Journal of Microencapsulation, February 2006; 23(1): 79–90
Characteristics of vitamin C encapsulated
tripolyphosphate-chitosan microspheres as affected by
chitosan molecular weight
K. G. DESAI1, C. LIU1, & H. J. PARK1,2
1
2
School of Life Sciences and Biotechnology, Korea University, Seoul, South Korea and
Clemson University, Clemson, SC, USA
(Received 17 January 2005; in final form 10 May 2005)
Abstract
In this paper, the effect of chitosan molecular weight on the characteristics (size, encapsulation
efficiency, zeta potential, surface morphology and release rate) of vitamin C encapsulated tripolyphosphate cross-linked chitosan (TPP-chitosan) microspheres. The molecular weight of chitosan had a
noticeable influence on the size, encapsulation efficiency, zeta potential, surface morphology and
controlled release behaviour of the vitamin C encapsulated TPP-chitosan microspheres. The mean
particle size and encapsulation efficiencies of TPP-chitosan microspheres were 3.1, 4.9 and 6.7 mm and
67.25, 60.43 and 52.74% for the microspheres prepared using low, medium and high molecular weight
chitosan, respectively. All the TPP-chitosan microspheres (low, medium and high molecular weight)
had positive charge on their surface. The zeta potential of the TPP-chitosan microspheres prepared
using low, medium and high molecular weight chitosan was 41.25, 40.84 and 39.13 mV, respectively.
The particle sizes of TPP-chitosan microspheres increased with increases in chitosan molecular weight.
Molecular weight of chitosan did not affect significantly the % yield of TPP-chitosan microspheres
prepared by spray-drying. The influence of chitosan molecular weight on the surface morphology of
vitamin C encapsulated TPP-chitosan microspheres was examined by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). It was observed that, as the molecular weight of
chitosan increases, TPP-chitosan microspheres with uniform spherical shape could be obtained. The
physical state of vitamin C (amorphous or crystalline) in TPP-chitosan matrix was studied by X-ray
diffraction (XRD) and it was found that vitamin C is dispersed at the molecular level (amorphous) in
the TPP-chitosan matrix. Release rate of the vitamin C from TPP-chitosan microspheres was
significantly affected by the chitosan molecular weight. The release rate decreased with increase in the
chitosan molecular weight. The release of vitamin C from TPP-chitosan microspheres followed
Fick’s law of diffusion.
Keywords: Vitamin C, chitosan, molecular weight, encapsulation, microspheres, tripolyphosphate,
controlled release
Correspondence: Hyun Jin Park, School of Life Sciences and Biotechnology, Korea University, 1, 5-Ka, Anam-Dong, Sungbuk-ku,
Seoul-136-701, South Korea. Tel: 82-2-3290-3450. Fax: 82-2-953-5892. E-mail: hjpark@korea.ac.uk
ISSN 0265-2048 print/ISSN 1464-5246 online ß 2006 Taylor & Francis
DOI: 10.1080/02652040500435360
80
K. G. Desai et al.
Introduction
The food industry is taking advantage of the technology of controlled release for food
additives including flavouring agents (flavour oils, spices, seasoning), sweeteners, colours,
nutrients (vitamins, amino acids, minerals), essential oils, acids, salts, bases, anti-oxidants,
anti-microbial agents, preservatives and ingredients with undesirable flavours (Kirby 1991,
Augustin et al. 2001, Desai and Park 2005). Controlled release helps to overcome both
the ineffective utilization and the loss of food additives during the processing steps
(Barbosa-Cánovas and Pothakamury 1995). The most commonly used method to achieve
controlled release in the food industry is microencapsulation (Kirby 1991, Augustin et al.
