Reducibly degradable hydrogels of PNIPAM and PDMAEMA: Synthesis, stimulus-response and drug release

Article abstract of DOI:10.1002/pola.24141

Reducibly degradable hydrogels of poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) were synthesized by the combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and click chemistry. The alkyne-pending copolymer of PNIPAM or PDMAEMA was obtained through RAFT copolymerization of propargyl acrylate with NIPAM or DMAEMA. Bis-2-azidyl-isobutyrylamide of cystamine (AIBCy) was used as the crosslinking reagent to prepare reducibly degradable hydrogels by click chemistry. The hydrogels exhibited temperature or pH stimulus-responsive behavior in water, with rapid response, high swelling ratio, and reproducible swelling/shrinkage cycles. The loading and release of ceftriaxone sodium proved the feasibility of the hydrogels as the stimulus-responsive drug delivery system. Furthermore, the presence of disulfide linkage in AIBCy favored the degradation of hydrogels in the reductive environment.

Full text of DOI:10.1002/pola.24141

Reducibly Degradable Hydrogels of PNIPAM and PDMAEMA:
Synthesis, Stimulus-Response and Drug Release
BO-YU ZHANG, WEI-DONG HE, LI-YING LI, XIAO-LI SUN, WEN-TAO LI, KE-REN ZHANG
Department of Polymer Science and Engineering, CAS Key Laboratory of Soft Matter Chemistry,
University of Science and Technology of China, Hefei, Anhui 230026, China
Received 29 March 2010; accepted 20 May 2010
DOI: 10.1002/pola.24141
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Reducibly degradable hydrogels of poly(N-isopro-
pylacrylamide) (PNIPAM) and poly(N,N-dimethylaminoethyl
methacrylate) (PDMAEMA) were synthesized by the combina-
tion of reversible addition-fragmentation chain transfer (RAFT)
polymerization and click chemistry. The alkyne-pending copoly-
mer of PNIPAM or PDMAEMA was obtained through RAFT
copolymerization of propargyl acrylate with NIPAM or
DMAEMA. Bis-2-azidyl-isobutyrylamide of cystamine (AIBCy)
was used as the crosslinking reagent to prepare reducibly
degradable hydrogels by click chemistry. The hydrogels exhib-
ited temperature or pH stimulus-responsive behavior in water,
with rapid response, high swelling ratio, and reproducible
swelling/shrinkage cycles. The loading and release of ceftriax-
one sodium proved the feasibility of the hydrogels as the stimu-
lus-responsive drug delivery system. Furthermore, the presence
of disulfide linkage in AIBCy favored the degradation of hydro-
C
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gels in the reductive environment. 2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 3604–3612, 2010
KEYWORDS: drug delivery systems; hydrogels; living radical po-
lymerization (LRP); reducible degradation; stimuli-sensitive
polymers; stimulus-response
INTRODUCTION Polymer-based intelligent hydrogels, which
could change their volume and porous structure under the
external stimulus, such as temperature, pH value, light, elec-
tric field, and chemical environment, have attracted great in-
terest in material science, polymer science, and biomedi-
cine.1–7 More and more intelligent hydrogels are leading
potential applications in biology and medicine fields, includ-
ing gene and drug delivery system,8 biochemical sensor,9 bio-
engineering,10 biocatalyst,11 and size-dependent separation.12
Recently, many research works are concerned with the devel-
opment of crosslinkers with degradable linkage to control
the hydrogel biodegradability.20,21 Various decomposable
crosslinkers, such as ester,22 peptide23 and anhydride,24 are
widely used in the synthesis of hydrogels. The resultant
crosslinked hydrogels are degradable to water-soluble poly-
mers. Disulfides represent an excellent choice for a degrad-
able crosslinker.25,26 As the disulfide can be cleaved by
reducing reagent such as 1,4-dithiothreitol (DTT),27 the
hydrogels prepared with disulfide crosslinker are reductively
decomposed. Disulfide-stabilized poly(methacrylic acid)
hydrogels was proposed as functional enzymatic cargo28 and
biochemical degradable gelators with disulfide group have
been synthesized by Armes et al.29 These gels show the
potential used in biology field.
