Journal of Biomaterials and Nanobiotechnology, 2011, 2, 201-206
doi:10.4236/jbnb.2011.23025 Published Online July 2011 (
Copyright © 2011 SciRes. JBNB
Self-Assembled Core-Shell Poly(Ethylene
Glycol)-POSS Nanocarriers for Drug Delivery
Kyu-Oh Kim1, Byoung-Suhk Kim2, Ick-Soo Kim2
1Department of Bioscience and Textile Technology, Shinshu University, Ueda, Japan; 2Nano Fusion Technology Research Group,
Faculty of Textile Science and Technology, Shinshu University, Ueda, Japan.
Received April 18th, 2011; revised May 5th, 2011; accepted May 15th, 2011.
In this work, novel nanostructured core-shell poly (ethylene glycol) (PEG)-polyhedral oligosilsesquioxane (POSS)
nanoparticles were used to encapsulate insulin as new drug delivery carriers. The morphologies, particle size and
potential of th e pure nanostructured core-shell PEG-POSS and th e corresponding insulin-load ed PEG-POSS nanopar-
ticles were investigated by transmission electron microscopy (TEM) and laser diffraction particle sizer. TEM analysis
demonstrated that pure and insulin-loaded self-assembled PEG-POSS nanoparticles were of spherical shape with
core-shell nanostructure, and were well-dispersed and uniform in size distribution. Insulin release test showed that in-
sulin was well-protected in side PEG-POSS nanoparticles at gastric pH for 2 hrs, and was released at intestinal pH (pH
6 - 7) where the absorption and activation of the drug are necessary. We therefore believe that such nanostructured
PEG-POSS nanoparticles could be useful as a potential carrier for insulin drug delivery systems.
Keywords: Self-Assembly, Amphiphilic, Nanoparticles, POSS, Insulin, Oral Delivery
1. Introduction
In a recent year, nanotechnology has been utilized to de-
velop new therapies and next generation nanosystems for
“smart” drug delivery [1]. A variety of organic/inor-
ganic nanomaterials and devices have been often used as
delivery vehicle to enhance the therapeutic activity by
prolonging drug half-life, improving solubility of hydro-
phobic drugs, reducing potential immunogenicity, and/or
releasing dugs in a sustained or stimuli-triggered fashion.
Insulin was known to treat diabetic. Current treatment
methods involve regular injections of insulin, which can
be both painful and inconvenient, thus leading to low
patient compliance [2]. In order to overcome this prob-
lem, the oral route is considered to be the most conven-
ient and comfortable means of drug administration for pa-
tients. However, oral administration of hydrophilic mac-
romolecules such as peptide/protein drugs is encountered
with many difficulties as the drugs have to confront va-
rious major barriers in the gastrointestinal (GI) tract.
Firstly, peptide/protein drugs get denatured readily by
low pH of gastric medium in the stomach. Secondly, dif-
ferent digestive enzymes in the stomach and small intes-
tine may lead to degradation of peptide/protein drugs [3].
A new carrier system is therefore required to protect pep-
tide/protein drugs from the harsh environment in the GI
We have recently developed the organic/inorganic hy-
brid materials, such as amphiphilic poly(ethylene glycol)
(PEG)-polyhedral oligosilsesquioxane (POSS) and poly
(vinyl alcohol) (PVA)-POSS hybrids incorporating POSS
macromers onto chain-ends or polymer backbone as
pendent groups, respectively [4-7]. Thanks to their am-
phiphilic properties (here, POSS is hydrophobic, PEG
and PVA hydrophilic), it can be expected that those
POSS-containing polymers can form the micelles by tai-
loring the composition ratio in polymers and solvent po-
larity, and this has important implications for drug deliv-
ery systems. For instance, we reported [8] that PVA-
POSS hybrid showed unagglomerated nanoparticles
within a diameter range of 60 - 90 nm, as confirmed by
atomic force microscopy (AFM) and bio-transmission
electron microscope (bio-TEM). The prepared nanoparti-
cles were found to improve the control release of anti-
cancer drug; paclitaxel as a model drug. However, there
are few reports on the solution properties and mi-
celles/nanoparticles formation of POSS-based polymeric
materials for drug delivery system [9-11]. In this report,
we prepare the hybrid core-shell nanostructured particles
Self-Assembled Core-Shell Poly(Ethylene Glycol)-POSS Nanocarriers for Drug Delivery
Copyright © 2011 SciRes. JBNB
composed of POSS as a hydrophobic inner core and PEG
as a hydrophilic outer shell by using dialysis approach,
and propose novel drug delivery carriers for protein
2. Experimental Section
2.1. Materials
Poly(ethylene glycol) (PEG) (molecular weight = 3.4 kDa,
Aldrich) was purified by repeating twice the process of
precipitation into n-hexane from chloroform solution.
