figure S2. Heat mediated helix formation. A photograph of the helix
formed after 2 h incubation in deionized water at 45 oC. After photopolymerization of
PAMPS, the patterned gel was exposed for 5 min to the air. The scale bar is 1 cm, t0 =
0.44 mm,
1 mm, = 45o. Experimental details are described in
Supplementary Methods.
Supplementary Figure S3. Reversibility of planar-to-helical transitions. a, A planar
patterned gel sheet floating in deionized water at 23 oC. b, A helix floating at the air-
liquid interface. The helix is formed following transfer of a sheet shown in (a) in a 1M
NaCl solution. c, A planar gel sheet formed by transferring a helix as in (b) in deionized
water at 23 oC. The scale bar is 0.5 cm. d, Variation in the number of turns, N, and pitch,
p, of the helix, following its repetitive 2 h-long incubation in deionized water and in a 1M
solution of NaCl. The error bars represent standard deviation calculated from 3
measurements. Following photopatterning, the top surface of the patterned gel was
exposed for 5 min to the ambient air. t0=0.44 mm,
= 1 mm, = 45o.
Experimental details are described in Supplementary Methods.
0
0.5
1
1.5
2
0
6
12
18
Tu
rns
of
heli
x Pitc
h (c
m)
C (M)NaCl
1 1 10 0
N
p (c
m)
a
b
c
d
Supplementary Figure S4. Deformation of the patterned gel sheet. a, A phototomask
used for gel patterning with stripes parallel to the long axis of the sheet. b, Corresponding
multi-roll hydrogel sheets formed after their 2 h incubation in a 1M NaCl solution. c, A
phototomask used for hydrogel patterning with stripes perpendicular to the long axis of
the sheet. d, A corresponding arc-like hydrogel sheet formed after its 2 h incubation in a
1M NaCl solution. Following polymerization, the gels were exposed for 5 min to the
ambient atmosphere. t0 = 0.44 mm,
1 mm. The scale bar is 1 cm.
b d
a c
= 0
Θ=0o
Θ=30o
Θ=45o
Θ=60o
Θ=90o
b
θ
65 mm
10 mm
1 mm1 mm
a
Θ=0o
Θ=30o
Θ=45o
Θ=60o
Θ=90o
b
θ
65 mm
10 mm
1 mm1 mm
a
b = 90o
Supplementary Figure S5. Helices with different chiralities formed without gradient
in composition across the gel sheet. a, Helix with both right- and left-handedness. b,
Helix with three sections with alternating right- and left-handedness. The scale bar is 0.5
cm. c, A fraction of helices with right-handed (RH), left-handed (LH) and helices with
both types of handedness (RH&LH, as shown in a,b), formed upon transfer without post-
polymerization air exposure of the gel sheet into a 1M NaCl solution. The results were
obtained for 102 helices. The gel sheet was patterned using a mask with the angle =
45o. t0 = 0.44 mm,
1 mm. Experimental details are described in
Supplementary Methods.
0
20
40
60
80
100
RH LH RH&LH
a
b
c
Supplementary Figure S6. Control of the chirality of the helix. a,b A left-handed and
a right-handed helices generated after post-polymerization exposure of the top and the
bottom surfaces of the patterned gel, respectively, to the ambient atmosphere for 5 min
and subsequent transfer of the gel sheet into a 1M solution of NaCl. The scale bar is 1
cm. The insets show the corresponding hand symbols. The gel sheet was patterned using
a mask with the angle = 45o. t0 = 0.44 mm,
1 mm. Experimental details
are described in Supplementary Methods.
a
b
Supplementary Figure S7. Variation in helix morphology in NaCl solutions with
varying ionic strength. Photographs of the left-handed helix formed in solutions at
CNaCl of 0.8M (top), 1.7M (middle), and 2.5M (bottom). The scale bar is 1 cm, t0=0.44
mm, w0
PG=1 mm, w0BG=1 mm, = 45
o. Experimental details are described in
Supplementary Methods.
Supplementary Figure S8. Effect of the ratio of widths of the stripes of PG-to-BG
on helix morphology. Photographs of the gel sheets patterned with PB and BG stripes
with different widths, after 24 h equilibration in a 1.2M solution of NaCl. White stripes
correspond to the PG regions. The insets indicate the ratio of the original widths of PG-
to-BG stripes (determined by the photomask). =45o, t0=0.44 mm. The scale bar is 1 cm.