Among intelligent hydrogels, poly(N-isopropylacrylamide)
(PNIPAM) and poly(N,N-dimethylaminoethyl methacrylate)
(PDMAEMA) based hydrogels are the most studied. Now the
research of PNIPAM based thermoresponsive hydrogels is
focused on biomedical field, such as the site-specific conjuga-
tion of vinyl sulfone-PNIPAM to N49C streptavindin,13 the
release of sodium salicylate from graft-type PNIPAM hydro-
gels14 and the utility of phase-separated PNIPAM as a pseu-
dostationary phase for capillary electrochromatography to
analyze the mutation of DNA.15 The hydrogels based on
PDMAEMA exhibit pH responsive behavior due to the proto-
nation of the tertiary amine groups,16 and there are many
reports on their synthesis and responsive behavior.17,18
Because PDMAEMA is a polycation, researchers also have
studied its application in gene delivery.19 Thus, preparing
PNIPAM or PDMAEMA hydrogels through easy ways and
applying them in biology field is a hot topic in recent years.
At present, click chemistry has gained much attention due to
the high efficiency and quantitative yield. Recent reports
about click chemistry indicate that it is an ideal method for
the synthesis of functional polymers.30,31 The combination of
click chemistry with living polymerization makes it easy to
prepare hydrogels with predetermined crosslinkage density.
Thus, the design of novel disulfide crosslinker suitable for
click reaction could provide a new method for preparing
reductively degradable hydrogels.
In general, the response rate of polymer hydrogels is gov-
erned by the thickness of polymer matrix skin layer.32 The
Correspondence to: W.-D. He (E-mail: wdhe@ustc.edu.cn)
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dense skin layer retards the outward diffusion of water dur-
ing the hydrogel collapse and decreases the swelling/shrink-
age rate of hydrogels.33 Usually, macroporous structure,
which is obtained by the addition of pore-forming reagents,
could increase the response rate of polymeric hydrogel.34
For example, Zhang et al. obtained PNIPAM macroporous gel
with poly(ethylene glycol) as the pore-forming reagent and
the gel exhibited the improved swelling rate and rapid
shrinkage rate.35
uum distillation, bisbromoisobutyrylamide of cystamine was
obtained (5.08 g, 89%).
Bis-bromoisobutyrylamide of cystamine (2.25 g, 5 mmol)
was added to the solution of sodium azide (0.65 g, 0.15 mol)
in DMF (30 mL) at room temperature. The reaction mixture
ꢀ
was stirred at 75 C for 48 h. After reaction, the resulting so-
lution was precipitated in 250 mL water. AIBCy was col-
lected by filtration and dried at room temperature under
vacuum for 24 h. (1.80 g, 96%)
This study aims to prepare intelligent hydrogels with rapid
response, adjustable drug release, and reducible degradation
by the combination of RAFT polymerization and click chem-
istry. The resultant hydrogels possess disulfide linkage, so
they could degrade into water soluble polymer in the pres-
ent of DTT. Also, these hydrogels have macroporous struc-
ture, which offers them with rapid response. Release of a
hydrophilic drug from the hydrogels demonstrates the stimu-
lus dependence and the potential in medical field.
RAFT Copolymerization of NIPAM or DMAEMA
with Propargyl Acrylate
After the solution of propargyl alcohol (1.10 g, 0.019 mol),
triethylamine (2.25 g, 0.020 mol), hydroquinone (0.050 g)
and THF (100 mL) was cooled in ice-water bath, acryloyl
chloride (1.80 g, 0.020 mol) was added dropwise in 20 min.
The mixture was stirred in ice-water bath for 1 h and at
room temperature for 15 h. After the filtration to remove the
solid salt, the solvent was removed by rotary evaporation.
Propargyl acrylate (ProA) was obtained by further vacuum
distillation (1.8 g, 88%).
