It Distribution
Resume:
Polymer Degradation and Stability ** (****) *** **9
Polycaprolactone microparticles and their biodegradation
D.R. Chen, J.Z. Bei, S.G. Wang*
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received 23 July 1999; accepted 19 August 1999
Abstract
Polycaprolactone (PCL) microparticles were prepared by an emulsi cation-solvent evaporation technique using various stabi-
lizers such as gelatin, hydrophobically modi ed polyacrylamide derivative (PAM) and poly(vinyl alcohol) (PVA). It was shown that
the particle size distribution of PCL microparticles was 13.4 4.7 (mm) in diameter with gelatin as stabilizer. The degradation
behavior of the PCL microparticles was determined and compared with that of PCL lm at pH 7.4 at 37 1 C with and without
lipase. The shape of PCL had no obvious e ect on its degradation rate. It suggested that homogeneous degradation dominated the
process. The degradation rate of PCL microparticles was enhanced in the presence of lipase enzyme. The degree of crystallinity of
PCL microparticles increased with degradation, proving preferential degradation in amorphous domains of the PCL microparticles.
# 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Polycaprolactone; Microparticles; Biodegradation; Crystallinity; Lipase
1. Introduction polymers due to its good drug permeability and bio-
compatibility [1 4].
A microcapsulated drug is one of the prospective drug As drug microparticles, the most important require-
delivery systems because it has obvious advantages, ment is that the matrix material should be biodegraded
such as improving the therapeutic e ect, prolonging the within a suitable period which is compatible with the
biological activity, controlling the drug release rate, drug release rate. In order to clarify the degradation
decreasing the administration frequency, and so on. behavior of the microparticles with drug release, and to
Especially It is especially important for anticancer further control the degradation rate of the polymer,
drugs, which can be target administration to increase pure PCL microparticles were designed as a model of
the drug concentration in the disease area with reduced the matrix. Degradation behaviors of the PCL micro-
toxicity of drug in the healthy area, as well as lowing the particles in vitro with and without lipase were deter-
side e ect of the drug. Therefore the study of micro- mined. The changes of morphology, molecular weight
particle preparation has attracted much interest for the and molecular weight distribution, as well as crystallinity
past decade. Biodegradable polymers, such as polylactide of the PCL microparticles before and after degradation
(PLA), polyglycolide (PGA) and polycaprolactone (PCL) were determined and compared by techniques of SEM,
show good biodegradability and biocompatibility. The GPC and X-ray di raction. The degradation behaviors
materials, as matrix material of microparticles, can be of PCL with di erent shape were also compared. Finally
decomposed into non-toxic and low molecular weight the degradation mechanism of PCL microparticles is
species with release of the drug and then metabolized or discussed.
absorbed by the organism. Therefore, many investiga-
tions have focused on the application of biodegradable
microparticles drugs in recent years. Among them, 2. Experimental
polycaprolactone is one of the widely used biodegradable
2.1. Materials
Polycaprolactone was purchased from Aldrich Che-
* Corresponding author.
mical Company. Gelatin (C.P. grade) was obtained
E-mail address: ******@*******.*** (S.G. Wang).
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0141-3910(99)00145-7
456 D.R. Chen et al. / Polymer Degradation and Stability 67 (2000) 455 459
2.5.2. Molecular weight determination
from Beijing Chaoyang District Xudong Chemical
Molecular weights of the PCL microparticles and
Plant, poly(vinyl alcohol) (PVA) (1700 of polymeriza-
PCL lm were determined by Gel Permeation Chromato-
tion degree and 88 %mol hydrolyzed) was from Beijing
graphy (GPC) technique using a Waters 510 high-
Organic Chemical Plant. Hydrophobically modi ed
performance liquid chromatograph, at a ow rate of 1.0
polyacrylamide derivative (PAM) was synthesized
ml/min through the Shodex GPC KF-800 series column
according to the reported technique [5]. Lipase (A.R.
with chloroform as eluent at 35 C. The weight-average
grade) was obtained from Shanghai Institute of Bio-
chemistry, Chinese Academy of Sciences. All the other molecular weight (Mw) and number-average molecular
chemicals were A.R. grade and used as received. weight (Mn), as well as the molecular weight dispersion
(Mw/Mn) of the PCL were calibrated by standard poly-
2.2. Preparation of PCL microparticles styrene samples.
PCL microparticles were prepared by an emulsi cation-
solvent evaporation technique [6]. Typically, the oil phase
was prepared as 1.0 g of PCL dissolved in 20 ml of
methylene chloride. Then the oil phase was added into
200 ml of 1% gelatin solution (external aqueous phase)
containing 0.05% of Tween-60 and stirred at 1200 rpm
for 1 min to form an O/W emulsion. The emulsion system
was continuously stirred for 2 3 h. Finally, the produced
microparticles were collected by centrifugation, washed
with distilled water and dried by freeze-drying to obtain
free owing powder-like PCL microparticles.