Experimental details are described in Supplementary Methods.
Supplementary Figure S9. Temperature-dependent deswelling of the structural
components of binary and ternary gels. Variation of the normalized weight of an
hydrogel sheet with increase in temperature for PNIPAm (), P(HEAm-co-NIPAm)
(H1) (), P(HEAm-co-NIPAm)/ PNIPAm (H2) (). Experimental details are described
in Supplementary Methods.
Supplementary Figure S10. Schematic of as-prepared gel sheet. The dark- and light-
blue colors correspond to the stripes of PG and BG, respectively.
Supplementary Figure S11. Variation in curvatures of PG and BG calculated from
energy minimization of Eq. S1 and S2. In the entire range of CNaCl the values of
() and () are of the order of ().
0
10
20
30
40
50
0 0.5 1 1.5 2 2.5
Cu
rva
ture
C (M)NaCl
Supplementary Figure S12. Determination of the number of turns of the helix. a,
The structural characteristics of the helix. b, The side view (left) and the front, cross-
section view (right) of the helix in a NaCl solution at high CNaCl. c, The side view (left)
and the front, cross-section view (right) of the helix in NaCl solution at low CNaCl.
N =φ
360
b
c
x
y
zA
B
x
y
zA
B
y
z
A
Bφ
A
y
z
B
φ
Side view Front view
L
p
N =L
p
a
Supplementary Figure S13. Evolution of helical structures. a, Photographs of the
patterned hydrogel taken at different time intervals after transferring it from deionized
water into a 1M NaCl solution. The scale bar is 1 cm. b, Variation in pitch p and the
number of turns N of the left-handed helix, plotted as a function of time. After
polymerization of PAMPS, the top sheet surface was exposed for 5 min to the ambient air.
t0 = 0.44 mm,
1 mm, = 45o.
p
a b
5 min
30 min
10 min
60 min
Supplementary Figure S14. Variation in the curvature of the PG and BG sheets. The
top surface of the sheets of PG () and BG () with dimensions of 65 mm × 5 mm × 0.4
mm were exposed for 5 min to the ambient atmosphere and immersed for 2 h into a NaCl
solution with a particular concentration.
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1 1.5 2 2.5
k (
cm
-1)
C (M)NaCl
a
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1 1.5 2 2.5
k (
cm
-1)
C (M)NaCl
a
Supplementary Figure S15. Characterization of the top and the bottom surfaces of
PAMPS hydrogel. a, ATR-FTIR spectra were collected for the top, air-exposed surface
(--) and the bottom, air-protected surface () of the film. The sheet was exposed to the
ambient air for 5 min. b, ATR-FTIR spectra collected for the top surface (--) and the
bottom surface () of the hydrogel film transferred in deionized water without air
exposure. The thickness of the gel film was 0.44 mm gel. The concentration of PAMPS
in the gel is 30 wt%. Each spectrum was generated from 70 scans at a scan rate of 10 kHz
and spectral resolution 4 cm-1
.
Wavenumber (cm-1)1300 1400 1500 1600
0.03
0.04
0.05
0.06
0.07A
bso
rpti
on
(ab
s. u
nit
s)
Wavenumber (cm-1)1300 1200 1100 1000
0.03
0.04
0.05
0.06
0.07
Ab
sorp
tio
n (a
bs.
un
its)
ba
Supplementary Figure S16. Shape transformations of poly(acrylamide-co-butyl
methacrylate) primary gel sheet patterned with stripes of pH-responsive polymers.
a, Schematic (left) and a roll shape of the gel (right) patterned with stripes of poly(N-
vinyl imidazole) at pH =2.3. b, Schematic (left) and planar shape of the gel (right)
patterned with stripes of poly(N-vinyl imidazole) at pH=9.5. c, Schematic (left) and a
planar shape of the gel (right) patterned with poly(methacrylic acid) at pH =2.3. d,
Schematic (left) and a helical shape of the gel (right) patterned with poly(methacrylic
acid) at pH =9.5. In (a-d) the stripes of poly(acrylamide-co-butyl methacrylate) (PG) are
shown with a dark-blue color. In (a,b) the light-blue color in the schematics corresponds
to poly(N-vinyl imidazole). = 0o. In (c,d) the light-purple color in the schematics
corresponds to poly(methacrylic acid). = 45o. The gel sheets were equilibrated for 18 h
in corresponding solutions. t0=0.44 mm,
1 mm. The scale bars are 0.5 cm.
pH = 2.3
pH = 9.5
a
b
c
d
Supplementary Tables
Supplementary Table S1. Effect of angle θ on the number of turns, N, the pitch, p,
and the radius, R, of the helix.*
*The standard deviations of N, p, and R were calculated from 3 measurements.