EXPERIMENTAL
Materials
2,20-Bipyridine, 2-bromoisobutyryl bromide and 1,4-dithio-
threitol (DTT, 99%) were purchased from Alfa Aesar and
used as received. Acryloyl chloride was purified by vacuum
distillation, and azodiisobutyronitrile (AIBN) was purified
by recrystallization from ethanol. N-Isopropylacrylamide
(NIPAM, Kohjin, Japan) was purified by recrystallization from
a benzene/n-hexane mixture (65/35 v/v). N,N-Dimethylami-
noethyl methacrylate (DMAEMA, Aldrich) was passed
through basic alumina column, then vacuum-distilled over
CaH2 before use. Cystamine dihydrochloride (Shanghai
Chemical Reagent, 98.5%) was purified by recrystallization
from a water/ethanol mixture (25/75 v/v). Triethylamine
was stirred with KOH for 12 h at room temperature, refluxed
with toluene-4-sulfonyl chloride and distilled before use.
Copper(I) bromide was washed with glacial acetic acid, fol-
lowed by washing with methanol and ethyl ether to remove
impurities, then dried under vacuum and kept under N2
atmosphere. THF was dried over sodium/benzophenone and
distilled just before use. Dibenzyltrithiocarbonate (DBTTC)
as chain transfer agent (CTA) was synthesized based on liter-
ature.36 All other reagents were of analytical grade and used
as received.
Proton nuclear magnetic resonance spectroscopy (1H NMR,
CDCl3, d, ppm): 2.45 (s, 1H, AC:CH), 4.63 (s, 2H,
AOCH2AC:CH), 5.85 (d, 1H, CH2¼¼CHA), 6.10 (q, 1H,
CH2¼¼CHA), 6.34 (d, 1H, CH2¼¼CHA).
Copolymer with low ProA content was synthesized via RAFT
copolymerization of NIPAM or DMAEMA with ProA using
DBTTC as CTA.37,38 The typical procedure was followed.
DMAEMA (1.57 g, 10 mmol), AIBN (1.64 mg, 0.01 mmol),
ProA (0.11 g 1 mmol) and DBTTC (0.029 g, 0.1 mmol) were
dissolved in THF (3.0 mL). The obtained solution was added
into a polymerization tube equipped with a magnetic stirring
bar. The mixture was degassed by three freeze-pump-thaw
cycles. After the tube was sealed under vacuum, the poly-
ꢀ
merization reaction proceeded under stirring at 80 C for 24
h. The tube was opened and the reaction mixture was pre-
cipitated into hexane. Poly(DMAEMA-co-ProA) (1.37 g, 87%)
was obtained after the dryness under vacuum at 25 ꢀC for
24 h. For the preparation of poly(NIPAM-co-ProA) (yield:
89%), diethyl ether was used as the precipitation medium.
1
The chemical structure of copolymers was determined by H
NMR spectroscopy and the relative molecular weight was
characterized by gel permeation chromatograph (GPC).
Synthesis of Bis-2-azidyl-isobutyrylamide
of Cystamine (AIBCy)
Synthesis of Reducible Intelligent Hydrogels
A solution of cystamine dihydrochloride (2.85 g, 0.013 mol)
in water (100 mL ) was cooled to 0–5 ꢀC in ice-water bath.
Then, the solution of 2-bromoisobutyryl bromide (6.00 g,
0.015 mol) in dichloromethane (10 mL) and NaOH aqueous
solution (2.00 g, 0.050 mol, 10 mL) were added simultane-
ously from two separated addition funnels in dropwise way.
The addition lasted for more than 1 h, and then the mixture
was stirred at room temperature for 2 h. After that, the mix-
ture was extracted with 5 ꢁ 30 mL methylene chloride. The
organic phase was collected and dried over anhydrous so-
dium sulfate. After the rotary evaporation and further vac-
with Disulfide Linkage
PNIPAN or PDMAEMA hydrogels were synthesized through
click reaction between alkyne-pending copolymer and azide-
containing AIBCy crosslinker. The typical procedure was fol-
lowed. P(NIPAM100-co-ProA10) (600 mg, the digits in sub-
script are the number of monomer units), AIBCy (85 mg),
CuBr (77 mg) and 2,20-bipyridine (0.25 g) were dissolved in
THF (3.0 mL). The mixture was deoxygenatedꢀ by three
freeze-pump-thaw cycles. After the reaction at 30 C without
stirring for 24 h, the resulted gel was purified by dialysis
against THF/water (1/1) for 2 d and water for 3 d. The final
DEGRADABLE HYDROGELS OF PNIPAM AND PDMAEMA, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Schematic illustration
of the synthesis of reducible
intelligent polymer hydrogel.
dry gel was obtained by the lyophilization in a freeze drier
(SIMENS FD5-3) for 48 h. (0.62 g, 95%)
and weighted after the excess water on the surface was wiped
off with filter paper. The results of three measurements were
averaged. Swelling ratio (Rs) was calculated by eq 1.