2001). Microencapsulation is defined as the technology of packaging solid, liquid
or gaseous materials in miniature sealed capsules that release their contents at controlled
rates under the influence of certain stimuli. The advantages of controlled release of food
ingredients are:
(a) The active ingredients are released at controlled rates over prolonged periods of time;
(b) Loss of ingredients, such as vitamins and minerals, during processing and cooking
can be avoided or reduced; and
(c) Reactive or incompatible components can be separated (Taylor 1983, Shahidi and
Han 1993).
Vitamin C is an essential nutrient involved in many physiologic functions. Nearly all
species of animals synthesize vitamin C and do not require it in their diets, but humans
cannot synthesize the vitamin C (Farajzadeh and Nagizadeh 2003). For practical purposes,
raw citrus fruits are good daily sources of vitamin C, since appreciable amounts in other
foods can be destroyed when cooked, in the presence of air and when in contact with traces
of copper (Kirby 1991, Farajzadeh and Nagizadeh 2003). Therefore, vitamin C is widely
used in various types of foods as a vitamin supplement and as an anti-oxidant (Jacobs et al.
2001). Vitamin C is distributed throughout the body with high concentrations in specific
tissues, such as lungs, white blood cells, pancreas, thyroid, spleen, brain, etc. This is an
important anti-oxidant that may reduce the risk of cancer by neutralizing reactive oxygen
species or other free radicals that can damage DNA (Jacobs et al. 2001). Since vitamin C
is responsible for many biochemical functions in the human body, it is mainly supplied by
the diet. In addition, vitamin C is also utilized in food or food additives and has been shown
to decrease LDL cholesterol in the plasma of hyper-lipidemia patients (Snyder and Malloy
1998, Jeserich et al. 1999, Mosinger 1999). Hence, microencapsulation and controlled
release of vitamin C would broaden its application range in food and pharmaceutical
industries.
The techniques used for microencapsulation are spray-drying, coating, extrusion,
liposome entrapment, coacervation and freeze drying. Of these methods, spray-drying is
the most commonly used method in the food industry. This is because the process is
economical, flexible in that it offers substantial variation in the encapsulation matrix,
adoptable to commonly used processing equipment and produces particles of good quality
(Shahidi and Han 1993, Barbosa-Cánovas and Pothakamury 1995, Augustin et al. 2001).
The spray-drying method is a one-stage continuous process, easy to scale-up and spraydrying production costs are lower than those associated with most other methods of
microencapsulation (Taylor 1983, Desai and Park 2005). The microcapsule sizes prepared
by the spray-drying method ranged from microns to several tens of microns and had a
relatively narrow distribution. Hence, it is a method of choice for the microencapsulation
of food ingredients in the food industry.
Preparation of chitosan based controlled release microspheres for vitamin C
81
The food additive (active agent) can be encapsulated using carbohydrates, gums, lipids,
proteins, polymers such as poly(vinyl) acetate (PVA), a fibre matrix made of polymers and/or
liposomes (Barbosa-Cánovas and Pothakamury 1995). Because few suitable polymers have
been approved for use in foods, certain food materials can be modified to increase their
porosity and to alter other characteristics, thus enabling their use as coating materials
in microencapsulation. Chitosan is a hydrophilic, biocompatible and biodegradable
polysaccharide of low toxicity which in recent years has been used for development of
oral drug delivery systems (Hejazi and Amiji 2003). Since it is a well-known dietary food
additive, it was selected as an appropriate encapsulating material for the vitamin C.
However, the encapsulating ability of chitosan varies with its molecular weight. Therefore,
in continuation of an ongoing programme of research on microencapsulation of vitamin C
in a chitosan-based matrix (Desai and Park 2005), this paper describes the effect chitosan
molecular weight has on the characteristics (size, encapsulation efficiency, zeta potential,
surface morphology and release rate) of thus prepared vitamin C encapsulated TPP-chitosan
microspheres.
Materials and methods
Materials
Vitamin C (99.5% purity) was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan).
Chitosan (low, medium and high molecular weight) and tripolyphosphate were purchased
from Sigma-Aldrich Chemie (Steinheim, Germany). All other chemicals were of analytical
grade and used as received. Ultrapure water (Millipore, USA) water was used throughout
the study.