θ N p (cm)
30o
45o
60o0.9±0.1
1.6±0.2
1.4±0.1
5.5±0.4
2.9±0.3
2.1±0.3
R (cm)
0.53±0.03
0.5±0.05
0.52±0.02
Supplementary Table S2. Variation in volume ratio of BG-to-PG in solutions with
varying concentration of NaCl.*
*The ratio of volumes of BG-to-PG was determined as VBG/ VPG = (fBG/fPG)3, where fBG
and fPG are the relative changes in linear dimensions of BG and PG (defined in the main
text).
0.00 3.6
0.10 3.6
0.30 3.8
0.50 4.2
0.75 9.4
1.00 11.5
1.25 11.4
1.50 10
2.00 8.5
2.50 7.2
CNaCl (M) VBG/VPG
Supplementary Table S3. Recipes used for the synthesis of PG and BG.
ComponentsFirst step (PG) Second step (BG)
NIPAm
AMPS
MBAA 1.0
14
-
0.25
20
-
V-50
Concentration (wt%)
1.0 0.50
Supplementary Table S4. Effect of the time of air-exposure of the patterned gel on
the number of turns and pitch of the helix.*
*The standard deviations of N and p were calculated from 3 measurements.
Exposure time (min) N p (cm)
5
10
15
20
1.5±0.1
1.5±0.1
1.4±0.1
1.5±0.1
2.9±0.1
2.9±0.1
2.8±0.1
2.9±0.1
Supplementary Notes
Supplementary Note 1. Theoretical modeling
1. Hydrogel sheet parameters
Due to the shrinkage of the PG and BG stripes in the solution of NaCl, their widths and
thickness change as
where t0 is the thickness of the as-prepared gel film, tPG and wPG are the thickness and the
width of PG stripes, respectively, and tBG and wBG are the thickness and the width of BG
stripes, respectively.
2. Generation and selection of curvature
General considerations
The composite hydrogel sheets are a special type of Non-Euclidean-Plates (NEPs)30,31
,
which are thin elastic sheets that are uniform across their thickness with in plane local
reference (or “rest”) lengths that are described by a non-Euclidean reference metric tensor
. Unlike previously studied cases, in the present work, in the patterned hydrogel sheets
is periodic on a short length scale along the y-direction and invariant along the x-
direction (Supplementary Fig. S10). In order to find the energy-minimizing
configuration, one should express the elastic energy in terms of the reference, and actual
metric and curvature tensors and subsequently, minimize it. This procedure will be
presented elsewhere. Here we simplify the expressions for the energy by using several
approximations that allow the derivation of an analytical expression (Eq. 1 in the main
text). This equation qualitatively predicts the variation in helix properties with properties
of small-scale structural components of the gel sheet. In addition, we relax some of the
approximations and provide a simplified form of the energy (Eq. S22 and S23) that can
be numerically minimized, in order to perform quantitative calculations.
Derivation of Eq. 1
We start with the energy density, W, of a plate, based on the theory of non-Euclidean
plates
( )
( )
(S1)
where
(
) is the isotropic homogenous elastic tensor,
and are the curvature and metric tensors of a given configuration, respectively, and
and are the reference metric and curvature tensors respectively, and
( ) is the
strain tensor.
In the experiments, the Poisson ratio is
and the spontaneous curvature is
. Using the in-plane strains, and curvatures, of a configuration, the energy
density is given by:
(
)
(
) (S2)
Assumption of (negligible strains in the y-direction) leads to
(
(
)) (S3)
Approximations
This section uses the notations introduced in the main text. In the main text, we solved
the problem for the case of a very large contrast in Young’s moduli of PG and BG,
similar to the natural fibrous tissue. Since the short PG stripes are significantly stiffer
than the long BG stripes, they were considered to be inextensible. In this case, all the
stretching energy is contained within the soft BG stripes. In addition, the bending energy
density is significantly higher for the rigid PG stripes, as it scales as , and the
curvatures in the x-direction of both soft and stiff stripes are of the same order (
).