Characterizations
1H NMR spectroscopy (300 MHz) measurements were per-
formed on Bruker DMX300 spectrometer in CDCl3 using
tetramethylsilane as internal reference. Fourier transform
infrared (FTIR) spectra were recorded on MAGNA-IR 750
spectrophotometer in KBr pellets. The relative molecular
weight and its distribution were determined on a Waters
150C GPC equipped with Waters 1515 HPLC pump, Waters
2414 differentia refractive index detector and three Ultra-
styragel columns (500, 103 and 104 Å in series) at 30 ꢀC,
using monodispersed polystyrene as calibration standard.
DMF was used as eluent at a flow rate of 1.0 mL/min. The
final freeze-dried gels were fractured carefully and fixed on
aluminum stubs. The interior morphology of the gels was
observed under scanning electron microscope (SEM, FEI-
SIRION 200). Before SEM observation, the hydrogel speci-
mens were coated with gold.
Ws ꢂ Wd
Rs ¼
ꢁ 100%
(1)
Wd
where Ws and Wd is the weight of swollen gel and dry gel,
respectively.
Swelling kinetics of PNIPAM hydrogel at different tempera-
tures was studied in distilled water with similar procedure
to that of PDMAEMA hydrogels.
Stimulus Response of Hydrogels with pH or
Temperature
After PDMAEMA hydrogels reached the swelling equilibrium
in buffer solution (pH ¼ 7.0) at 25 ꢀC, they were quickly
transferred to the buffer solution with pH of 9.0. The sam-
ples were withdrawn at the determined intervals and
weighted after wiping off the excess water on the surface.
Thus, the shrinkage kinetics of PDMAEMA hydrogels with pH
increase was observed. After that, the swelling-shrinkage
The Swelling Kinetic of Intelligent Hydrogels
For the investigation of the swelling kinetic of PDMAEMA
gels at different pH values (pH ¼ 4.0, 7.0, and 9.0), dry
PDMAEMA gels (diameter: 2 cm, thickness: 0.5 cm) were
immersed into buffer solutions at 25 ꢀC. At the different
determined intervals, the swollen hydrogels were taken out
ꢀ
cycles between pH ¼ 7.0 and 9.0 were repeated at 25 C by
keeping PDMAEMA hydrogels in two buffer solutions
alternatively.
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40 ꢀC. The swelling-shrinkage cycles of PNIPAM hydrogels
ꢀ
between 20 and 40 C were carried out.
Drug Loading and Release Behavior of Intelligent
Hydrogels
Ceftriaxone sodium was chosen as the model drug to test
the drug loading and release behaviors of hydrogels. The dry
PNIPAM or PDMAEMA hydrogels (50 mg) was swollen in
ceftriaxone sodium aqueous solution (10.0 mg/g, 10 g) at
20 ꢀC for 12 h. After the excess water on the surface was
wiped off with filter paper, the drug-swollen hydrogel was
weighted. The loading amount (mg) of ceftriaxone sodium
for 50 mg hydrogels was determined.
The drug release was carried out at different pHs or temper-
atures. For PDMAEMA hydrogels, the drug-loaded hydrogels
were kept in buffer solution (100 mL, pH ¼ 7.0 or 9.0). For
PNIPAM hydrogels, the drug-loaded hydꢀ rogel was kept in
ꢀ
distilled water at the temperature of 20 C or 40 C. At each
of determined time intervals, 2 mL solution was taken out
for UV-vis (Hitachi U-3010) measurement (k ¼ 272 nm) and
its concentration of ceftriaxone sodium was determined
using the calibration built on the absorbance of ceftriaxone
sodium solutions with known concentrations. At each sam-
pling, 2 mL fresh solution was added to keep the constant
volume of medium.
