Smart hydrogels co-switched by hydrogen bonds and π-π stacking for continuously regulated controlledrelease system

Article abstract of DOI:10.1002/adfm.200901245

A series of hydrogels with continuously regulatable release behavior can be achieved by incorporating hydrogen bonding and π-π stacking co-switches in polymers. A poly(nitrophenyl methacrylate-co-methacrylic acid) hydrogel (NPMAAHC) for control over drug

Full text of DOI:10.1002/adfm.200901245

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Smart Hydrogels Co-switched by Hydrogen Bonds and
p–p Stacking for Continuously Regulated Controlled-
Release System
By Fang Li, Yingchun Zhu,* Bo You,* Donghui Zhao, Qichao Ruan, Yi Zeng,
and Chuanxian Ding
is generally based on non-covalent dynamic
bonding interactions, e.g., hydrogen bond-
ing,[1–3] hydrophobic,[8] p–p stacking,[9,10]
and electrostatic interactions,[11,14] which
respond to stimuli such as pH,[2,7] tem-
perature,[3,7] ultraviolet[6] or visible lights,[8]
chemical substances[1] or electric field,[11,14]
respectively. In this work, we incorporate
both hydrogen-bonding and p–p stacking
interactions in polymer hydrogels to
achieve a dual switched on–off control over
drug release for oral administration.
Oral administration is the most conve-
nient route of chemotherapy.[15,16] Drugs
for oral administration need protection
against the hostile acidic environment
and high concentrations of proteolytic and
metabolytic enzymes in the stomach, and
their release needs to be controlled to fit the
pharmacokinetics. Poly(methacrylic acid)
(PMAA) as a well known controlled release
hydrogel works on the basis of hydrogen
bonding between carboxylic acid groups
(COOH) in the crosslinking network. The
A series of hydrogels with continuously regulatable release behavior can be
achieved by incorporating hydrogen bonding and p–p stacking co-switches in
polymers. A poly(nitrophenyl methacrylate-co-methacrylic acid) hydrogel
(NPMAAHG) for control over drug release is fabricated by copolymerizing 4-
nitrophenyl methacrylate and methacrylic acid using ethylene glycol
dimethacrylate as a crosslinker. The carboxylic acid groups and nitrylphenyl
groups form hydrogen bonds and p–p stacking interactions, respectively,
which act as switches to control the release of guest molecules from the
polymers. As revealed by the simulated gastrointestinal tract drug release
experiments, the as-synthesized NPMAAHG hydrogels can be regulated to
release only 4.7% of drugs after 3 h in a simulated stomach and nearly 92.6%
within 43 h in the whole digestive tract. The relation between the release
kinetics and structures and the mechanism of the smart release control are
analyzed in terms of diffusion exponent, swelling interface number, drug
diffusion coefficient, and velocity of the swelling interface in detail. The results
reveal that the release of guest molecules from the hydrogels can be
continuously regulated for systemic administration by controlling the ratio of
the hydrophilic hydrogen bonds and the hydrophobic p–p stacking switches.
hydrophilic nature of PMAA limits the ability to effectively load
hydrophobic and large macromolecular drugs.[17–19] The introduc-
tion of hydrophobic units can increase the load of hydrophobic
drugs and reduce the toxic burst release effects.[20–24] Currently
used hydrophobic units are usually long-chain alkyl groups,
hydrophobic esters, or cyclodextrins which form hydrophobic
domains by van der Waals forces with a lower interaction energy.
Incorporation of hydrophobic segments overcomes the issue of
loading hydrophobic drugs, but it is still a problem to have a long
sustained controlled drug release that corresponds to the transit
time in the gastrointestinal tract (3–4 h in stomach and 36–48 h for
whole digestive tract).[25,26]
1. Introduction
Hydrogels have attracted great attention because of their smart
response to specific changes in environmental conditions.[1–10]
Controlled-release drug delivery is one of the promising
applications of smart gels.[4,11–14] The smart behavior of hydrogels
[*] Prof. Y. Zhu, Dr. F. Li, Dr. D. Zhao, Dr. Q. Ruan, Prof. Y. Zeng,
Prof. C. Ding
Key Laboratory of Inorganic Coating Materials,
Shanghai Institute of Ceramics
Chinese Academy of Sciences
Shanghai 200050 (P. R. China)
E-mail: yzhu@mail.sic.ac.cn
The p–p stacking interaction is a strong physical interaction
between aromatic groups within
[9,10,23]
a distance of 3.645–
The p–p stacking interactions formed by
˚
3.928 A.
Prof. B. You
N-(fluorenyl-9-methoxycarbonyl)-amino (Fmoc-amino) acids can
cooperate with short peptide derivatives to form a fibrous hydrogel
for application in biological sensing.[27] Other substances such as
phenylalanine (Phe)[28] and modified N-fluorenylmethoxycarbonyl-
amino acids[10,29] are used for cell culture and tissue engineering
applications. The key role that aromatic interactions play the
Department of Materials Science and Advanced
Coating Research Center of China, Educational Ministry
Fudan University
Shanghai 200433 (P. R. China)
E-mail: boyou2005@yahoo.com.cn
DOI: 10.1002/adfm.200901245
Adv. Funct. Mater. 2010, 20, 669–676
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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biological field lie in their free energy of
formation of an ordered and directional
structure as well as good biocompatibility.