2.3. Preparation of PCL lm
The PCL lm was prepared by solution casting using
chloroform as solvent. 8 %wt of PCL solution was cast
into a poly(tetra uoroethelene) mold at room tempera-
ture. After solvent evaporation in air, the lm was
removed from the mold and dried in vacuum at room
temperature for 24 h. The lm was 0.30 mm in thickness.
2.4. Biodegradation
The biodegradation test was performed at 37 1 C.
150 mg of the PCL microparticles were immersed in pH
7.4 PBS medium with or without 1 mg/ml of lipase. The
test-tube was shaken at regular intervals and the bu er
medium was renewed every 3 days. After predetermined
periods of time the samples were taken out by cen-
trifugation from the bu er, then washed with distilled
water and dried in vacuum at room temperature.
2.5. Measurements
2.5.1. Morphology observation
PCL microparticles were well dispersed in distilled
water and then air-dried onto a piece of aluminum foil.
The samples were coated with gold and then observed
with a Hitachi S-530 scanning electron microscope. The
PCL microparticles were also observed with an
OLYMPUS IMT-2 Inverted light microscope and
photographed by Interference contrast method. Then
the particle size and the particle size distribution of the Fig. 1. E ect of stabilizers on morphology of PCL microparticles. (a)
microparticles were calculated by computer. gelatin; (b) PAM; (c) PVA.
D.R. Chen et al. / Polymer Degradation and Stability 67-200*-***-***-***
2.5.3. Crystallinity measurement think it was caused by di erent properties of the stabilizers.
The crystallinity of PCL microparticles was examined Because of the hydrophilic/hydrophobic balance and e ect
by a D/max-2400 X-ray di ractometer with a CuKa of the hydrogen bond of the gelatin, the stabilizer gelatin
source at room temperature. kept e ective protection from aggregation of the emulsion
microdroplets. As a result, the produced microparticles
were smaller with an even particle size distribution.
3. Results and discussion However PVA turned out to be more hydrophilic, while
PAM was more hydrophobic than gelatin. As a result,
3.1. Preparation of PCL microparticles they could not protect the emulsion microdroplets from
coagulation e ectively. So uneven particle size distribution
The PCL microparticles were prepared by the emulsi- was formed and irregular microparticles were produced.
cation-solvent evaporation technique [6] using gelatin,
PAM or PVA as stabilizers. Morphology of the produced 3.2. E ects on degradation rate of PCL
microparticles was observed by SEM technique (Fig. 1).
It could be seen that all of the microparticles prepared 3.2.1. E ect of sample form
with di erent stabilizer appeared to show similar spherical The changes of molecular weight of the PCL micro-
form. However, in the case of gelatin as stabilizer, the particles and PCL lm with degradation time were
microparticles possessed smooth surface and smaller measured by GPC and are shown in Table 2. By com-
particle size (13.4 4.7 mm in diameter), as well as narrow parison of the changes of weight-average molecular
particle size distribution (Fig. 2). Compared with that weight of both samples of lm and microparticles, it can
using PAM as stabilizer, the particle size distribution of be seen that the weight average molecular weight for
the produced PCL microparticles was not uniform. In both samples decreased with degradation time. The
the case of using PVA as stabilizer, the shape of produced reduction of Mw ( Mw) for the microparticles sample
microparticles changed to be irregular and some of the was a little more than that of the lm sample. This
microparticles have broken. The particle size distribution means that in these conditions both samples had degraded
of the microparticles which were prepared by di erent but the degradation rate of microparticles was a little
stabilizer is shown in Table 1. faster than that of lm.
This result indicated that depending on the stabilizer The molecular weight distribution of the PCL micro-
used, di erent particle size, particle size distribution and particles and PCL lm was calculated from the GPC
morphology of microparticles could be produced. We data as shown in Table 3. The molecular weight dis-
tribution of both lm and microparticles samples
increased; that the microparticles had a little bigger
Table 2
Dependance of change of molecular weight on sample form with
degradation under pH 4.7 at 37 1 C
Mw%a
Degradation time
(weeks)
Film Microparticles
Without lipase Without lipase With lipase
2 3.8 3.0 6.7
4 8.6 9.0 13.0
7 12.8 15.8 27.6
Fig. 2. Particle size distribution of PCL microparticles using gelatin as a
Mw%=(Mw0 Mwt)/Mw0 100; Mw determined by GPC.
stabilizer.
