Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
Abstract
To minimize the agro-environmental pollution, completely biodegradable
potato starch graft polymethylacrylate co graft polyvinyl alcohol (St-g-PMA-g-
PVA) based formulation has been prepared under microwave irradiation.
Successful grafting of poly(vinylalcohol) onto potato starch graft
poly(methylacrylate) backbone was confirmed with FTIR, SEM, TGA and DSC.
Present study deals with formulation of St-g-PMA-g-PVA based agrochemical
(urea) delivery system for their controlled release. Formulation characteristics like
entrapped efficiency of urea, equilibrium water absorbency and diffusion rate of
urea loaded hydro gel was studied. The introduction of hydrophilic PMA-PVA
content increase the swell ability of starch matrix and the release rate of urea
from loaded hydro gels could be controlled by adjusting the graft efficiency.
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
1. Introduction
Controlled drug delivery technology has now emerged as a truly
interdisciplinary science aimed at improving human health and received attention
in expression of a growing awareness for that substances ranging from drugs to
agricultural chemicals which are often excessively toxic and sometimes futile
when applied by conventional method, thereby preventing any adverse effect
associated with traditional drug administration and deliver the drug to specific
sites for a preferred extent.
Over the past decades carbohydrates biodegradable graft copolymers
have been explored to convince the necessities of particular sector of polymer
industry for drug delivery [1].
Despite of several polymers being used in the preparation of drug deliver,
natural polymers give the impression of obvious choice due to their excellent
biocompatibility low toxicity high enzymatic degradability and unique mechanical
properties [2-5]. On the other hand synthetic polymers provide many desired
advantageous properties with wide choice availability thus the combination of
natural and synthetic polymers may provide mechanical stability and biological
acceptability, acquiring from synergistic properties of both materials for controlled
drug delivery [6-9]. In order to achieve this, the properties of natural and synthetic
polymers have been modified by grafting, blending and other means [10-13]. Grafting of vinyl monomers onto natural polymers has been widely accepted [14-16]. Among the various chemical and physical combination method, grafting have
practical and academic interest for CR of drugs because they provide a
convenient route for the modification of properties to meet specific needs.
Recently hydrophilic starch graft copolymers with high swell ability have
been broadly used to formulate a CR device for highly water soluble drugs to
slow down the release of agrochemical and nutrients in agricultural application
[17-20]. Such graft copolymers showed superior performance over the
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
conventional individual polymers for CR device and consequently the range of
application has grown rapidly for such class of materials.
Among the various agrochemicals nitrogen is the most vital nutrient for
crops. Urea is widely used because of its high nitrogen content and low cost
production but, the high solubility of urea causes large economic and resource
losses along with environmental pollution [21-24]. Controlled release technique
could effectively resolve these problems. The delivery of agrochemicals by CR
formulation offers with economical advantage. A large amount of work has been
done and method encapsulation various agrochemicals with in modified starch
matrices [25-26].
Early report showed that controlled release of various herbicides such as
simazine, 2, 4, 5 trichlorophenoxy acetic acid, triazine and furan by using
polymeric matrices have been studied [27-30]. Polymeric metrics such as corn
starch, St-g-poly butyl acrylate [6] and alginate gel have been used for thiram
release [31]; St-g-polylactide, [7] and St-g-polyacrylic acid [17] have been used
for urea release. Thus, grafting of synthetic monomers onto Starch backbone
constitutes a powerful means for improving starch properties.
In the present work an attempt of grafting polyvinyl alcohol onto St-g-PMA
matrix made to develop a delivery system to enhance the mechanical strength of
natural graft copolymers and to overcome the biological draw backs of synthetic
polymers. These composites were formulated into hydrogel beads and
subsequently loaded with agrochemicals.
2. Experimental
2.1. Material and Method
Domestic microwave oven model no LG Intellocook TM MS-1947 C was
used for the synthesis having 2450 MHz microwave frequency. Distilled water
was used throughout the study. Urea used in this experiment purchased from
Merck, India, is of industrial grade. P-dimithylamino benzaldehyde, hydrochloric
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
acid and other solvents were of analytical grade. The St-g-PMA was previously
prepared as described in chapter second used as raw material for the synthesis
of St-g-PMA-g-PVA matrix.
The encapsulating matrix, St-g-PMA-g-PVA was synthesized and
characterized as describe in chapter fourth. The starch surface was firstly
modified by reacting the –OH groups on the starch with poly(methylacrylate) and
then the further grafting of poly(vinyl alcohol) onto St-g-PMA-g-PVA surface in
presence of K2S2O8 as initiator under influence of microwave irradiation. After
completion of the reaction the untreated PVA was washed with water and dried
till the constant weight under vacuums ovens at 40 0C. Small rounded beads
were cut from this matrix using a circular whole cutter.
These rounded beads were used for the controlled release of agrochemical
encapsulated within the matrix.
2.2. Swelling Equilibrium
A weighed quantity of the composite matrix was immersed in distilled water at
room temperature to reach equilibrium. The swelled samples were taken out from
the water and wiped with filter paper to remove the excess water. The equilibrium
water absorbency (Qev) of matrix was determined by weighing the swollen
samples. The Qev of matrix was calculated using following equation:
Qev (g/g) = M2-M1
M1
Where M2 is the weight of swelled samples and M1 is the weight of dried
samples. Qev expressed in gram/g.