Isocyanatopropyldimethylsilylcyclohexyl-polyhedral oli-
gosilsesquioxane (POSS macromer) was purchased from
Tomen Plastics Co., Japan. Dibutyl tin dilaurate (DBTDL;
Aldrich, 95% purity) as a catalyst for urethane formation
was used as received. Amphiphilic PEG3.4k- POSS tel-
echelic studied herein was synthesized by direct urethane
linkage between the diol end groups of PEG homopoly-
mers and the monoisocyanate group of POSS macromers
as catalyzed by DBTDL. 1H-NMR results confirmed that
the amphiphilic PEG3.4k-POSS telechelic was success-
fully prepared. Evidence for the formation of urethane
linking groups comes from the emergence of a weak
proton signal at about 4.26 ppm, accompanied by the
disappearance of a proton signal of the -CH2-NCO group
(3.15 ppm). That is, the level of incorporation of POSS
macromers in the amphiphilic PEG3.4k-POSS telechelic
could be determined quantitatively by the monitoring of
the resonances for the cyclohexyl groups of POSS mac-
romers. A degree of end functionalization was found to
about 2.1, indicating quite consistent with the feed ratio.
The detailed synthesis and characterization are described
in our previous reports [4,12]. Toluene was dried over
CaH2 and then distilled under nitrogen prior to use. A
sample of 100 mg of insulin, from bovine pan- creas (ac-
tivity: 25 USP units/mg, secondary activity: 2500 units)
were purchased from Sigma-Aldrich. All chemicals were
of analytical purity or higher quality and were used
without further purification.
2.2. Preparation of Insulin-Loaded Hybrid
PEG-POSS Particles
Insulin solution was prepared by dissolving 100 mg of
insulin in 10 ml of 0.01 N HCl solution. Insulin encapsu-
lation was carried out on the basis of self assembly proc-
ess. Briefly, 40 mg of PEG3.4k-POSS was fully dis-
solved in 10 ml of THF. Then, 0.5, 0.9, 1.3 ml insulin so-
lutions was added dropwise into the PEG3.4k-POSS so-
lutions, respectively. The mixture was poured into di-
alysis tube (spectra/Por 6, MWCO: 3.5 kD) and dialyzed
against distilled water at room temperature under mag-
netic stirring for 1 day. Afterwards, the distilled water
was exchanged at least 3 times in order to remove the
THF and HCl residues. To determine the loading effi-
ciency of insulin in hybrid PEG-POSS nanoparticles, the
amount of free insulin in supernatants was assayed by
UV-vis spectrophotometry. The drug loading efficiency
was calculated using the equations listed below [13,14].
Loading Efficiency %
total amount of insulin added
amount of free insulin100%
total amount of insulin added
2.3. Characterization
Particle size and potential of pure and insulin-loaded
PEG-POSS nanoparticles were measured depending on
pH of the mixture by laser diffraction particle sizer
(Nano-ZS, Malvern Instrument Ltd., UK). FI-IR analysis
of the prepared nanaoparticles and insulin was carried
out using an IRPrestige-21 (Shimadzu Co., Japan) Trans-
mission Electron Microscopy (TEM) images were taken
out on a JEM-2100 LaB6 microscope (JEOL, Japan) op-
erating at an accelerating voltage of 160 kV to observe
the morphologies of the obtained pure and insulin-loaded
PEG-POSS nanoparticles. The insulin-loaded PEG-POSS
nanoparticles dispersed in water was directly dropped
onto a carbon-coated copper grid (200-A mesh, Nissin
EM Co., Ltd.), followed by drying at room temperature.
Finally, the samples were dried in vacuum oven. The av-
erage particle size was determined from the TEM micro-
graphs using an image analysis software (Image J, Na-
tional Institutes of Health, Bethesda, U.S.).