Table 3
Table 1 Comparison of molecular weight distribution of di erent PCL samples
Comparision of particle size distribution of PCL microparticles using with degradation under pH 7.4 at 37 1 C
di erent stabilizers
Sample Lipase Degradation time (weeks)
Sample Stabilizer Concentration Particle size
0 1 3 9
no. of stabilizer (%wt) distribution (mm)
Film Without 1.58 1.60 1.60 1.63
1 Gelatin 1.00 13.4 4.7
Microparticles Without 1.58 1.62 1.62 1.64
2 PAM 0.25 18.5 17.4
With 1.58 1.60 1.66 1.84
3 PVA 3.00 20.3 19.1
458 D.R. Chen et al. / Polymer Degradation and Stability 67 (2000) 455 459
molecular weight distribution explained the similar low degradation rate due to higher crystallinity. In the
degradation rule as shown in Mw. presence of lipase, the PCL microparticles appeared to
be slightly porous on the surface only after 3 weeks of
3.2.2. E ect of lipase degradation [Fig. 3 (c)]. After 9 weeks channels and
Morphology of PCL microparticles before and after pores could be observed on the surface of the micro-
degradation with and without lipase was compared. It particles [Fig. 4 (b)]. This demonstrates that the degrad-
can be seen that in the condition of without lipase, ation of PCL microparticles is accelerated by lipase.
the morphology of PCL microparticles sample was The changes of molecular weight of PCL micro-
unchanged even when degraded for 9 weeks [Fig. 3(a) particles after 5 weeks of degradation with and without
and (b)]. It revealed that PCL microparticles possessed a lipase were determined by GPC and shown in Fig. 5. It
can be seen that without lipase the number-average
molecular weight (Mn) reduced with degradation from
30 000 [Fig. 5 (a)] to 26 700 [Fig. 5 (b)], but in the pre-
sence of lipase, the Mn reduced to 22 800 [Fig. 5 (c)], and
the peak of the GPC curve became much wider. The
degree of degradation was estimated from the reduction
of molecular weight and the change in molecular weight
distribution. After 5 weeks of degradation the reduction
rate of molecular weight of PCL microparticles in the
case of with lipase was 24%, which is much faster than
that without lipase (only 11%). The change of mole-
cular weight distribution is shown in Table 3. The
molecular weight distribution of PCL microparticles
was also wider when lipase was introduced. This is more
evidence to demonstrate that degradation rate of PCL
Fig. 4. Comparison of morphology of PCL microparticles: (a) original
Fig. 3. Morphology changes of PCL microparticles with degradation PCL microparticles by interference contrast microscope; (b) SEM
under pH 7.4 at 37 1 C: (a) original; (b) after 9 weeks without lipase; photograph of PCL microparticles degraded for 9 weeks under pH 7.4
at 37 1 C.
(c) after 3 weeks with lipase.
D.R. Chen et al. / Polymer Degradation and Stability 67-200*-***-***-***
Table 4
The change of crystallinity of PCL microparticles with degradation
under pH 7.4 at 37 1 C
Crystallinitya
Degradation time Lipase
(weeks) Xc 0 51
10 Without 51
10 With 62
a
Determined by X-ray di raction.
PCL was a partly crystalline polymer, the crystalline
structure leading to the striped appearance. Since prefer-
ential degradation of the PCL microparticles occurred
in amorphous domains of the microparticles, as a result
the crystalline part of the particles remained.
The degree of crystallinity of the PCL microparticles
Fig. 5. GPC spectra of PCL microparticles before and after degrada-
tion under pH 7.4 at 37 1 C: (a) original; (b) after 5 weeks without was measured by X-ray di raction as shown in Table 4.
lipase; (c) after 5 weeks with lipase.
The degree of crystallinity for the original PCL micro-
particles was 51% [8], but after degradation for 9 weeks, the
crystallinity of the remained microparticles was unchanged
for those without lipase, while it increased to 62% for
microparticles has been accelerated by lipase. It also
those with lipase. This conformed to the results shown
proved the biodegradability of PCL and the speci c
in Fig. 4 that the degradation of PCL microparticles
enzymatic action of the lipase [7].
happened rst in the amorphous area of the micro-
particles, as a result the crystallinity of the residual
3.3. Degradation mechanism of PCL
particles increased. This is evidence that the degradation
rate of PCL could be accelerated by reducing the crys-
By comparison of degradation rate of both lm-like
tallinity of the polymer as shown in PCL copolymers [9].
and microparticles-like PCL, it can be seen that
although the speci c area of the microparticles was
much larger than that of lm (about 67 times), the
Acknowledgement
degradation rate of the PCL lm was not obviously
di erent from that of microparticles. It revealed that
The authors are indebted to the High Technology
surface area had no great in uence on the degradation
Research and Development Program of China for
rate of the PCL sample. Thus the homogeneous degra-
nancial support (Project No. 863-715-002-0210).
dation of the PCL polymer in the degradation process
can be suggested.
The Interference contrast picture of the original PCL
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