Methods
Determination of molecular weight of chitosan. The average molecular weight of chitosan
was determined by a batch mode method using a multi-angle laser light scattering
(MALLS) photometer according to Chen and Tsaih (1998). Every sample was repeated
five times in an identical manner. The obtained values are presented in Table I.
Determination of degree of deacetylation of chitosan. The % N-deacetylation of chitosan was
determined by the 1NMR spectroscopy method (Hirai et al. 1991, Lavertu et al. 2003).
The obtained values are presented in Table I.
Table I. Viscosity, molecular weight and degree of deacetylation of chitosan used for the microencapsulation
of vitamin C.
Chitosan
Low Mw
Medium Mw
High Mw
Viscosity
(cps)
Molecular
weight
DOD
(%)
20
200
800
7.270 105
1.336 106
1.743 107
84.94
82.10
81.29
Brookfield viscosity provided by Aldrich.
Degree of N-deacetylation of chitosan.
82
K. G. Desai et al.
Preparation of vitamin C encapsulated TPP-chitosan microspheres. Microencapsulation
of vitamin C in a TPP-chitosan matrix was achieved by the spray-drying technique as
described previously (Desai and Park 2005). Briefly, the required volume (usually 250 ml) of
chitosan (low or medium or high molecular weight) solution (1% w/v) was prepared using
aqueous acetic acid solution (0.5% v/v). Vitamin C (2.5 g) was dissolved in 10 ml of
ultrapure water (Millipore, USA). The vitamin C solution was then added to the chitosan
solution and homogenized at 8000 rpm for 10 min using a Young Ji HMZ 20DN stirrer
(Hana Instruments). To the above prepared mixture, 10 ml of 1% w/v aqueous solution of
TPP was added dropwise under constant stirring at 8000 rpm for 20 min using a Young Ji
HMZ 20DN stirrer. TPP was used as a cross-linking agent. Spray-drying was then
performed using a SD-04 spray drier (Lab Plant, UK), with a standard 0.5 mm nozzle. Inlet
temperature, liquid flow rate and compressed spray air flow were set at 170 C, 2 ml min1
and 10 l min1, respectively.
Encapsulation efficiency. Twenty five milligrams of vitamin C encapsulated TPP-chitosan
microspheres were dissolved in 100 ml 0.1 N HCl. The sample was passed through a 0.2 mm
filter (Millipore, USA) and then vitamin C content was assayed by measuring the
absorbance at 244 nm (lmax of vitamin C in 0.1 N HCL) after suitable dilution using an UV
spectrophotometer (Shimadzu 1601PC, Japan). Experiments were performed in triplicate
(n ¼ 3) and encapsulation efficiencies were calculated as follows.
Encapsulation efficiency (%) =
Calculated vitamin C concentration
100
Theoretical vitamin C concentration
ð1Þ
Measurement of particles size. TPP-chitosan microspheres prepared by spray-drying
exhibited quick swelling in liquid medium and, hence, sizes could not be determined
using a laser diffraction technique in a particle size analyser. Therefore, the particle size was
determined by microscopy. Briefly, in each determination, 5 mg of chitosan microspheres
were taken on a glass slide and sizes of 100 particles were measured by using biological
microscope (Olympus, Japan). Mean size of 100 particles was considered as size of the
microspheres from one determination and it was repeated for three times and then mean
particle size of the microspheres was calculated from three independent determinations.
Zeta potential. TPP-chitosan microspheres concentration of 0.3% w/v was made
by dispersing microspheres in KCl solution (pH 7). The zeta potential of TPP-chitosan
microspheres was recorded using laser doppler anemometry (Malvern Zetasizer, UK). Zeta
potential measurements were done in triplicate.