Reducible Degradation of Intelligent Hydrogels
The reducible degradation of hydrogels was studied by using
DTT to cleave disulfide linkage of hydrogels. For example,
PDMAEMA hydrogels were immersed in DTT aqueous solu-
tion (0.1 M) and kept under nitrogen atmosphere for 5 d.
The gel-to-sol transition was recorded by a digital camera.
RESULTS AND DISCUSSION
Herein, we reported one method to prepare reducible hydro-
gels. We synthesized P(NIPAM-co-ProA) or P(DMAEMA-co-
ProA) by RAFT copolymerization, and obtained polymer
FIGURE 1 1H NMR spectrum of AIBCy (a), P(DMAEMA-co-ProA)
(b), and P(NIPAM-co-ProA) (c) in CDCl3.
For the study of shrinkage of PNIAPM hydrogels, they were
firstly immersed in distilled water at 20 ꢀC. After they
reached the swelling equilibrium, the swollen PNIPAM
hydrogels were then quickly transferred to distilled water at
FIGURE 2 FTIR spectrum of AIBCy, PNIPAM gel with 10 mol %
ProA, PDMAEMA gel with 5 and 10 mol % ProA.
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TABLE 1 Synthesis of the Copolymersa and the Reducible Gelsb
GPC Results
Main Monomer ProA (mmol) Molar Ratio in Copolymerc Mn
Gel Preparation
Copolymer Code
Mw/Mn AIBCy (mg) Geltation
P(DMAEMA-co-ProA)-1 DMAEMA
P(DMAEMA-co-ProA)-2 DMAEMA
1.0
0.5
1.0
0.5
Mm:ProA ¼ 10
Mm:ProA ¼ 20
Mm:ProA ¼ 10
Mm:ProA ¼ 20
c
16,700 1.32
15,900 1.37
12,100 1.23
11,700 1.29
72
36
85
43
Yes
Yes
Yes
No
P(NIPAM-co-ProA)-1
NIPAM
NIPAM
P(NIPAM-co-ProA)-2
a
RAFT copolymerizations were carried out with main monomer (10.0
mmol) in the presence of DBTTC (0.1 mmol) and AIBN (0.01 mmol) in
THF (3.0 mL) at 80 ꢀC for 24 h.
The copolymer composition was determined by 1H NMR. Mm stands
for the main monomer, which is DMAEMA and NIPAM respectively.
b
The click crosslinkage reaction of copolymer (600 mg) and AIBCy was
performed in the presence of CuBr (77 mg) and bipyridine (0.25 g) in
THF (3.0 mL) at 30 ꢀC without stirring for 24 h.
hydrogels through click chemistry with disulfide-containing
crosslinker. The overall procedure was illustrated in Scheme 1.
Figure 1(b) shows 1H NMR spectrum of P(DMAEMA-co-
ProA), The copolymer composition was determined by com-
paring the integration of DMAEMA peak (a) at 4.0 ppm
and ProA peak (b) at 4.7 ppm. The ratio of a:b is 10:1 or
20:1, which is consists with the molar ratio of charged
monomers. Figure 1(c) shows 1H NMR spectrum of
P(NIPAM-co-ProA) and the integration ratio of PNIPAM peak
(a) at 4.0 ppm to ProA peak (b) at 4.7 ppm is 10:1 or 20:1,
which is also in agreement with the monomer ratio. The
details about the synthesis of the copolymers are summar-
ized in Table 1. The molecular weight and its distribution of
the copolymers were characterized by GPC and the result is
also listed in Table 1.
Synthesis of Reducible Intelligent PNIPAM
or PDMAEMA Hydrogels
The disulfide-diazide crosslinker of AIBCy was synthesized
through two steps. Firstly, the amino groups of cystamine
react with bromoisobutyryl bromide to introduce bromide
groups. Secondly, the halogen groups are transformed into
azide groups. Figure 1(a) shows 1H spectrum of AIBCy in
CDCl3. The signals of a at 3.71 ppm and b at 2.82 ppm are
described to the methylene groups of cystamine, and that of
c at 1.98 ppm to the methyl group from 2-bromoisobutyryl
group. The integration ratio of a:b:c equals 1:1:3, in agree-
ment of the structure of AIBCy. IR analysis of AIBCy shows
an absorption peak at 2100 cmꢂ1, which corresponds to the
asymmetric stretching vibration of the azide group (Fig. 2).