Herein, we pioneer a strategy of incorporating
p–p stacking interactions in a PMAA hydrogel
system to achieve a co-switched controlled
release of drugs that is consistent with
the transit time in the gastrointestinal tract.
The stronger physical interactions between the
aromatic groups performed as p–p stacking
switches retain the release of drugs and
consequently change the drug release kinetics.
The hydrophobic nature of the p–p stacking
interactions can provide
a hydrophobic
domain for storage of hydrophobic drugs.
The p–p stacking interactions are introduced
using 4-nitrophenyl methacrylate (NPMA),
which is synthesized by reacting acyl chloride
with 4-nitrophenol. Then NPMA is copolymer-
ized with methacrylic acid (MAA) using
ethylene glycol dimethacrylate (EGDMA) as
crosslinker to prepare a poly-(nitrophenyl
methacrylate-co-mathacrylate acid) hydrogel
(NPMAAHG). Simulated gastrointestinal tract
drug release experiments are carried out to
investigate the drug delivery property of
NPMAAHG by using ibuprofen (IBU) as a
model drug. Scanning electron microscopy
(SEM), UV-vis spectroscopy, and X-ray diffrac-
tion (XRD) are used to investigate the p–p
stacking interactions and the crystallographic
Figure 1. Schematic illustration of the preparation, structure, and co-switched drug release
mechanism of the NPMAAHG hydrogels. a) Synthesis of the functional comonomer NPMA.
b) Copolymerization of NPMA with MAA to obtain the NPMAAHG hydrogel by using EGDMA as a
crosslinker. The carboxylic acid groups and NPMA segments form hydrogen bonding and p–p
stacking interactions, respectively, which act as switches to control the release of guest molecules.
c) Drugs are closed in NPMAAHG at low pH (1.4); hydrogen bonding and p–p stacking
interactions perform as off-status switches to retard the release of drugs. d) Drugs partially
release from the NPMAAHG networks owing to the opening of the hydrogen bond switch in basic
solution; the p–p stacking switch still remains. e) Drugs further release owing to the opening of
the p–p stacking switch caused by the swelling of the hydrogels.
nature of the as-synthesized hydrogels. In addition, the pH
sensitivity and drug release kinetics properties of the as-prepared
hydrogels are also examined.
molecules effectively. In the high pH (7.4) solution, the ionization
of the carboxylic acid groups leads to the invalidity of the hydrogen
bondswitch, butthep–pstacking switchstillremains. Thus, drugs
are partially released at the initial stage (Fig. 1d). The ongoing swell
of the hydrogel disables the p–p stacking interactions, and drugs
are further released (Fig. 1e). In the low pH (1.4) solution, the
hydrogen bonds and p–p stacking interactions act as physical
crosslinks to resist the release of guest molecules from their
network(Fig. 1c). Thepolymerization iscarriedoutinordinarytest
tubes under protection of N2 for about 24 h at 60 8C. After the
productissoakedinawater/acetone(50:50 v/v)solventmixturefor
one week, a colorless and transparent hydrogel of NPMAAHG is
obtained (Fig. 2). By changing the molar ratios of NPMA and MAA
from 1:2, to 1:4, to 1:8, NPMAAHGs are synthesized and noted as
NPMAAHG-2, NPMAAHG-4, and NPMAAHG-8, respectively.
2. Results and Discussion
2.1. Design and Preparation of Functional Hydrogels
To achieve a long sustained control over drug release suitable for
systematic administration, hydrophobic aromatic groups with
strong interaction forces are incorporated into the EGDMA
crosslinked network of a PMAA hydrogel. Figure 1 displays the
synthetic route used to prepare the targetted hydrogels. First, a
functionalized comonomer NPMA is synthesized by modifying 4-
nitrophenol with methacryloyl chloride (Fig. 1a). The functiona-
lizedmonomeristhencopolymerizedwithMAAusingEGDMAas
a crosslinker to prepare a NPMAAHG hydrogel (Fig. 1b). The
carboxylic acid groups and NPMA segments in NPMAAHG form
hydrogen bond and p–p stacking interactions, respectively, which
act as switches to control the release of guest molecules (Fig. 1c).
