Shape-shifting micro- and nanopatterns controlled by temperature

Article abstract of DOI:10.1021/ja304860q

Herein, features that alter their shape to form a different pattern upon an external trigger are described. Electron-beam lithography was used to fabricate micrometer- and nanometer-sized surface immobilized poly(triethylene glycol methacrylate) (pTEGMA) that exhibits significant thermal responsivity; the resulting hydrogels collapsed by up to 95% of their height upon addition of heat. Multicomponent features composed of both the thermoresponsive polymer and nonresponsive poly(ethylene glycol) (PEG) were then prepared. Upon increase in temperature, only the thermally responsive component of the pattern collapsed, causing a significant and predictable alteration in the overall pattern. Reversible micrometer- and nanometer-sized square-to-triangles, squares-to-checkerboards, smiles-to-neutral face, and zeros-to-ones shapes were shown.

Full text of DOI:10.1021/ja304860q

Communication
pubs.acs.org/JACS
Shape-Shifting Micro- and Nanopatterns Controlled by Temperature
Christopher M. Kolodziej and Heather D. Maynard*
Department of Chemistry and Biochemistry and the California NanoSystems Institute, University of California, Los Angeles, 607
Charles E. Young Drive South, Los Angeles, California 90095, United States
*
S Supporting Information
There are several examples of surfaces that hide or reveal
34−37
ABSTRACT: Herein, features that alter their shape to
form a different pattern upon an external trigger are
described. Electron-beam lithography was used to fabricate
micrometer- and nanometer-sized surface immobilized
poly(triethylene glycol methacrylate) (pTEGMA) that
exhibits significant thermal responsivity; the resulting
hydrogels collapsed by up to 95% of their height upon
addition of heat. Multicomponent features composed of
both the thermoresponsive polymer and nonresponsive
poly(ethylene glycol) (PEG) were then prepared. Upon
increase in temperature, only the thermally responsive
component of the pattern collapsed, causing a significant
and predictable alteration in the overall pattern. Reversible
micrometer- and nanometer-sized square-to-triangles,
squares-to-checkerboards, smiles-to-neutral face, and
zeros-to-ones shapes were shown.
words or patterns upon an external trigger.
Herein, for the
first time we describe a straightforward approach that exploits
electron beam (e-beam) lithography to produce patterns that
reversibly change shape upon addition of heat.
We designed a system whereby patterns composed of
contiguous thermally responsive and nonresponsive hydrogels
were produced such that when the temperature is raised, the
responsive component collapses causing a change in the overall
shape. Polymers with side-chain oligoethylene glycol units that
38,39
are thermoresponsive were utilized.
The lower critical
solution temperature (LCST) of these polymers, where in
water the polymers go through a phase transition and become
insoluble and collapse, can be finely tuned by altering the
40
polymer structure. When attached to a surface, the thermal
27,41,42
responsivity is retained.
In this study, micro- and
nanopatterns containing regions of thermally responsive
pTEGMA and regions of nonresponsive star PEG were
produced by e-beam lithography. This fabrication technique
allows for arbitrary pattern formation at both the micrometer
here is tremendous interest in materials that reversibly
43
T
alter their properties depending on the environment. For
example, shape-memory materials respond to a stimulus to
undergo a significant physical deformation to a predetermined
and nanometer scale. E-beam radiation readily causes PEG-
based polymers to cross-link to both silicon surfaces and
4
3,44
polymer films,
forming hydrogels via a radical mecha-
1
−6
45
shape. Adaptive hydrogels, often inspired by nature, undergo
nism. The resulting hydrogels are stable on surfaces for long
significant mechanical changes to alter viscosity or modulus in
time periods, and thermal and photoresponsive patterns have
7
,8
46,47
the presence of a trigger. Actuating materials exhibit large
displacements in structure, typically involving large forces,
been prepared by e-beam lithography.
Importantly for this
study, although it is a serial technique, the alignment capability
for e-beam lithography is high, allowing two different polymers
to be patterned touching each other.