Scanning electron microscopy. The surface morphology of vitamin C encapsulated
TPP-chitosan microspheres was examined by means of an Hitachi (Japan) scanning
electron microscope. The powders were previously fixed on a brass stub using double-sided
adhesive tape and then were made electrically conductive by coating, in a vacuum, with a
thin layer of platinum (3–5 nm), for 100 s and at 30 W. The pictures were taken at an
excitation voltage of 15 kV and a magnification of 8.0, 1.2, 3.0 or 4.5 k.
Transmission electron microscopy. The shape of vitamin C encapsulated TPP-chitosan
microspheres was examined by TEM. TPP-chitosan microspheres sample was added on a
formvar coated grid. The shape of the microspheres was examined using a Philips TECNAI
12 transmission electron microscope (The Netherlands) at an accelerating voltage of 120 kV.
Preparation of chitosan based controlled release microspheres for vitamin C
83
XRD. X-ray powder diffraction patterns of pure vitamin C and vitamin C encapsulated
TPP-chitosan microspheres prepared using different molecular weight chitosan were
obtained at room temperature using a Philips X’ Pert MPD diffractometer (Philips,
The Netherlands), with Co as anode material and graphite monochromator, operated at a
voltage of 40 kV. The samples were analysed in the 2 angle range of 2–60 and the process
parameters were set as: scan step size of 0.025 (2), scan step time of 1.25 s and time
of acquisition of 1 h.
In vitro release studies. The in vitro release of vitamin C from TPP-chitosan microspheres
was determined using a USP model dissolution apparatus (TW-SM, Wooju Scientific, Co.,
Korea). In order to suspend the microspheres in the dissolution medium, microspheres
equivalent to 25 mg of vitamin C were taken into a cellulose dialysis bag (previously soaked
in dissolution medium) containing 3 ml of dissolution medium and tied to the paddle.
The in vitro release studies of vitamin C were carried out at a paddle rotation of 100 rpm
in 500 ml of phosphate buffer (pH 7.4 and 25 C). In order to facilitate compatible
environment for released vitamin C, the temperature of the dissolution medium was
maintained at 25 0.1 C until the end of the study. An aliquot of the release medium (5 ml)
was withdrawn through a sampling syringe attached with a 0.2 mm filter at pre-determined
time intervals (0.5, 1, 2, 3, 4, 5 and 6 h) and an equivalent volume of fresh dissolution
medium which was pre-warmed at 25 C was replaced. Collected samples were analysed for
vitamin C content by measuring the absorbance at 265 nm using UV spectrophotometer
(Shimadzu 1601PC, Japan). In vitro release studies were performed in triplicates (n ¼ 3)
in an identical manner.
Results and discussion
Microencapsulation of vitamin C in low, medium and high molecular weight
chitosan microspheres
Chitosan, a natural linear biopolyaminosaccharide, is obtained by alkaline deacetylation
of chitin, which is the second abundant polysaccharide next to cellulose. Properties such as
biodegradability, low toxicity and good biocompatibility make it suitable for use in food and
pharmaceutical formulations. Chitosan microspheres are the most widely studied drug
delivery systems for the controlled release of drugs viz. antibiotics, anti-hypertensive agents,
proteins, peptides and vaccines (Hejazi and Amiji 2003). Due to the easy availability of free
amino groups in chitosan, it carries a positive charge and, thus, in turn reacts with many
negatively charged surfaces/polymers. Chitosan can form gels by interacting with different
types of divalent and polyvalent anions. Using this property, many researchers have prepared
cross-linked chitosan microspheres or films to sustain the release of drugs.