Thus the energies per unit length are
(S4)
(
) (S5)
In addition, for bending energy minimization, the curvatures in the x- and y-
directions should be comparable, leading to
(S6)
The composite gel sheets are sufficiently thin to buckle, but not very thin, that is,
they are beyond, but close to the buckling threshold. As a result, the curvature across a
stripe is roughly constant and the stripe has an arc shape in the y-direction (Fig. 3c).
Under such conditions, , the deviation of the centerline of the BG stripes from R (Fig.
3d) is
(S7)
The ratio between the lengths of BG and PG stripes is therefore
(S8)
While the ratio of lengths of the free stripes of (the “rest lengths”) is
(S9)
The strain in the BG stripes, , is the difference between the rest length ratio, , and
the actual length ratio, , per unit length of the BG stripe,
(S10)
and the stretching energy (Eq. S4) is then given by
(
)
(S11)
The total elastic energy in our approximation is
(
)
(S12)
Eq. S8 leads to
(
)
(S13)
The first term is the geometric stretching term, while the second one reflects the bending
term. The simplest estimation of the selected radius of the gel sheet is obtained by
equalizing the bending and stretching energies
(
)
(S14)
Neglecting 4-th orders in
we obtain
( ) (
)
(S15)
and by solving for R
( )
(S16)
Substituting the relations
( ) (S17)
we obtain
( )
(S18)
which is similar to Eq. 1 up to a numerical factor.
If instead of equating the energies, we minimize the energy E (Eq. S13), the result is
(
( ) ) (S19)
Rearranging it using the relation (Eq. S17) yields
( (
)
( ) )
(S20)
Since the second term in the denominator is small (in our experiments it varies in the
range from 0.01 to 0.1), we can expand Eq. S20) to second order in it and obtain
( )
(S21),
which is Eq. 1 in the main text.
This simple analytical expression captures the qualitative dependence of R on
the properties of small-scale structural components of the gel sheet. However, for
quantitative calculations it is preferred to avoid some of the used approximations and to
numerically minimize the energy. The assumptions of inextensibility of the PG stripes
and of the equality of curvatures in the x- and y- directions can be removed. In this case,
the energy includes the different curvatures of PG and BG regions in the y-direction,
and , respectively, as in Eq. S3. In addition, the stretching energy of the PG stripes
and the bending energy of the BG stripes are included in the expression for the elastic
energy and the bending and the stretching energies (ESt and EBend, respectively) are
22
BG
BG
BG
PGBGBGBG
22
PG
PG
BG
PGPGPGPGSt
811
2
1
811
2
1
w
f
ftwE
w
f
ftwEE
yxyx
(S22)
BG2BG23
BGBGBG
PG2PG23
PGPGPGBend12
1
12
1yxyxyxyx twEtwEE
(S23)
where
.
The total energy, can be minimized numerically, thereby
leading to the adjustment of , and
and yielding helix parameters that are close
to the experimental ones. Such energy minimization can be used to check the validity of
our earlier approximations (when deriving Eq. 1). Supplementary Fig. S11 shows that
although in the intermediate range of CNaCl there is a difference between the different
curvatures, they are all on the order 1/R in the entire range of CNaCl.
Remark 1. The imposed metric is not continuous, as it 'jumps' between the PG and
the BG stripes. This feature, however, is not crucial for the generation of curvature and
the formation of the helical structure. Even a continuous metric of the form
(( ⁄ )
( ⁄ ) ) (where A takes the role of , and takes the
role of the 'stripes' widths) generates helical structures that are qualitatively similar to
those observed in the experiments. It is only for very thin sheets with that the
qualitative differences between the continuous and discontinuous cases are expected.
Remark 2. In the main text, we used the approximation
, thereby assuming
an approximately constant curvature along the y-direction within each BG stripe. This
assumption is valid as long as the sheets are not much thinner than the stripes width. In
the experiments, we have
, which justifies the approximation (see also Fig. 3c,
main text).
3. Structural characteristics of the helix
Helix parameters, such as the pitch, p, and number of turns, N, are determined by the
value of radius R, the angle , and the sheet length, X as
~cos
2
~
~tan2
R
XN
Rp
(S24)
We note however, that in this expression is not the angle in the as-prepared gel sheet,
and is not the length of the as-prepared sheet. Due to the anisotropic shrinkage of the
entire sheet (the largest contraction normal to the stripes and the smallest contraction
along them), for
2BGPG
0
BG
2
BGPG
2
PGBGPG
BG
3
BGPG
3
PG
~tan1
2cos
~
tan2~
tan
ffXX
EfEfff
EfEf
(S25)
where and are the relative changes in dimensions of PG and BG (Fig. 2a) and
is the initial length of as-prepared sheet.