Thermal actuation of pTEGMA hydrogels prepared by e-
beam lithography was confirmed on micro- and nanopatterned
surfaces prior to preparing multicomponent patterns (Figure
9
−11
induced by an environmental change.
term used to describe materials that fold into three-dimensional
Microorigami is a
1
2−16
objects upon exposure to a stimulus.
Often these smart/
intelligent systems are reversible, and stimuli include heat, light,
9
,10,17−19
electricity, magnet, or binding of a (bio)molecule.
1
). pTEGMA was synthesized by conventional radical polymer-
These hydrogel materials are candidates for a myriad of exciting
applications including sensors, diagnostics, coatings, miniature
devices, lab-on-a-chip designs, drug delivery vehicles, and
ization (number-average molecular weight (M ) of 22 000 Da;
n
polydispersity index (PDI) of 2.10; LCST of 52 °C, see
Supporting Information (SI) for synthesis, characterization, and
confirmation of thermoresponsivity of bulk hydrogels). Dilute
solutions of the polymer were then spin-coated onto silicon
wafers and patterned by e-beam lithography into arrays of 5 μm
circles (Figure 1 a,b). The micropatterned substrates were
mounted on top of a Peltier heater and imaged by atomic force
microscopy (AFM) in water at both 22 and 65 °C (Figure
2
0,21
nanomachines.
With the advent of micro- and nanotechnology fabrication
2
2
approaches, patterned switchable surfaces are possible.
Responsive micropatterns have been prepared, and some
23−26
interesting shapes have been made.
Switchable nanostruc-
tures have been fabricated that either actuate or deform
directionally to produce interesting patterns and to control fluid
1
1,27−30
1
6
b,c). The hydrogels had an average maximum height of 553 ±
4 nm at 22 °C (Figure 1d,g) and collapsed when heated to 65
flow.
Features that respond to a variety of different
stimuli to reversibly swell have been exploited for applications
°
C to an average maximum height of 29 ± 6 nm (Figure 1e,h).
as diverse as microfluidic valves and surfaces that manipulate
2
3,26,31−33
This represents a 95 ± 1% reduction in swelling. Upon cooling
and probe cells.
There have been exciting examples of
patterns that actuate in a certain direction or materials that fold
1
1−16,27−30
or close.
Yet producing arbitrary shapes that
Received: May 18, 2012
Published: July 17, 2012
reversibly change pattern remains a significant challenge.
©
2012 American Chemical Society
12386
dx.doi.org/10.1021/ja304860q | J. Am. Chem. Soc. 2012, 134, 12386−12389

Journal of the American Chemical Society
Communication
Figure 1. Patterning and actuation of pTEGMA hydrogels. pTEGMA
film that was spin-coated on a silicon wafer is patterned by e-beam
lithography (a). The uncross-linked polymer is rinsed away to reveal
the discrete hydrogels (b). Upon heating to 65 °C, the hydrogels expel
water and actuate vertically (c). AFM images of pTEGMA
micropatterned hydrogels at 22 °C (d,g) and at 65 °C (e,h) show a
dramatic decrease in height as the pTEGMA actuates. Following
cooling to 22 °C (f,i), the pTEGMA microgels regain their initial
height. Features 5 μm in diameter.
Figure 2. Micropatterned shape-shifting pTEGMA/eight-arm PEG
patterns. AFM imaging of square-to-triangle pTEGMA/PEG micro-
patterns show square hydrogels at 22 °C (a,c). Upon heating to 65 °C
the pTEGMA regions collapse revealing triangular 8-arm PEG
hydrogels (b,d). Following cooling back to 22 °C the pTEGMA
regions reswell, and the hydrogels again become squares (a,e). AFM
imaging of block-to-checkerboard pTEGMA/8-arm PEG micro-
patterns shows a single hydrogel block at 22 °C (f,h). Upon heating
to 65 °C the pTEGMA regions collapse revealing a checkerboard
pattern of 8-arm PEG hydrogels (g,i). Following cooling back to 22 °C
the pTEGMA regions reswell, and the hydrogel again becomes a
continuous block (f,j). Scale bars = 5 μm.