The word chitosan refers to a large number of polymers, which differs in their degree
of N-deacetylation (40–98%) and molecular weight (50 000–2 000 000 Da). These two
characteristics are very important to the physico-chemical properties of the chitosans and,
hence, they have a major effect on the biological properties (Sannan et al. 1976). The
molecular weight of chitosan has a remarkable influence on the size, encapsulation
efficiency, zeta potential, surface morphology and controlled release behaviour of drugloaded microspheres (Hejazi and Amiji 2003). It is, therefore, necessary to establish
the effect of chitosan molecular weight on the characteristics of thus prepared
vitamin C encapsulated TPP-chitosan microspheres. Hence, this paper mainly focused
on the influence of chitosan molecular weight on the properties of vitamin C encapsulated
84
K. G. Desai et al.
TPP-chitosan microspheres. Such information will help a food formulator while choosing
the appropriate chitosan for the encapsulation of vitamin C.
Effect of chitosan molecular weight on size, % yield, encapsulation efficiency and
zeta potential of vitamin C encapsulated TPP-chitosan microspheres
Three chitosan materials (low, medium and high molecular weight) were used in the present
study. The effect of molecular weight of chitosan on the size, % yield, encapsulation
efficiency and zeta potential of TPP-chitosan microspheres is presented in Table II. The
results of the present study indicated that the size, encapsulation efficiency and zeta potential
were remarkably influenced by the molecular weight of chitosan. For instance, the size of the
vitamin C encapsulated TPP-microspheres prepared using low, medium and high molecular
weight chitosan was 3.1, 4.9 and 6.7 mm, respectively. As the molecular weight of chitosan
increases, size of the TPP-chitosan microspheres increased. It is well-known that the
viscosity of the polymer solution increases with increase in its molecular weight. Therefore,
under the same preparation conditions, the droplets formed from the higher viscosity
chitosan solution will be larger in size and result in larger microspheres. Pavenetto et al.
(1993) prepared PLA microspheres by a spray-drying method. At same polymer
concentration (1.25%), the particles size was similarly increased with polymer molecular
weight. In the preset study, the encapsulation efficiencies of the TPP-chitosan microspheres
ranged from 52.74–67.25%. Encapsulation efficiency decreased with increase in the
molecular weight of chitosan. This is in agreement with the report of Filipovic-Grcic et al.
(2003), where the encapsulation efficiency of spray-dried chitosan microspheres decreased
for carbamazepine with increases in chitosan molecular weight. The influence of chitosan
molecular weight on the surface charge of vitamin C encapsulated TPP-chitosan
microspheres was determined by measuring the zeta potential. The zeta potential
of the vitamin C encapsulated TPP-chitosan microspheres did not vary significantly with
increase in the molecular weight of chitosan. However, zeta potential of vitamin C
encapsulated TPP-chitosan microspheres decreased slightly with an increase in the
molecular weight of chitosan.
Effect of chitosan molecular weight on the surface morphology of
vitamin C encapsulated TPP-chitosan microspheres
The surface morphology of vitamin C encapsulated TPP-chitosan microspheres prepared
using low, medium and high molecular weight chitosan is presented in Figure 1. It can be
emphasized that non-cross-linked chitosan microspheres did not exhibit a smooth surface
(Figure 1a). Non-cross-linked chitosan microspheres had wrinkles on their surface. Vitamin
C encapsulated TPP-chitosan microspheres prepared using low, medium and high
molecular weight chitosan exhibited a smooth surface. This can be clearly observed from
Table II. Effect of chitosan molecular weight on particle size, encapsulation efficiency, % yield and zeta potential
of vitamin C encapsulated TPP-chitosan microspheres (M S.D., n ¼ 3).