4. The addition of a spontaneous curvature
When one of the surfaces of the patterned gel sheet is exposed to the ambient atmosphere
and polymerization of PAMPS is quenched, the sheet develops a spontaneous curvature
within the BG regions (Supplementary Fig. S14. This sheet is no longer a NEP and the
bending energy of the BG stripes should be measured, compared to their isotropic
reference curvature, . We therefore replace the term for the bending energy of the
binary stripes in Eq. S23 with
S
BG
S
2
S
BG2
S
3
BGBGBG12
1 yxyxtwE
(S26)
The other terms in Eq. S23 remain unchanged.
Supplementary Methods
1. Characterization of helix morphology
Supplementary Fig. S12a illustrates the structural characteristics of the helix: the length L
of the helix, the pitch p (the distance between the two neighboring turns of the helix), the
number of turns N, and the relation between them. In NaCl solutions, the value of L was
influenced by CNaCl. Thus both N and p were required to characterize helix morphology.
Alternatively, the value of N was determined as N=/360, where is the degree of
rotation (Supplementary Fig. S12b and c) and the pitch was determined as p =L/(/360).
This method enabled the determination of the value of N and p in solutions with a low
CNaCl, in which the gel sheet did not form a full turn.
2. Imaging of the cross-section of the walls of the helical structures
The imaging of the cross-section of walls of the helices in a NaCl solution at CNaCl=1M
was carried out in transmission mode using an optical microscope (Olympus BX51) with
a 2x objective. An optical micrograph was analyzed using Image-Pro Plus 5.0 software
(Media Cybernetics, Inc.). The simultaneous observation of the inner and outer surface
topographies of the helix wall (shown in Fig. 3c) was achieved by moving the focal plane
of the microscope to view the cross-section at the edge of the helix.
3. Formation of helices by subjecting the gel sheet to elevated temperature
Helices were generated using heating-induced differential shrinkage of PG and BG. In
the control experiments, we determined the ratios of dimensions of PG and BG at 45 oC
to those at room temperature to be 0.61 and 0.98, respectively. The PG samples exhibited
a strong contraction in the range of 30-32 oC, the Lower Critical Solution Temperature of
PNIPAm21
. The BG samples did not shrink owing to the strong retention of water by the
large content of PAMPS23
.
Prior to the formation of the helix, a gel sheet comprising PG and BG stripes was
exposed for 5 min to the air, and subsequently, immersed into a large volume of
deionized water at 25 oC. After 2 h incubation, the gel sheet exhibited weak bending to
the direction of the air-exposed surface. The temperature of the water was gradually
increased to 45 oC. After 2 h, the gel sheet transformed into a helix with N=1.2 and
p=4.48 cm (Supplementary Fig. S2).
4. Reversibility of planar-to-helical transitions
The reversibility of the formation of the helical structures was examined by the multiple-
time transfer of the patterned gel between a 1M NaCl solution and deionized water. In
each liquid medium, the gel was incubated for ~2 h to achieve a stable configuration.
Supplementary Fig. S3a-c shows planar-to-helical transitions of the free-floating gel
sheet patterned with BG and PG stripes, following its transfer from a 1M NaCl solution
to deionized water and again, in a 1M NaCl solution, respectively.
Supplementary Fig. S3d shows the number of turns, N, and the pitch, p, of the
helix, which were measured in a series of experiments. In a 1M NaCl solution, the planar
gel sheet formed a compact, highly reproducible helical structure with p=2.97±0.18 mm
and N=1.44±0.06. In deionized water the helix relaxed and exhibited weak bending.
5. Evolution of helixes
Supplementary Fig. S13a illustrates the evolution of the helical structure following the
transfer of the patterned hydrogel sheet from water into a 1M NaCl solution. During the
swelling process, the NaCl ions diffused into the gel matrix, leading the differential
shrinkage of PG and BG stripes and the change in their Young’s moduli. The increase in
the number of turns N and the reduction of pitch p of the helix with incubation time is
illustrated in Supplementary Fig. S13b. After 1 h, the helix reached a stable
configuration. Reversible formation of helices is shown in Supplementary Videos S1 and
S2.
6. Effect of the oblique angle θ on the deformation of the patterned hydrogel sheet
We examined the effect of the angle θ between the long axis of the patterned hydrogel
sample and the stripes of PG and BG on the hydrogel structure in the NaCl solution.