This confirms the successful synthesis of the AIBCy.
PNIPAM and PDMAEMA gels were obtained through the click
reaction between alkyne groups of copolymers and azide
groups of AIBCy, which was firstly prepared and used by our
group. The amount of alkyne groups is little less than azide
groups (90 mol %). After thorough dialyze, the gels have no
absorbance at 2100 cmꢂ1 in IR spectra (Fig. 2). Herein, the
crosslinkage density is defined as the molar ratio of ProA to
DMAEMA or NIPAM. As shown in Table 1, the click crosslink-
age between P(DMAEMA-co-ProA) (5 mol % and 10 mol %
P(NIPAM-co-ProA) and P(DMAEMA-co-ProA) were synthe-
sized through RAFT copolymerization. As one of living poly-
merizations, RAFT polymerization could be used to easily
prepare the polymers with well defined structure.39,40
FIGURE 3 SEM micrographs of the morphology of polymer hydrogels produced from (PDMAEMA-co-ProA) with 5 mol % ProA (a),
(PDMAEMA-co-ProA) with 10 mol % ProA (b), and (PDNIPAM-co-ProA) with 10 mol % ProA (c).
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FIGURE 5 Swelling kinetics of PNIPAM hydrogels with 10 mol %
ProA at 20 ꢀC and 40 ꢀC.
The Swelling Kinetics of Reducible Intelligent Hydrogels
After the thorough dialysis and freeze-dryness, the swelling
kinetics of reducible intelligent hydrogels was investigated to
evaluate their water-absorption behavior. Figure 4 shows the
swelling kinetics of PDMAEMA hydrogels with two crosslink-
age densities at 20 ꢀC in buffer solution with different pHs.
Figure 5 shows the swelling kinetics of PNIPAM gel from
P(NIPAM-co-ProA) with 10 mol % ProA at 20 and 40 ꢀC in
distilled water. From the figures, we can find that all the
hydrogels have excellent ability to uptake water with swel-
ꢀ
ling ratio of 1000–2000% at 20 C and pH 7.0, which might
be attributed to the macroporous structure of polymer net-
works. Usually, there are two ways for the hydrogels to
uptake water. One is the water binding by polymer matrix
via hydrogen bond and another is the capillary action.41 As
for our gels, the pore volume is much larger than the poly-
mer volume, so the capillary action is dominated. Their mac-
roporous structure offers them enough space to hold large
amount of water.
FIGURE 4 Swelling kinetics of PDMAEMA hydrogels at differ-
ent pHs [(a) 10 mol % ProA, (b) 5 mol % ProA].
of ProA to DMAEMA) and AIBCy leads to the formation of
bulk gels. On the contrary, the click crosslinkage between
P(NIPAM-co-ProA) produce the bulk gel only at 10 mol % of
ProA to NIPAM. No formation of bulk gel was observed
P(NIPAM-co-ProA) with 5 mol % of ProA to NIPAM.
Interior Morphology of Reducible Intelligent Hydrogels
The interior morphology of reducible intelligent hydrogels
was observed and their SEM images reveal the macroporous
morphology with averaged pore diameter of ꢃ50 lm as
shown in Figure 3. The pores are regular and arrange along
the same direction, suggesting the controlled porosity struc-
ture of the gels. Generally, some pore-forming agents, such
as PEG and cellulose, are needed in preparing macroporous
hydrogels. Herein, we prepare macroporous networks with-
out them. As expected, PDMAEMA gel from P(DMAEMA-co-
ProA) with 10 mol % ProA has smaller pore size than that
from 5 mol % ProA. It indicates that the porosity structure
could be modulated simply by the copolymer composition.
This macroporous structure might improve the property of
intelligent hydrogels and impart the gels with a rapid
response rate in swelling/shrinkage experiment.