Molecular weight
of chitosan
7.270 105
1.336 106
1.743 107
Mean particle
size (mm)
Encapsulation
efficiency (%)
Yield (%)
Zeta
potential (mV)
3.1 0.2
4.9 0.4
6.7 0.3
67.25 2.6
60.43 1.5
52.74 1.3
59.26 1.4
60.56 1.8
58.46 2.1
41.25 3.4
40.84 1.3
39.13 1.5
Preparation of chitosan based controlled release microspheres for vitamin C
(a)
(b)
(c)
(d)
85
Figure 1. Scanning electron microscopic pictures of non-cross-linked non-loaded chitosan microspheres (a) and vitamin C encapsulated TPP-chitosan microspheres prepared using low (b), medium
(c) and high (d) molecular weight chitosan (volume of 1% w/v TPP solution employed for cross-linking
process is 10 ml).
the SEM pictures (Figure 1b, c and d). However, TPP-chitosan microspheres prepared
using medium and high molecular weight chitosan depicted more uniform spherical shape
(Figure 1c and d) than those prepared using low molecular weight chitosan (Figure 1b). In
other words, with increase in the viscosity of the chitosan, TPP-chitosan microspheres with
uniform spherical shape could be prepared by a spray-drying method. Some of the porous
TPP-chitosan microspheres were formed from low molecular weight chitosan (Figure 1d).
Shape of the vitamin C encapsulated TPP-chitosan microspheres was further confirmed by
TEM and the pictures are presented in Figure 2. It can also be noticed that uniform
spherical shape microspheres could be obtained from even non-cross-linked chitosan
(Figure 2a). In the case of TPP-chitosan microspheres, medium and high molecular weight
chitosan microspheres (Figure 2c and d) were having a more uniform spherical shape than
the low molecular weight chitosan microspheres (Figure 2b).
Physical state of vitamin C in TPP-chitosan matrix
The physical state of the core material (crystalline or amorphous) in the polymeric matrix
can be studied by XRD studies (Filipovic-Grcic et al. 2003). Therefore, in this paper,
86
K. G. Desai et al.
(a)
(b)
(c)
(d)
Figure 2. Transmission electron microscopic images of non-cross-linked non-loaded chitosan
microspheres (a) and vitamin C encapsulated TPP-chitosan microspheres prepared using low (b),
medium (c) and high (d) molecular weight chitosan (volume of 1% w/v TPP solution employed
for cross-linking process is 10 ml).
the physical state of the vitamin C (crystalline or amorphous) in TPP-chitosan matrix was
studied by XRD studies. The X-ray diffractograms of pure vitamin C and vitamin C
encapsulated TPP-chitosan microspheres prepared using different molecular weight
chitosan are presented in Figure 3. From Figure 3(a), it is evident that vitamin C exhibited
characteristic crystalline peaks at 2 of 10.3, 14.09, 17.3, 25.24, 40.29, 48.19 and 54.3,
indicating the presence of crystalline vitamin C. The characteristic crystalline peaks of
vitamin C disappeared after microencapsulation in the TPP-chitosan matrix prepared using
low, medium and high molecular weight chitosan (Figure 3b, c and d), but instead only
placebo TPP-chitosan microspheres pattern was obtained. This indicates that vitamin C
Preparation of chitosan based controlled release microspheres for vitamin C
87
d
c
b
a
2θ
Figure 3. XRD spectra of pure vitamin C (a) and vitamin C encapsulated TPP-chitosan
microspheres prepared using low (b), medium (c) and high (d) molecular weight chitosan (volume
of 1% w/v TPP solution employed for cross-linking process is 10 ml).
is dispersed at the molecular level (amorphous form) in the TPP-chitosan matrix and, hence,
no crystals were found in the vitamin C encapsulated TPP-chitosan microspheres.
Effect of chitosan molecular weight on the release rate of vitamin C from
TPP-chitosan microspheres
The effect of chitosan molecular weight on the release rate of vitamin C from TPP-chitosan
microspheres having the same cross-linking density is depicted in Figure 4. It is evident
that the TPP-chitosan microspheres prepared using low molecular weight chitosan
(i.e. 7.270 105) exhibited a higher vitamin C release rate than those prepared using
medium (1.336 106) and high (1.743 107) molecular weight chitosan. These results are
in agreement with the reports of Al-Helw et al. (1998). Although the volume of TPP solution
(10 ml) used in all the preparations for cross-linking reaction was the same, the vitamin C
release rates were remarkably different. The chain length between cross-links in the higher
molecular weight chitosan is expected to be quite long, whereas the lower molecular weight
chitosan possesses smaller free chains. Thus, in the microspheres with the longer chain
segments can be held responsible for the slower release rate of vitamin C from the higher
molecular weight chitosan microspheres. In addition, lower release rates of vitamin C
with increase in the molecular weight of chitosan can also be attributed to the increased
viscosity of the chitosan. This behaviour was predictable, taking into the account the
relationship between the molecular weight and viscosity (Adusumilli and Bolten 1991).