After two-step photopolymerization, the upper glass slide of the reaction cell was
removed, the patterned gel was exposed to the ambient air for 5 min, and subsequently,
immersed into a 1M NaCl solution for 2-3 h. For θ=0o and θ=90
o, the patterned gels
formed a multi-layer roll and an arc, respectively (Supplementary Fig. S4b and d). For
30≤ θ≤60o, the hydrogel sheet transformed into a helix (Fig.1b) with the structural
characteristics presented in Supplementary Table S1.
7. Control of helix handedness
When a patterned gel sheet was transferred without air-exposure into the solution of NaCl,
right- and left-handed helices formed with close-to-equal probability, or even on the two
ends of the same gel sheet (Supplementary Fig. S5).
To achieve control of helix chirality, after photopolymerization of PAMPS, one of
the glass slides from the polymerization cell was removed and the corresponding surface
of the patterned gel was exposed to the ambient atmosphere to quench polymerization by
molecular oxygen24
. In this manner, the concentration of PAMPS was selectively reduced
at the air exposed surface. The importance of this step was evaluated in a series of control
experiments conducted with a patterned gel and with non-patterned PG and BG. A left-
handed helix formed in the solution of NaCl when the top surface of the patterned gel
was exposed to the air (Supplementary Fig. S6a). A right-handed helix was generated
when the bottom surface of the gel was exposed to the air (Supplementary Fig. S6b).
Both types of helices were formed from the gel patterned with the same mask at =45o.
We conclude that the exposure of a particular surface of the gel to the ambient
environment determined the handedness of the helix, since the air-exposed surface of the
patterned gel formed the inner surface of the helix.
To understand the contribution of the PG and BG in the development of gel
curvature, we carried out experiments with the corresponding individual non-patterned
gels. Following the polymerization procedure and a 5 min-long exposure of PG and BG
to the ambient atmosphere, the gel sheets were immersed for 2 h into NaCl solutions with
varying concentrations. The BG sheet bent in the direction of the air-exposed surface,
whereas the PG remained almost planar (Supplementary Fig. S14). The bending of BG
was reversible: the gel acquired a planar shape when incubated in deionized water and
developed an arc-like shape when transferred into a solution of NaCl. The curvature, ,
of the BG sheet was determined as =1/R, where R is the radius of the arch formed by the
gel along the long axis of the sheet.. The value of increased with increasing CNaCl. The
curvature of the BG sheet was significantly smaller than the curvature of the helical
structure formed by the patterned gel sheet in NaCl solutions with similar concentration.
To determine the difference in polymer content at the top (air-exposed) and the
(air-protected) surface of the PAMPS hydrogel film, we conducted Attenuated Total
Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) experiments. We
prepared a film from the monomer mixture (the recipe is given in Supplementary Table
S3) and irradiated it for 70 s with ultraviolet light. The top surface of the film was
exposed to the ambient air for 5 min, the gel was transferred into a large volume of
deionized water for 12 h and subsequently, dried for 15 h at room temperature. In the
control experiment, the hydrogel film was transferred in deionized water without air
exposure.
The bottom and the top surfaces of the films were examined using a Fourier
Transform Infrared spectrometer (Veratex 70, Bruker Inc.). A single beam absorption
spectrum was collected using the spectrum of air as background. Supplementary Fig. S15
shows IR spectra in the spectral window from 850 to 1350 cm-1
, in which the two
dominant bands at 1200 and 1040 cm-1
correspond to the sulfonate groups of the
PAMPS32
. The air-exposed surface showed a lower intensity of both bands than the air-
protected surface. In contrast, the top and the bottom surfaces of the gel film transferred
in deionized water without air-exposure exhibited the same intensity of the two dominant
bands.
We also examined the conditions of photopolymerization of PAMPS at the top
and the bottom surface of the film. During photopolymerization of PAMPS, the
thermocouple-measured temperatures were ~1.2 and ~0.7 oC at the top and the bottom
surfaces, respectively. The intensities of ultraviolet irradiation (measured using a
radiometer (AccuMAX XRP-3000)) were ~8.6 and 8.1 mW/cm2 at the top and bottom of
the gel, respectively. Thus we concluded that the insignificant difference in
polymerization conditions across the gel sheet did not produce the gradient in gel
composition across the film, in agreement with IR characterization of the film surfaces.