2.4. pH Effects on Insulin Release from
PEG-POSS Nanoparticles
The pH of the insulin-loaded nanoparticle suspensions
was varied at the ranging from pH 2.5 to pH 7.4 to study
the release behaviors of the insulin from PEG-POSS
nano-particles at room temperature. The insulin-loaded
PEG-POSS nanoparticles treated with centrifugation 3000
rpm/15 min were placed into 10 ml of PBS buffer solu-
tion (pH 2.5) and incubated at 37˚C for 2 hrs. 0.5 mL of
the supernatants, which was isolated by centrifugation
(2000 rpm/1 min), was taken by every twenty minutes
for the measurement of the released amount of insulin
from PEG-POSS nanoparticles and replaced by fresh
medium. Afterwards, the same insulin-loaded PEG-
POSS nanoparticles were again isolated by centrifugation
(3500 rpm/15 min) and were transferred to 10 mL of
phosphate buffer at pH 7.4 and incubated at 37˚C for 3
hrs. The released amount of insulin from PEG-POSS
nanoparticles were monitored by UV-vis spectrophotome-
try. A plot of cumulative release with time was reported,
Self-Assembled Core-Shell Poly(Ethylene Glycol)-POSS Nanocarriers for Drug Delivery
Copyright © 2011 SciRes. JBNB
Cumulative release %100%
 (2)
where At is the absorbance of the characteristic peak at
275 nm (ε = 5.53 mM–1·cm–1) [15] at time t, and AMAX is
the maximum absorbance of this peak.
3. Results and Discussion
In our previous paper [4-7], we have reported that the
synthesized organic/inorganic PEG-POSS and PVA-
POSS hybrid materials were found to be amphiphilic,
and thereby resulted in self-assembling into core-shell
nanostructured micelles with hydrophobic inner-core
(POSS moieties) and hydrophilic outer-shell (PEG or
PVA moieties) in aqueous media. Therefore, hydropho-
bic drugs, such as insulin used in this work can be easily
entrapped into the core. Scheme 1 shows the schematic
illustration of self-assembling process, which leads to
self-aggregation into core-shell nanostructured particles
(i.e., flower-like micelles) encapsulating insulin inside
the hydrophobic core in aqueous solution. Here, it is
worth mentioning that the formation of self-assembled
nanostructures using POSS-PEG-POSS telechelic (ABA
triblock copolymers) depends on two competing forces:
the entropy loss due to loop formation of the central
block in the micelle corona and the interfacial energy
penalty that accompanies the transfer of insoluble block
from the micelle corona to the solution [16].
3.1. Characterization of Insulin-Loaded
PEG-POSS Nanoparticles
Figure 1 shows TEM images of pure and insulin-loaded
self-assembled PEG-POSS nanoparticles. It can be clearly
seen that insulin-loaded PEG-POSS nanoparticles were
of spherical shape with core-shell nanostructure, and were
well-dispersed and uniform in size distribution unlike
pure PEG-POSS nanoparticles. The size of pure self-
assembled PEG-POSS nanoparticles was about 15.9 ±
1.3 nm, while the insulin-loaded self-assembled PEG-
POSS nanoparticles (loading content ~ 0.9 mg) were 330
± 80 nm, as measured by TEM analysis, suggesting suc-
cessful encapsulation of insulin molecules into a hydro-
phobic core. In addition, the loading efficiency (LE) of
insulin in the insulin-loaded PEG-POSS nanoparticles
with an added insulin contents of 5, 9, 13 mg was found
to be 52.6%, 70.5% and 76.5%, indicating that PEG-
POSS nanoparticles have good loading capability of hy-
drophobic drug, insulin [17,18]. In addition, FT-IR
analysis demonstrated the existence of insulin in insulin-
loaded PEG-POSS nanoparticles (Figure 2). The char-
acteristic peaks at 1651 cm–1 and 1531 - 1514 cm–1 re-
gion were observed respectively, corresponding to C–N
stretching and N-H bending modes of amide II region in
pristine insulin [19]. The Si-O-Si peak in the insulin-
loaded PEG-POSS nanoparticles was shifted toward
higher wavenumber, compared to pure PEG-POSS nano-
particles. This suggests that there are strong interact-
tions between insulin and PEG-POSS segments [20].
3.2. pH Effects on Insulin-Loaded PEG-POSS
The -potential and size of the obtained hybrid nanopar-
ticles was investigated at various pH and the results were
shown in Figure 3. The -potential of pure PEG-POSS
nanoparticles was close to constant zero -potential values
as increasing pH, whereas the insulin-loaded PEG-POSS
nanoparticles obviously exhibited increased negative
charges, presumably due to ionized carboxyl groups in an
encapsulated insulin (Figure 3(a)) [21]. Moreover, it is
also expected that such negative charged nanoparticles
may be less aggregated and show good dispersion-sta-
bilities because of an electrostatic repulsion. Therefore,
the resultant negatively charged insulin-loaded PEG-POSS
nanoparticles exhibit well-dispersed nanoparticles, which
can help them to be taken up by cells more easily than
aggregated ones [8]. As expected, unlike pure PEG-POSS
nanoparticles, the size of insulin-loaded PEG-POSS
nanoparticles clearly increased as increasing pH (loading
content ~ 0.9 mg, Figure 3(b)).