p–p stacking interactions between NPMA segments cover an
2.2. Drug Release Behavior of NPMAAHG Hydrogels
Figure 3 displays the simulated gastrointestinal release of IBU
from NPMAAHG-4, which is synthesized with a NPMA-to-MAA
molar ratio of 1:4. The drug storage of the hydrogels is 33.45%
(386.87 mg of ibuprofen per gram of NPMAAHG-4) loaded in
40 mg mLꢂ1 IBU solution. Drug-loaded NPMAAHG-4 is first
placed in simulated gastric fluid (SGF) (pH 1.4) for 3 h, the transit
time of substances in the stomach. It is then transferred into
simulated intestinal fluid (SIF) (pH 7.4) until all drugs are released
˚
effective area of 7.74 ꢀ 3.96 A (Fig. S1 in the Supporting
Information). Crosslinked by EDGMA, the distance between
˚
two neighboring alkyl chains is confined to ꢁ10.79 A. Considering
the size of ibuprofen molecules as 10.29 ꢀ 5.24 A,[30] the structure
˚
of NPMAAHG ensures the storage and maintenance of drug
670
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Figure 4. The in vitro drug release of IBU-loaded NPMAAHG hydrogels.
a) NPMAAHG-8 in pH 7.4 buffer solution. b) NPMAAHG-4 in pH 7.4 buffer
solution. c) NPMAAHG-2 in pH 7.4 buffer solution.
in SIF (pH 7.4). It takes approx. 13, 29, and 39 h to achieve 90%
drugreleaseforNPMAAHG-8, NPMAAHG-4, andNPMAAHG-2,
respectively. The release ratedecreases with theincreasing content
of the NPMA segment. That is to say, the release rate is slowed
down by increasing the p–p stacking interactions in NPMAAHG.
It usually takes 2–14 h to achieve 80% drug release for traditional
hydrophobic-modified acrylic acid and methacrylic acid hydro-
gels.[21–24] Remarkably,thehydrogenbondingandp–pstackingco-
switch in the network of the hydrogel successfully prolongs the
release time for oral drug administration and make continuously
regulated drug release possible.
Figure 2. Image of the NPMAAHG hydrogels. The hydrogel in the bottom
of the bottle (note bottle is inverted) shows a transparent colorless nature.
2.3. Structural Characterization of NPMAAHG Hydrogels
To clearly understand the unique release behavior of the hydrogel,
the structure and p–p stacking interactions and swelling behavior
of the hydrogels are studied in detail. 1H NMR spectra of the as-
synthesized comonomer NPMA is displayed with the structure of
the target molecule in Figure 5a. The chemical shift at 2.12 ppm
correspondstotheprotonsofalkylgroupsandthepeaksat5.85and
6.34 ppm are owing to the protons on the vinyl group. Signals are
found with a higher chemical shift because of the electron-
withdrawing nitrylgrouponthephenyl ring. In addition, thepeaks
at7.27and8.28 ppmarerelatedtotheprotonsonthephenylgroup.
FTIR spectroscopy analyses of NPMA and NPMAAHG-4 hydro-
gels are presented in Figure 5b. The peaks at 1530.6 and
1347.1 cmꢂ1 in the bottom curve of Figure 5b correspond to
the NꢂO asymmetrical stretching vibration of the nitryl group.
Thepeakaround1740.7 cmꢂ1 istheC¼Ostretchingvibration, and
the strong absorption bands at 1600, 864.9, and 744.5 cmꢂ1 are a
result of the CꢂH out-of-plane motion of the phenyl ring.
Figure 3. Release of IBU from the NPMAAHG-4 hydrogel system by
simulating the gastrointestinal tract. After 3 h incubation in SGF at pH
1.4, the drug-loaded NPMAAHG-4 hydrogel is transferred to SIF at pH 7.4
until all drugs are released. Inset: drug release in SGF at pH 1.4.
from the system at 37 8C. Only 4.7% of the IBU molecules are
releasedduringthefirst3 hinSGF, andthenthedrugsarereleased
increasingly after transferring the hydrogel from the SGF to the
SIFsolution. It takes 43 h in the SIFsolution to achieve 92.6% drug
release and 48 h to achieve 98% drug release, which accords well
with the transit time through the gastrointestinal tract. The slow
drug release in the SGF (pH 1.4) is presented as an insert in
Figure 3. Therefore, by introducing functional NPMA segments,
we have a successful control over the drug delivery during the
transit through the gastrointestinal tract.