FIGURE 6 Shrinkage kinetics of PDMAEMA hydrogels at pH
variation from 7.0 to 9.0 and PNIPAM gel at temperature varia-
tion from 20 to 40 ꢀC.
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ꢀ
40 C. The swelling ratio dramatically decreases from 1200%
to 150% in about 30 min. It should be assigned to responsive
properties of PNIPAM segment in gel network. Because a
majority of gel space is pore capacity, PNIPAM chains only
take up a little volume. After it is immersed in water, the size
of pore enlarges quickly through capillary action. At the tem-
perature above PNIPAM LCST, the shrinkage of PNIAM indu-
ces the exclusion of the most absorbed water from the hydro-
gel, thus skin of the fully shrunk gel seems dry. Due to a little
physical absorption of water by PNIPAM gel, the gel could
have 150% swelling ratio at shrinkage equilibrium. This rapid
and full shrinkage of PNIPAM hydrogels could help to devise
immediate responsive drug delivery system.
To testify these polymer hydrogels can be used circularly
and have the dynamistic responses of shrinkage and swelling
behavior via pH or temperature variation, their shrinkage-
swelling circles are investigated and the results are showed
in Figure 7.
FIGURE 7 Shrinkage-swelling circles of PDMAEMA hydrogels
between pH 7.0 and 9.0, and PNIPAM hydrogel between 20
and 40 ꢀC.
Figure 7 shows the shrinkage-swelling circles of PDMAEMA
hydrogel with 10 mol % ProA between pH 7.0 and 9.0.
Based on the feature of PDMAEMA, PDMAEMA hydrogels ex-
hibit pH responsive behavior. Equilibrium swelling ratio and
swelling rate decrease while pH changes from 7.0 to 9.0.
This might be cause by two reasons. Firstly, the hydrophilic-
ity of PDMAEMA chains decreases gradually with the
increase of pH value,42 leading to the decrease of binding
interaction between water and polymer matrix. Secondly,
electrostatic repulsion between PDMAEMA segment
decreases while the amount of ammonium groups from
PDMAEMA decreased drastically.43 Thus, the pore size
becomes smaller and the capacity of hydrogels for water
uptake decreases. It also shows that the saturated swelling
ratio also decreases with increasing crosslinking density.
This is because the hydrogel with low crosslinking density
has larger pore, thus it could uptake more water at quicker
rate than the hydrogel with high crosslinking density.
Stimulus Response of Intelligent Hydrogels
with pH or Temperature
Figure 6 shows the shrinkage kinetics of PDMAEMA hydro-
gels when environment pH changes from 7.0 to 9.0. Since
the hydrophilicity of PDMAEMA hydrogels become worse
and the decrease of electrostatic repulsion between
PDMAEMA segments induce the shrinkage of hydrogel net-
work, the swelling ratio of hydrogels decreases. After 40 and
60 min, the shrinkage equilibrium reaches with swelling ra-
tio of 900 and 600% for PDMAEMA hydrogel with 5 and
10 mol % ProA, respectively. The swelling ratio at shrinkage
equilibrium keeps relative high, due to the fact that
the PDMAEMA is a weak base with pKa about 7.0 and is
water-bound within wide pH range. When pH is 9.0, it is
partially hydrophilic and still could absorb water somewhat.
As shown in Figure 6, PNIPAM hydrogel undergoes the
shrinkageꢀ after the environment temperature changes from
ꢀ
20 to 40 C. PNIPAM homopolymer has LCST about 32 C in
aqueous solution, thus PNIPAM hydrogel also shrinks their
volume when the temperature rises above LCST. From the
figure, we could find that PNIPAM gel shrinks very quickly at
FIGURE 8 (a) Drug release of PDMAEMA hydrogels with time
at different pHs, (b) Drug release of PNIPAM hydrogels with
time for at two temperatures.
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FIGURE 9 Reducible degradation of PDMAEMA hydrogel with 10 mol % ProA in DTT aqueous solution (0.1 M) [(A) 1 d, (B) 3 d,
and (C) 5 d].