to 22 °C in water, the pTEGMA hydrogels regained their initial
height, with an average maximum height of 559 ± 64 nm
(
Figure 1f,i). This demonstrates that the phase change is
reversible. Likewise, nanopatterns were thermally responsive
and collapsed by 79 ± 5% (see SI, Figure S8). The overall
percent change is less than for the microgels; however, this is
likely due to limitations in the accuracy of the height
measurement of the small features. Micron-sized hydrogels of
the nonresponsive eight-arm PEG polymer hydrogels were also
fabricated. Upon increase in temperature, the height change
was approximately 20% ± 14% (see SI, Figure S7). This value is
significantly less than the pTEGMA gels and, with the error of
the measurement, not statistically significant. Thus, the eight-
arm PEG could be utilized as a relatively nonresponsive
component.
then patterned in the pTEGMA film around each 8-arm PEG
feature to produce a contiguous square-shaped feature (Figure
2a). AFM images of the pTEGMA/8-arm PEG micropatterns
in water at 22 °C showed uniform square features (Figure 2c).
Upon heating to 65 °C, the pTEGMA portions of the patterns
expelled water and collapsed to 21 ± 1% of their initial height
(from 162 ± 14 to 34 ± 5 nm), while the 8-arm PEG portions
remained hydrated to a greater degree, at 73 ± 4% of their
initial height (from 236 ± 22 to 172 ± 17 nm), thus appearing
as triangles (Figure 2b,d). When the patterns were again cooled
to 22 °C, the pTEGMA portions rehydrated, regaining their
initial heights (165 ± 11 nm) and returning the shapes of the
features to squares (Figure 2e). In a similar fashion, squares
that changed to checkerboards were fabricated (Figure 2f−j),
and the height changes upon heating are summarized in the SI.
The heating and cooling cycles were repeated for each of the
above, and it was found that after the second and third cycles,
respectively, the thermally responsive polymer lost approx-
imately 20% of its original height (see SI, Figure S9). The
origin of this is not known and may be due to hydrolysis of the
polymer side chains. Still the features are fully reversible for at
least one cycle.
Micropatterns that contained both eight-arm PEG and
pTEGMA were made in order to investigate changing the
shape of a surface feature in a predictable manner with
temperature. Two different sets of shapes were investigated:
square features that change to triangles and a solid block that
changes to a checkerboard pattern as the temperature is
increased (Figure 2). The square-to-triangle multicomponent
patterns were created by first spin-coating silicon wafers
containing prefabricated gold electrodes for use as alignment
marks with a 0.38% w/v solution of 8-arm PEG. This
concentration was determined experimentally to produce a
film of the same height as the 1% w/v solution of pTEGMA
(
see SI, Figure S6). The final relative heights of the surface
features typically do not correspond exactly to relative original
film heights, likely due to differences in polymer structure and
required dosage to cross-link; yet, uniform relative film heights
do result in more uniform multicomponent features. E-beam
lithography was then used to write 5 × 5 μm isosceles triangles
composed of 8-arm PEG. The substrates were rinsed to remove
the uncross-linked 8-arm PEG, and then spin-coated with a 1%
w/v solution of pTEGMA. Two 2.5 × 5 μm right triangles were
48
E-beam lithography is capable of high resolution, and
multicomponent nanoscale features are readily accessed. To
test this, actuating smiley faces and nanopatterns that convert
between zeroes and ones were fabricated (Figure 3). First, four
1
2387
dx.doi.org/10.1021/ja304860q | J. Am. Chem. Soc. 2012, 134, 12386−12389

Journal of the American Chemical Society
Communication
when initially patterned. This may be due to a more loosely
cross-linked structure following partial hydrolysis of TEG side
chains during the heating/cooling cycle or removal of uncross-
linked polymer during the first cycle.
Collectively, the results demonstrate that micrometer- and
nanometer-sized shape-shifting patterns can be achieved.