By increasing the viscosity of the chitosan, the diffusion of the vitamin C through the gel into
88
K. G. Desai et al.
the release medium was retarded. Chitosan with higher molecular weight has more acetyl
groups on molecular chain. The presence of acetyl groups leads to significant decrease in
charge density as well as its intra-molecular hydrogen bonds and makes the chitosan
molecules exist in an extended form. So, high deacetylated degree molecules tend
100
Cumulative release (%)
80
60
40
Low molecular weight
Medium molecular
weight
High molecular weight
20
0
0
1
2
3
4
5
6
Time (h)
Figure 4. The influence of chitosan molecular weight on the release rate of vitamin C from TPPchitosan microspheres (volume of 1% w/v TPP solution employed for cross-linking process is 10 ml).
120
y = 45.919x - 10.27
R2 = 0.9641
Cumulative release (%)
100
y = 44.03x - 13.827
R2 = 0.9828
80
y = 40.991x - 14.513
60
R2 = 0.9848
40
Low molecular weight
20
Medium molecular weight
High molecular weight
0
0
0.5
1
1.5
Time
2
2.5
3
(h1/2)
Figure 5. Higuchi plot of vitamin C encapsulated TPP-chitosan microspheres prepared using
different (low, medium and high) molecular weight chitosan.
Preparation of chitosan based controlled release microspheres for vitamin C
89
to be a random coil in the solution, which will result in less entanglement and weaken the
structure to hold the vitamin C. Therefore, microspheres prepared from higher deacetylated
degree chitosan were higher in release percentage than those prepared with lower
deacetylated degree chitosan.
Mechanism of vitamin C release from TPP-chitosan microspheres
In the present study, an effort has been made to understand the mechanism of vitamin C
release from chitosan microspheres prepared using different molecular weight chitosan. In
order to understand the release mechanism, the release study data obtained were fit to the
Higuchi equation (Bravo et al. 2004).
Q ¼ kH t 1=2
ð2Þ
where Q is the amount of vitamin C release at time t, kH the Higuchi rate constant.
The dissolution data were plotted as the percentage of vitamin C release against the square
root of time (see Figure 5). Linearity was observed with the plots since the correlation
coefficient (R2) ranged from 0.9641–0.9848. However, good correlation (R2) 0.9848
was observed with high molecular weight chitosan than those of medium (0.9828) and
low (0.9641) molecular weight chitosan. This indicates that the release of vitamin C from
TPP-chitosan microspheres followed Fick’s law of diffusion.
Conclusions
The influence of chitosan molecular weight on the characteristics of vitamin C encapsulated
TPP-chitosan microspheres was thoroughly studied. The study indicated that molecular
weight of chitosan had a noticeable influence on the size, encapsulation efficiency, zeta
potential, surface morphology and release rate of the TPP-chitosan microspheres. The
particle size increased with increase in the molecular weight of chitosan. Encapsulation
efficiencies and release rate decreased with increase in the molecular weight of the chitosan.
The vitamin C could be homogeneously dispersed (amorphous form) in the TPP-chitosan
matrix by the spray-drying method. Vitamin C release from TPP-chitosan microspheres
followed Fick’s law of diffusion.
Acknowledgements
This study was supported by a grant of the Korea Health 21 R & D Project, Ministry
of Health & Welfare, Republic of Korea (A050376).
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