2.3. Encapsulation of hydro gel beads
Encapsulation of urea was carried out by immersing accurately weighed
quantity of prepared beads (on dry basis) in saturated solution of urea at room
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
temperature to reach swelling equilibrium. After attaining equilibrium, the swollen
beads were taken out and the liquid on the samples surface was absorbed by a
filter paper. The water was evaporated slowly at 40 0 C over three days. Before
the release experiment the samples were washed with water rapidly in order to
remove the urea exposed onto surface of beads.
2.4. Encapsulation Efficiency
Total weight percentage of encapsulated material within the matrix
represents the encapsulation efficiency of the matrix. To determine the actually
encapsulated amount of urea, the samples were weighed and wash with 20 ml of
water to remove the excessively surface adhered urea. Then urea content in
water was measured spectrophotometrically at 420 nm [32]. The encapsulation
efficiency was calculated according to the given equation:
EE (%) = [1 - W2 ] X100% [W0xC]
Where W0 is weight of loaded urea samples, W2 is the urea exposed on
surface of the hydrogel and C is the urea content of the hydrogel calculated from
the feed composition.
2.5. Urea Release study
In vitro release of urea from St-g-PMA-g-PVA hydrogels were studied by
keeping dried and loaded 200 mg of samples in 500 ml distill water at 25 0C. Two
ml of solution were withdrawn at regular interval and same volume of water was
added to make the volume constant. The amount of urea released was
measured by UV-spectrophotometer at 420 nm [32].
2.6. Surface Morphology
The surfaces of the polymeric hydrogels prier and after loading with urea
were observed with SEM. Fig (2) shows the SEM pictures of the urea loaded gel
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
matrix. It is obvious from this fig (1) that grafting of polyvinyl alcohol was
uniformly onto grafted starch backbone. SEM of St-g-PMA-g-PVA gel revealed
that grafting of PVA and PMA led to physical and chemical cross linking; as well
defined pores are visible in these micrographs. It is supposed that these pores
are the regions of water permeation and interaction sites of external stimuli with
the hydrophilic groups of the graft copolymers.
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
Fig 1: SEM of St-g-PMA-g-PVA matrix
Fig 2: Urea encapsulated St-g-PMA-g-PVA matrix
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
3. Result and Discussion
St-g-PMA used for grafting reaction was previously prepared in our
laboratory and characterized. After drying the St-g-PMA, the free carboxylic
groups (>C=O) of PMA reacts with OH groups of PVA to obtains the graft
copolymer under influence of microwave irradiation and grafting reaction
produces very low quantity of homopolymer. The grafted product and PVA could
be easily separated by cold water treatment in which PVA is soluble.
St-g-PMA-g-PVA gel matrix shows (table 1) better water holding
capacities, which was obviously greater than that of poly(methylacrylate) grafted
starch matrix. Poly vinyl alcohol is a hydrophilic polymer; responsible for higher
water holding capacity. Maximum urea loading was obtained when the St-g-PMA
and PVA contents are in equal ratios.
Table- 1
S. No St-g-PMA: PVA (25: 75)
St-g-PMA: PVA (50:50)
St-g- PMA:PVA (75: 25)
St-g-PMA
Water Loding %
6.1 4.2 2.5 1.5
Urea Loding %
43.70 69.98 32.1 14
The gel matrix encapsulates urea to larger extent due to presence of voids
in gel matrix. Fig 2 shows typical SEM micrographs of urea loaded hydro gels, in
which voids are clearly seen. Urea loading capacities were increased with the
increase of PVA contents and maximal loading capacity was reached up to 50%,
when grafting efficiency of PVA was 50%.
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
The influences of the graft modification and grafting ratio on the release
rate of urea from gel matrix are shown in table 1. The urea release was generally
reduced from the obtained gel matrix compared with St-g-PMA matrix. The
following equation was used to predict the diffusion nature of urea into gel matrix.
F = MT/M0 = Ktn
Where MT/M∞ is the fractional release of urea in time t, k is the constant
related to the structure of the network and exponent n is diffusion exponent
characteristic of the release mechanism. For normal fickion diffusion the value of
n=0.5, case II diffusion n=1.0 and non fickion n=0.5-1.0.
Diffusion exponent n and gel characteristic constant K for the release of
urea have been calculated from the slope and intercept of the straight line
obtained by the plots of ln F versus ln t. From the fig 3 it can be clearly seen that
values of n ranges between 0.8 to 0.9 and urea release from gel matrix was
assumed to be Non fickion in diffusion character.
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
Figure 3: Kinetic release of urea
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
Figure 4: Urea released %
The maximum release of urea from St-g-PMA-g-PVA gel matrix was about
98.38 % after 26 hours (fig 5). In early stage release rate is very fast and attained
almost maximum values after a time of about 6-7 hours. The observation of plot
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
reveals that in early six hour time period the release rate has a linear behavior
and then after release rate becomes slower attaining almost complete release in
ten hours time period.
The gel matrix releases the entirely encapsulated urea in a very controlled
and sustained manner, which is the primary requisite for the use of agro-
chemicals to control the environment and health hazard.
4. Conclusion Starch based semi interpenetrating polymer networks are synthesized in
presence of monomers of methyl acrylate and poly vinyl alcohol. Swelling rates
were measured. The amount of release of the urea is studied by
sphetrophotometric method. Although increasing the amount of initial St-g-PMA
copolymer leads to a stronger structure. It results in a decrease in the swelling
rate and also releases rates. As the conc. of PVA increases, the mechanical
strength of the network increases and releases rate of urea was reduced up to
lower extent.
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Controlled release of urea from biodegradable starch/ Polyvinyl Alcohol matrix
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