Micron-sized squares-to-triangles and squares-to-checkerboards
and nanosized smiles-to-neutral faces and zeros-to-ones were
demonstrated. Yet, e-beam lithography forms arbitrary shapes,
and this technique should be readily translated to a large
number of different forms. Thus, many different applications,
such as encryption and miniature substrates for spatial and
temporal cell manipulation, can be envisioned. The latter
should be possible, particularly since the LCST of the patterned
polymer is readily altered in a predictable manner to
40
physiologically relevant temperatures. The features were
fabricated of thermally responsive and nonresponsive PEG
polymers by sequentially exposing films to e-beam radiation.
Upon increase in temperature, only the pTEGMA portions of
the features collapsed. This caused a predictable and reversible
change in overall pattern in both micrometer- and nanometer-
sized features. Because e-beam lithography can be used to
43
cross-link many different polymers onto the same surface,
features containing regions with different LCSTs should be
possible. This could lead to shapes that change several times
and morph as the temperature is increased. Patterns that
respond to a variety of different stimuli, such as pH or light,
should also be possible.
Figure 3. Nanopatterned shape-shifting pTEGMA/8-arm PEG
hydrogels. AFM imaging of a smiley-face pTEGMA/8-arm PEG
nanopattern shows the smiley-face at 22 °C (a,c). Upon heating to 65
°
C the pTEGMA regions collapse removing the “smile” (b,d).
Following cooling back to 22 °C the pTEGMA regions reswell, and
the hydrogels again form a smiley-face (a,e). AFM imaging zero-to-one
pTEGMA/8-arm PEG nanopatterns show hydrogel zeroes at 22 °C
ASSOCIATED CONTENT
■
*
S
Supporting Information
1
(
f,h). Upon heating to 65 °C the pTEGMA regions collapse revealing
-arm PEG hydrogels in the shape of ones (g,i). Following cooling
Experimental procedures, H NMR spectra, bulk hydrogel
characterization, nanoactuation, thermal cycling. This material
is available free of charge via the Internet at http://pubs.acs.org.
8
back to 22 °C the pTEGMA regions reswell, and the hydrogels again
become zeroes (f,j). Scale bars =1 μm.
AUTHOR INFORMATION
Corresponding Author
maynard@chem.ucla.edu
■
lines composed of eight-arm PEG were written on silicon
wafers in the shape of a face. pTEGMA lines were then added
to form a smiley face (Figure 3a). AFM images of the
nanopatterned hydrogels at 22 °C in water showed the pattern
Notes
The authors declare no competing financial interest.
(
Figure 3c). Upon heating to 65 °C, the pTEGMA portions of
the pattern collapsed to 48 ± 5% of their initial height (from 42
2 to 20 ± 2 nm), while the 8-arm PEG portions remained
hydrated to a greater degree, at 87 ± 3% of their initial height
ACKNOWLEDGMENTS
±
■
Funding was provided by the National Science Foundation
(CAREER CHE-0645793). The authors thank Adam Stieg for
helpful discussions on AFM measurements. We acknowledge
the use of the Scanning Probe Microscopy facility at the Nano
and Pico Characterization Laboratory at the California
NanoSystems Institute.
(
3
3
from 62 ± 2 to 54 ± 3 nm), removing the “smile” (Figure
b,d). Cooling back to 22 °C restored the smiley face (Figure
e). The pTEGMA portions rehydrated to their initial height
(
40 ± 2 nm). Multicomponent nanopatterns that convert
between representations of zeroes and ones were also created.
First, lines composed of eight-arm PEG were written on silicon
wafers followed by three pTEGMA lines to complete the shape
of the zero (Figure 3f). AFM images of the pTEGMA/8-arm
PEG in water at 22 °C showed an array of zeroes (Figure 3h).
When heated to 65 °C, the pTEGMA portions of the pattern
collapsed to 48 ± 5% of their initial height (from 56 ± 1 to 27
REFERENCES
■
(1) Leng, J.; Lan, X.; Liu, Y.; Du, S. Prog. Mater. Sci. 2011, 56, 1077−
1135.