PDMAEMA hydrogel presents an asymmetrical change of
swelling ratio in each circle. The shrinkage lasts for about
1 h to reach its equilibrium while the swelling process needs
about 11 h to achieve its equilibrium status. However, the
shrinkage-swelling circle can be repeated with high reprodu-
cibility for thirty times. As for PNIPAM hydrogel with
10 mol % ProA, its shrinkage-swelling circles are quite dif-
ferent form PDMAEMA hydrogel. PNIPAM hydrogel shows a
symmetrical shrinking-swelling behavior when changing
temperature. This rapid response consists with the result in
Figure 6. Only in 1 h, PNIPAM hydrogel can get to swelling/
shrinkage balance status. From the figure, we find that PNI-
PAM gel still shows a quickly responsive behavior in 10
circles. This stable and repeatable responsive behavior
endows the hydrogels with the potential in gene or drug
delivery system.
to the shrinkage of PNIPAM hydrogel at the temperature
above LCST.
Reducible Degradation of Intelligent Hydrogels
Because these intelligent hydrogels are prepared by using
disulfide crosslinker, they can be degraded by DTT. The
decrosslinkage of intelligent gels was carried out in water
with DTT as the cleavage reagent. The hydrogels firstly broke
down into tiny fragments in DTT aqueous solution. After 5
days, the intelligent hydrogels were totally dissolved in
water. The reducible degradation of PDMAEMA hydrogel
with 10 mol % ProA is illustrated in Figure 9. Because the
rate of gel degradation is much lower than drug release rate,
this reducible degradation of intelligent hydrogels exercises
little influence to the drug release.
CONCLUSIONS
The Loading and Release of Ceftriaxone Sodium
Reducibly degradable intelligent hydrogels of PNIPAM and
PDMAEMA were successfully synthesized by the combination
of RAFT polymerization and click chemistry. SEM observation
displayed the regular macroporous morphology of the resultant
hydrogels. The final hydrogels exhibit temperature or pH stim-
ulus-response with swelling-shrinkage reproducibility in water.
The macroporous structure imparts the hydrogels with rapid
response and high swelling ratio. The loading and release of
ceftriaxone sodium prove the feasibility of the hydrogels as the
stimulus-responsive drug delivery system. Furthermore, the
experiment of reaction with DTT testifies that the hydrogels
could be degraded in the reductive environment, which can
facilitate the removal of empty hydrogels, as well as might
improve the release of encapsulated drug molecules.
Ceftriaxone sodium, one of the third generation of cephalo-
sporin drugs, is well dissolved in water and has been chosen
as a model drug in loading and release experiment.44 It has
strong UV-vis absorption at 273 nm, thus its concentration
in water can be easily determined. After the hydrogels
(50 mg) were saturately swollen in the aqueous solution of
ceftriaxone sodium, the amount of loaded drug was 9.5 mg,
4.5 mg, and 5.0 mg for PDMAEMA hydrogel (50 mg) with
5 mol % ProA, 10 mol % ProA, and PNIPAM hydrogel (50 mg)
with 10 mol % ProA, respectively. Then, the drug release
experiments of drug-loaded hydrogels were carried out at
various environments.
Figure 8(a) shows the drug release behavior of PDMAEMA
hydrogels as function of time. The hydrogels with different
crosslinking degrees demonstrate the varied drug release
profiles. PDMAEM hydrogel with low crosslinking density
releases the drug at faster rate. It might be caused by larger
pore size, which leads to better drug holding/releasing of
the hydrogel. The drug release in alkalic environment is
quicker than that in neutral system, caused by the shrinkage
of PDMAEMA hydrogel. Figure 8(b) shows the drug release
behavior oꢀf PNIPAM gel. It is obvious that the drug release
This work was financially supported by NSFC (20934005),
Ministry of Science and Technology of China (2007CB936401),
the HKSAR Earmarked Research Grant (403706), and National
Undergraduate Innovative Experiment Project of China Educa-
tion Ministry. W.-T. Li and K.-R. Zhang thank the Chemistry
Experiment Center of USTC for the convenience of
measurements.
ꢀ
rate at 20 C is slower than 40 C and the maximum amount
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ꢀ
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