1
Combining the H NMR spectra, we can see that the functional
NPMA comonomer has been successfully synthesized. The
presence of OꢂH stretching at 3421.6 cmꢂ1 in the top curve of
Figure 5b clearly indicates the intercalation of MAA segments into
the network of hydrogels. All the typical absorption of NPMA
segments are weakened after being incorporated in PMAA
hydrogels. The cleavage peaks at 1141.8 and 1102.4 cmꢂ1 are
attributed to the CꢂO stretching mode of the ester groups. The
The drug release rate from NPMAAHG can be regulated by
varying the ratio of the hydrogen bonds to p–p stacking
interactions. Figure 4 presents the in vitro drug release curves
of IBU-loaded NPMAAHG-2, NPMAAHG-4, and NPMAAHG-8
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Figure 5. a) 1H NMR spectra of NPMA and b) FTIR spectra of NPMA (bottom curve) and NPMAAHG-4 hydrogels (top curve). Peaks corresponding to the
typical groups are singled out for further identification.
copolymerization of the monomers is revealed by the disappear-
ance of the absorption peak at 3116.7 cmꢂ1 owing to the CꢂH
stretching vibration on unsaturated ethylenic bonds and the
appearance of the CꢂH vibration absorption on an alkyl group
around2930 cmꢂ1. NPMAsegmentshadbeenintroducedintothe
hydrogelsystemofNPMAAHGasshownbytheweakenedpeaksat
1530.6 and 1347.1 cmꢂ1 owing to the NꢂO asymmetrical
stretching vibration of nitryl.
The lamella-like surface morphology of the as-prepared
NPMAAHG-4 can be seen from the scanning electron microscopy
(SEM) image (Fig. 6a). The formation of this unique surface
structure may be attributable to the freeze drying process. When
water is extracted from the hydrogels during the freeze drying
process, the hydrogen bonding and p–p stacking interactions
between the MAA and NPMA segments are formed, which lead to
an ordered interconnected 3D network similar to peptide–peptide
interactions as shown in Figure S1b.[31–34] X-ray diffraction (XRD)
reveals that the p–p stacking interactions between NPMA
segments has been preserved after swelling in the solution of
pH 1.4.[36] The characteristic absorption of band IIIin nitrylphenyl
groups is observed at l ¼ 280 nm after the hydrogel is soaked in a
solution of pH 7.4 for 3 days, which means that the nitrylphenyl
groups are in an isolated state. In addition, the absorption peak of
the hydrogel is at 295 nm after soaking in a pH 7.4 solution for 7 h,
which is red-shifted by 15 nm in comparison to that of the isolated
nitrylphenyl groups (l ¼ 280 nm), which indicates that the p–p
stacking interactions between nitrylphenyl groups are partially
dissociated. It is reasonable to conclude that the p–p stacking
interactions help to preserve the shrinkage of the hydrogel, which
are dissociated following the swelling of hydrogels because of the
ionization of the cardoxylic acid groups in the network.
˚
2.4. The Swelling Properties of the NPMAAHG Hydrogels
displays the periodic structure with reflections at 5.4 and 2.7 A,
which should be the separation between the planes of alkyl chains
and p–p stacking groups in the NPMAAHGhydrogels in Figure 6a
(insert). The extracted water forms ice layers within the hydrogels
and finally evaporates, which may lead to a lamella-like
morphology.[31–34] The effect of water crystallization during
freezing promotes the organization of these laminate structures
into oriented regularly spaced stacks, which
The swelling properties of the NPMAAHG-4 hydrogels are
determined in buffer solutions from pH 1.4 to 9 with an ionic
strength of 0.1 M at 37 8C (Fig. 7a). After 1 h incubation, the
swelling ratios (SRs) are 1.0, 1.3, 2.9, 3.3, 4.8, 7.1, and 8.0 when
exposed to solutions with pH values of 1.4, 3, 4, 5, 6, 7.4, and 9
gives rise to rather brittle freeze-dried hydro-
gels. Such a lamella-like structure can be
formed during the freeze drying process even
if there are no hydrophobic domains.[34] The
presence of an absorbance peak at 1630 cmꢂ1
in the FTIR spectra reveals hydrogen bond-
ing interactions in the lamella structure
(Fig. 5b).[35] The formation of p–p stacking is
shown by UV-vis spectra in Figure 6b.
UV-vis spectra (Fig. 6b) reveal an absorption
peakat310 nmwhenthehydrogelissoakedina
pH 1.4 solution for 3 days, which is 30 nm
longer than the characteristic absorption of
band III in the nitrylphenyl group
(l ¼ 280 nm). The red shift of the characteristic
absorption band of the nitrylphenyl group
Figure 6. a) SEM images of the NPMAAHG-4 hydrogels. Insert is the XRD patterns of the
synthesized NPMAAHG-4 hydrogels. b) UV-vis absorption spectra of the NPMAAHG-4 hydrogels
in pH 1.4 buffer solution after 3 days incubation (dotted lines) as well as in pH 7.4 buffer solution
for 7 h (solid lines) and 3 days (dashed lines).
672
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2010, 20, 669–676