(2) Behl, M.; Razzaq, M. Y.; Lendlein, A. Adv. Mater. 2010, 22,
3
(
(
(
4
388−3410.
3) Osada, Y.; Matsuda, A. Nature 1995, 376, 219−219.
4) Lendlein, A.; Langer, R. Science 2002, 296, 1673−1676.
5) Lendlein, A.; Jiang, H. Y.; Junger, O.; Langer, R. Nature 2005,
±
2 nm), while the 8-arm PEG portions remained hydrated to a
greater degree, at 81 ± 3% of their initial height (from 82 ± 2
to 66 ± 2 nm), displaying an array of ones (Figure 3g,i). Upon
cooling back to 22 °C, the pTEGMA portions of the
nanopatterns rehydrated and swelled to 60 ± 1 nm, returning
the features to the shapes of zeroes (Figure 3j). Interestingly,
after the full heating and cooling cycle, the height of the
pTEGMA portions of the hydrogel was slightly greater than
34, 879−882.
(
6) Kainuma, R.; Imano, Y.; Ito, W.; Sutou, Y.; Morito, H.; Okamoto,
S.; Kitakami, O.; Oikawa, K.; Fujita, A.; Kanomata, T.; Ishida, K.
Nature 2006, 439, 957−960.
(7) Shanmuganathan, K.; Capadona, J. R.; Rowan, S. J.; Weder, C.
Prog. Polym. Sci. 2010, 35, 212−222.
1
2388
dx.doi.org/10.1021/ja304860q | J. Am. Chem. Soc. 2012, 134, 12386−12389

Journal of the American Chemical Society
Communication
(
8) Capadona, J. R.; Shanmuganathan, K.; Tyler, D. J.; Rowan, S. J.;
Weder, C. Science 2008, 319, 1370−1374.
9) Alarcon, C. d. l. H.; Pennadam, S.; Alexander, C. Chem. Soc. Rev.
005, 34, 276−285.
10) Ahn, S. K.; Kasi, R. M.; Kim, S. C.; Sharma, N.; Zhou, Y. X. Soft
Matter 2008, 4, 1151−1157.
11) Sidorenko, A.; Krupenkin, T.; Taylor, A.; Fratzl, P.; Aizenberg, J.
(44) Krsko, P.; Sukhishvili, S.; Mansfield, M.; Clancy, R.; Libera, M.
Langmuir 2003, 19, 5618−5625.
(
2
(
(45) Sofia, S. J.; Merrill, E. W. J. Biomed. Mater. Res. 1998, 40, 153−
163.
(46) Tirumala, V. R.; Divan, R.; Ocola, L. E.; Mancini, D. C. J. Vac.
Sci. Technol., B 2005, 23, 3124−3128.
(
(47) Saaem, I.; Tian, J. Adv. Mater. 2007, 19, 4268−4271.
(
80.
48) Kolodziej, C. M.; Maynard, H. D. Chem. Mater. 2012, 24, 774−
Science 2007, 315, 487−490.
7
(
(
(
12) Ionov, L. Soft Matter 2011, 7, 6786−6791.
13) Rothemund, P. W. K. Nature 2006, 440, 297−302.
14) Leong, T. G.; Randall, C. L.; Benson, B. R.; Zarafshar, A. M.;
Gracias, D. H. Lab Chip 2008, 8, 1621−1624.
15) Randhawa, J. S.; Keung, M. D.; Tyagi, P.; Gracias, D. H. Adv.
Mater. 2010, 22, 407−410.
16) Stoychev, G.; Puretskiy, N.; Ionov, L. Soft Matter 2011, 7,
(
(
3
(
(
277−3279.
17) Mendes, P. M. Chem. Soc. Rev. 2008, 37, 2512−2529.
18) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.;
Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299,
71−374.
19) Ehrick, J. D.; Deo, S. K.; Browning, T. W.; Bachas, L. G.;
3
(
Madou, M. J.; Daunert, S. Nat. Mater. 2005, 4, 298−302.