Based on the control experiments, we ascribe the different content of PAMPS at
the air-exposed and air-protected surfaces to the inhibition of polymerization by
molecular oxygen at the air-exposed surface24
. Following a 70 s-long photoirradiation,
the polymerization and crosslinking of PAMPS were not complete, and exposure of the
gel surface to the ambient atmosphere quenched polymerization. When the film was
transferred into water or into the NaCl solution, the unreacted monomer diffused from the
film, and the bottom glass-protected gel surface had a higher content of PAMPS. The
bending of the gel sheet originated from the stronger swelling of the polymer-rich
surface, with the curvature increasing with CNaCl (Supplementary Fig. S14).
To examine the effect of the variation in the time of exposure of the patterned gel
to the air, following polymerization, we removed the top cover glass, exposed the gel to
the air for various time intervals, and immersed it overnight into a NaCl solution with a
CNaCl of 1M. Supplementary Table S4 shows that the number of turns and pitch of the
helices did not change with the time of air-exposure of the hydrogel sheet in the time
range from 5 to 20 min.
8. Synthesis of the ternary bistable gel
A hydrogel with dimensions of 75 mm × 50 mm × 0.25 mm was synthesized in an
aqueous medium by copolymerizing N-hydroxyethylacrylamide (HEAm) and NIPAm in
a molar ratio 3:7 (the total monomer concentration was 16 wt%). A photoinitator V-50
and a crosslinking agent MBAA were each used at the concentration of 1 wt%. The
reaction mixture was introduced into the reaction cell comprising two 1 mm-thick glass
slides separated with a 0.25 mm-thick silicone rubber spacer. The cell was exposed for 0 s
to ultraviolet light irradiation (365 nm, Hönle, UV Print at intensity 9 mW/cm2). The
resulting P(HEAm-co-NIPAm) hydrogel (denoted as H1) was washed in deionized water.
A sheet of H1 was swollen for 18 h in an aqueous solution of NIPAm (14 wt%),
MBAA (1 wt%) and V-50 (1wt %), and exposed to ultraviolet light irradiation for 60 s
through a photomask containing black stripes (2 mm wide, 1 mm apart) passing at an
angle =45o with respect to the long axis of the mask. Following photopolymerization of
NIPAm in the light-exposed regions, the sheet contained 2 mm-wide stripes of H1 and 1
mm-wide stripes of the P(HEAm-co-NIPAm)/PNIPAm hydrogel (denoted as H2). The
composite gel sheet was washed with deionized water and swollen for 18 h in a solution
of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (20 wt%), MBA (0.25 wt%) and
V-50 (0.5 wt %). After swelling, the widths of stripes of H1 and H2 became 2.1 and 1.05
mm, respectively. The swollen sheet was exposed to ultraviolet light irradiation for 60 s
through a photomask containing black stripes (2.1 mm wide, 1.05 mm apart) passing at
=45o with respect to the long axis of the mask. The mask was laterally shifted to
polymerize 1.05 mm-wide stripes of the P(HEAm-co-NIPAm)/PAMPS hydrogel
(denoted as H3) in the light-exposed regions of H1. After washing in deionized water, the
resulting gel sheet was composed of the stripes of H1, H2 and H3.
9. Temperature-mediated deswelling of the components of ternary gels
The PNIPAm gel was prepared using photoirradiation of a solution of NIPAm (14 wt%),
MBAA (1 wt%) and V-50 (1 wt%) for 25 s. The P(HEAm-co-NIPAm) gel (H1) was
synthesized by photoirradiating a solution containing 16 wt% of a HEAm:NIPAm
mixture (3:7 molar ratio, respectively), 1 wt % of MBAA and 1 wt% of V-50 for 25 s.
The P(HEAm-co-NIPAm)/PNIPAm gel (H2) was prepared by swelling H1 in an aqueous
solution of NIPAm (14 wt%), MBA (1wt%) and V-50 (1wt%), followed by a 70 s
photoirradiation. All the gels were thoroughly washed in deionized water to remove an
unreacted monomer prior to measurements. A hydrogel composed of P(HEAm-co-
NIPAm)/PAMPS (indicated as H3 in Fig. 5a) did not show any change in the temperature
range from 25 to 80 oC.
Disks of H1, H2 and PNIPAm gel with a diameter of 21 mm and a thickness of 2
mm were cut from the corresponding gel sheets and incubated for 1 h in deionized water
at a particular temperature. The weight of the gel disk was rapidly measured.