3.3. Insulin-Release Study
Figure 4 shows insulin release behavior from insulin-
loaded PEG-POSS nanoparticles at different pH values
of 2.0 and 7.4. At pH = 2.0, even though a tiny release of
insulin was observed, probably due to a weakly physical-
adsorbed insulin, PEG-POSS nanoparticles appeared to
have higher insulin retention capacity at low pH. Af-
terwards, following a pH change to 7.4, a dramatic re-
lease of insulin was observed during the first hour fol-
lowed by a relatively sustained release, due to a fast dis-
sociation of insulin from PEG-POSS nanoparticles at
higher pH. This suggests that insulin was well-protected
inside PEG-POSS nanoparticles at gastric pH for 2 hrs,
and was released at intestinal pH (pH 6-7) where the ab-
sorption and activation of the drug are necessary. We
believe that such PEG-POSS nanoparticles could be used
as a potential carrier for insulin drug delivery systems.
The release data have been further studied [22] by fitting
the cumulative fraction release data, Mt/M to an empiri-
cal Equation (1). The drug release behavior according to
diffusion controlled mechanism is usually governed by
the following Equation (3),
where M is the total amount of insulin in dosage form,
Self-Assembled Core-Shell Poly(Ethylene Glycol)-POSS Nanocarriers for Drug Delivery
Copyright © 2011 SciRes. JBNB
Scheme 1. Schematic illustration of core-shell nanostructured PEG-POSS nanoparticles loaded with insulin.
Figure 1. TEM images of pure (a, b, c) and insulin-loaded (d, e, f) PEG-POSS nanoparticles at different magnifications.
Figure 2. FTIR spectra of (a) pristine insulin, (b) pure PEG-POSS nanoparticles, and insulin-loaded PEG-POSS nanoparti-
cles with different amount of insulin of (c) 0.5, (d) 0.9 and (e) 1.3 mg.
Self-Assembled Core-Shell Poly(Ethylene Glycol)-POSS Nanocarriers for Drug Delivery
Copyright © 2011 SciRes. JBNB
Figure 3. Zeta-potential (a) and particle size (b) of pure ()
and insulin-loaded () PEG-POSS nanoparticles with re-
spect to the pH.
Figure 4. Insulin release from insulin-loaded PEG-POSS
nanoparticles produced with an insulin mass of 0.9 ml in gas-
tric pH 2 simulated fluids for 2 hrs followed by additional 16
hrs in intestinal pH 7.4 simulated fluids at 37˚C.
Mt is the amount of insulin released at time t, k is kinetic
constant, and n is diffusion or release exponent constant.
Using the least-squares procedure, n value was estimated
to about 0.87, suggesting anomalous diffusion or non-
fickian diffusion. This finding refers to combination of
both diffusion and erosion controlled rate release. Ac-
cordingly, it is expected that insulin release from PEO-
POSS nanostructured nanoparticles was pH-controlled,
accompanying the swelling of insulin-loaded PEG-POSS
4. Conclusions
We have successfully prepared the pure and insulin-
loaded nanostructured core-shell poly(ethylene glycol)
(PEG)-polyhedral oligosilsesquioxane (POSS) nanopar-
ticles via self-assembly process. TEM analysis demon-
strated that pure and insulin-loaded self-assembled PEG-
POSS nanoparticles were of spherical shape with core-
shell nanostructure, and were well-dispersed and uniform
in size distribution. Such PEG-POSS nanoparticles
showed a good loading capability of hydrophobic drug,
insulin. It was found that insulin was well-protected in-
side PEG-POSS nanoparticles at gastric pH for 2 hrs, and
was released at intestinal pH (pH 6 - 7) where the ab-
sorption and activation of the drug are necessary. As a
result, insulin release from PEG-POSS nanoparticles was
5. Acknowledgements
The authors acknowledge the support of Shinshu Univer-
sity Global COE Program “International Center of Ex-
cellence on Fiber Engineering”. This paper is dedicated
to the first principal, Chotaro Harizuka, on the occasion
of 100th anniversary of Faculty of Textile Science and
Technology, Shinshu University
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