(
(
20) Bath, J.; Turberfield, A. J. Nat. Nanotechnol. 2007, 2, 275−284.
21) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Mueller, M.; Ober,
C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban,
M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010,
9
, 101−113.
22) Hook, A. L.; Voelcker, N. H.; Thissen, H. Acta Biomater. 2009,
, 2350−2370.
23) Beebe, D. J.; Moore, J. S.; Bauer, J. M.; Yu, Q.; Liu, R. H.;
Devadoss, C.; Jo, B. H. Nature 2000, 404, 588−590.
24) Hyun, J.; Lee, W. K.; Nath, N.; Chilkoti, A.; Zauscher, S. J. Am.
Chem. Soc. 2004, 126, 7330−7335.
25) Plunkett, K. N.; Zhu, X.; Moore, J. S.; Leckband, D. E. Langmuir
006, 22, 4259−4266.
26) Kaehr, B.; Shear, J. B. Proc. Natl. Acad. Sci. U.S.A. 2008, 105,
850−8854.
27) Jonas, A. M.; Hu, Z.; Glinel, K.; Huck, W. T. S. Nano Lett. 2008,
, 3819−3824.
28) Pokroy, B.; Epstein, A. K.; Persson-Gulda, M. C. M.; Aizenberg,
J. Adv. Mater. 2009, 21, 463−469.
29) Pokroy, B.; Kang, S. H.; Mahadevan, L.; Aizenberg, J. Science
009, 323, 237−240.
30) Shields, A. R.; Fiser, B. L.; Evans, B. A.; Falvo, M. R.; Washburn,
S.; Superfine, R. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 15670−15675.
31) Yu, C.; Mutlu, S.; Selvaganapathy, P.; Mastrangelo, C. H.; Svec,
(
5
(
(
(
2
(
8
(
8
(
(
2
(
(
F.; Frechett, J. M. J. Anal. Chem. 2003, 75, 1958−1961.
(32) Pelah, A.; Jovin, T. M. ChemPhysChem 2007, 8, 1757−1760.
(33) Na, K.; Jung, J.; Kim, O.; Lee, J.; Lee, T. G.; Park, Y. H.; Hyun, J.
Langmuir 2008, 24, 4917−4923.
34) Hu, Z. B.; Chen, Y. Y.; Wang, C. J.; Zheng, Y. D.; Li, Y. Nature
998, 393, 149−152.
35) Ionov, L.; Minko, S.; Stamm, M.; Gohy, J. F.; Jerome, R.; Scholl,
(
1
(
A. J. Am. Chem. Soc. 2003, 125, 8302−8306.
(36) Kim, H. S.; Crosby, A. J. Adv. Mater. 2011, 23, 4188−4192.
(37) Kim, J.; Yoon, J.; Hayward, R. C. Nat. Mater. 2010, 9, 159−164.
(38) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36,
8
(
312−8319.
39) Ishizone, T.; Seki, A.; Hagiwara, M.; Han, S.; Yokoyama, H.;
Oyane, A.; Deffieux, A.; Carlotti, S. Macromolecules 2008, 41, 2963−
967.
2
(
(
40) Lutz, J. F.; Hoth, A. Macromolecules 2006, 39, 893−896.
41) Jonas, A. M.; Glinel, K.; Oren, R.; Nysten, B.; Huck, W. T. S.
Macromolecules 2007, 40, 4403−4405.
42) Laloyaux, X.; Fautre, E.; Blin, T.; Purohit, V.; Leprince, J.;
Jouenne, T.; Jonas, A. M.; Glinel, K. Adv. Mater. 2010, 22, 5024−5028.
43) Christman, K. L.; Schopf, E.; Broyer, R. M.; Li, R. C.; Chen, Y.;
Maynard, H. D. J. Am. Chem. Soc. 2009, 131, 521−527.
(
́
(
1
2389
dx.doi.org/10.1021/ja304860q | J. Am. Chem. Soc. 2012, 134, 12386−12389