Supplementary Fig. S9 shows the variation in the weight of the gel disk at a particular
temperature normalized by the gel weight at 25 oC. The corresponding dehydration
temperatures were 34 oC (PNIPAm), 34 and 53
oC (H1) and 53
oC (H2).
10. Differential swelling of ternary gels triggered at varying pH
A planar sheet of poly(acrylamide-co-butyl methacrylate) gel (PG) was prepared from the
15 wt% mixture of comonomers containing 60 mol% of acrylamide and 40 mol% of
butyl methacrylate, 5 wt% of MBAA and 1 wt% of V-50 in a 1:1 vol. mixture of water
and dimethylformamilde. The solution was placed between the two glass plates separated
by a 0.4 mm-thick silicon spacer and exposed to ultraviolet irradiation for 45 s. The gel
sheet was thoroughly washed with a water/dimethylformamilde mixture and with
deionized water, and subsequently, immersed for 18 h in an aqueous solution containing
22.5 wt% of N-vinyl imidazole, 0.5 wt% of MBAA and 1 wt% of V-50. After 90 s
photopolymerization of N-vinyl imidazole through a photomask ( = 0o), the gel was
washed with deionized water for 24 h, and immersed for 18 h in an aqueous solution
containing 15 wt% methacrylic acid, 0.5 wt% of MBAA and 1 wt% of V-50. Following
incubation, the gel was irradiated for 45 s through a photomask (=45o) to initiate
selective photopolymerization of methacrylic acid. The gel sheet was washed with
deionized water for 24 h. The patterned gel was subsequently cut into 5 cm 1 cm
pieces and immersed in acidic (pH=2.3), close-to-neutral (pH=6.0) and basic (pH=9.5)
solutions. Shape transformations of the gel sheet under these conditions are illustrated in
Fig. 6.
Control experiments were conducted for poly(acrylamide-co-butyl methacrylate)
gel sheets (PG) patterned either with stripes of poly(N-vinyl imidazole), or with stripes
polymethacrylic acid. In the composite gel, these stripes formed the regions of BG. The
compositions of the corresponding reaction mixtures were identical to those described
above.
In the first series of experiments, the primary gel poly(acrylamide-co-butyl
methacrylate) was patterned with N-vinyl imidazole at =0o. Supplementary Fig S16a
and b (left) shows the schematics of the stripes of PG and BG at pH of 2.3 and 9.5,
respectively. At pH=2.3, the regions of BG swelled, leading to the transformation of the
gel sheet into a roll (Supplementary Fig S16a, right). At pH=9.5, the dimensions of the
stripes of PG and BG were similar, and the patterned gel remained flat (Supplementary
Fig S16b, right).
In the second series of experiments, the poly(acrylamide-co-butyl methacrylate)
gels was patterned with stripes of poly(methacrylic acid) at =45o. Supplementary Fig.
S16c shows that the regions of PG and BG had similar dimensions at pH=2.3, resulting in
a planar shape of the gel sheet. Equilibration of the patterned sheet at pH=9.5, caused
expansion of the stripes of BG and the corresponding planar-to-helical transition of the
patterned gel (Supplementary Fig. S16d).
11. Preparation of Supplementary Movies
Hydrogels were immersed in an aqueous environment in a glass Petri dish, which was
placed below a tripod-mounted digital camera (CASIO EX-F1). Lighting was provided
by a LED, with its beam aimed parallel to the bottom of the Petri dish. Time lapse movies
were acquired from a series of images using controller software (EX-F1 Controller) that
was connected to the digital camera through a USB connection. The controller software
was set to record images of the hydrogel sheets at particular time intervals, along with
other parameters including shutter speed, aperture settings, zoom and focusing options.
Generally, time intervals were 10-20 s for the first 5 min after hydorgel transfer to a
particular medium, to capture rapid shape evolution. As the changes in the hydrogel
morphology slowed down, the time interval between image acquisition progressively
increased. Images were arranged into videos using video editing software (Pinnacle
Studio 14).
Supplementary References
30. Efrati, E., Sharon, E. & Kupferman, R. J. Mech. Phys. Solids 57, 762-775 (2009).
31. Sharon E. & Efrati, E. Soft Matter 6, 5693-5704 (2010).
32. Durmaz, S. & Okay, O. Acrylamide/2-acrylamido-2-methylpropane sulfonic acid
sodium salt-based hydrogels: synthesis and characterization. Polymer 41, 3693-
3704 (2000).