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Compression profiles of different molecular weight chitosans
Faisal Alakayleh, Iyad Rashid, Mahmoud Al-Omari, Khaldoun Al-Sou¡!–[INS][’]–¿’¡!–[/INS]–¿od, Babur Z. Chowdhry, Adnan A. Badwan
PII: S0032-5910(16)30253-4DOI: doi: 10.1016/j.powtec.2016.05.019Reference: PTEC 11659
To appear in: Powder Technology
Received date: 16 November 2015Revised date: 7 May 2016Accepted date: 14 May 2016
Please cite this article as: Faisal Alakayleh, Iyad Rashid, Mahmoud Al-Omari, Khal-doun Al-Sou¡!–[INS][’]–¿’¡!–[/INS]–¿od, Babur Z. Chowdhry, Adnan A. Badwan, Com-pression profiles of different molecular weight chitosans, Powder Technology (2016), doi:10.1016/j.powtec.2016.05.019
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Compression profiles of different molecular weight
chitosans
Faisal Alakayleh1, Iyad Rashid
2, Mahmoud Al-Omari
2, Khaldoun Al-Sou’od
3, Babur Z.
Chowdhry4, Adnan A. Badwan
2*
1 Department of Pharmaceutics and Pharmaceutical Technology, College of Pharmacy,
Petra University, Amman, Jordan
2 The Jordanian Pharmaceutical Manufacturing Co., Naor, Jordan
3 Department of Chemistry, Al-Albait University, Irbid, Jordan
4 Faculty of Engineering & Science, University of Greenwich, Medway Campus,
Chatham Maritime, Kent, UK
*Corresponding author: Dr Adnan Badwan, e-mail: [email protected]
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Abstract
The compression behavior of hydrophilic chitosan has been investigated as a function of
molecular weight (MW), ranging from 8-100 kDa. Powder packing, plasticity, elasticity,
brittle-fracture nature were evaluated using force displacement curves and
Heckel/Kawakita analysis upon compression of different MW chitosan powders. The
compacts produced were subjected to ejection work determination and tensile strengths
analysis, the later using Leuenberger equation. Molecular modeling was used to correlate
the MW-binding energy dependence on the physical strength of the chitosan compacts.
The high work of compression of the low MW (≤30 kDa) chitosans is accounted for their
brittle nature upon compression. At high compression forces (> 3kN), the low MW
chitosan compacts exhibited slightly lower tensile strength due to their increasing extents
of elastic recovery and ejection work. Lower particle size of chitosan enhances lower
fragmentation, packing, and frictional tendency that can be related to an increase in
plastic deformation and compacts tensile strength. In molecular modeling, the calculated
higher binding energies of the high MW chitosan (88 kDa) than that of the low MW
chitosan (8 kDa) may further explain its rigid, complex nature when compared to low
MW chitosan. Upon compression, increasing MW of chitosan is manifested as higher
energies for densification which in turn determines the extent of helical packing from
intermingled molecular to open structure as the MW is decreased. Such changes in
structural configuration are suggested to contribute to the transfer from plastic to brittle-
fracture nature with high packing extent of chitosan upon compression when the MW is
reduced. The foregoing may justify the slow diclofenac Na drug release upon dissolution
testing when low MW chitosan is used in tablet preparation.
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Keywords: chitosan, molecular weight, compression force, molecular modeling
1. INTRODUCTION
Chitosan powders have been the subject of various investigations as matrices for
immediate/controlled release solid dosage forms [1-9]. Since chitosan is prepared from
chitin [10,11], the final molecular weight (MW) of chitosan depends on the source or
initial MW of chitin and the deacetylation time and temperature [12]. However, Hwang
KT et al [13] demonstrated the complexity of variables, e.g. temperature and time,
involved to control the depolymerization and/or the deacetylation of chitosan. Many of
these variables were found to manipulate the effective surface charge, electrokinetic
mobility, and colloidal stability of chitosan especially upon interfacial interaction inside
an aqueous environment [14,15]. Unfortunately, the literature itself lacks sufficient
handling of the physicochemical investigations as a function of chitosan MW.
Investigations were mostly concerned with applications with few justifications, if present,
on the role of chitosan MW at the molecular and morphological level [16-25]. These
justifications were confined to structure-MW dependence and supported by the fact that
the MW is equivalent to the number of repeating units of D-glucosamine. Onto each of
these units, interactive amine groups (NH2) are bonded. The general observation
conclusively illustrates that the longer the chain length of chitosan, the more NH2 binding
sites are available. Consequently chitosan binding affinity, uptake capacity, rheological
properties, particle morphology and subsequently surface area, are all strongly dependent
on the number of NH2 groups in a specific polymer mixture, i.e. average MW.
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The scientific inputs become even shallower when chitosan is present in the powder state.
In this context, the compression behavior of chitosan powder was not understood to be
MW related [1,5,6,26-27]. In order to correlate such compression dependency, the
particle morphology and density have to be introduced for analysis when the MW of
chitosan is varied. These two parameters play a significant role in the packing and
arrangement upon powder compression [28]. Fortunately, some attention has been given
to try to understand the complex relationship between the MW of chitosan and the
structural changes and consequently the powder physical properties, all as a function of
chain length (or MW). For example, Goycoolea et al pointed out that chitosan of longer
chain lengths tends to form ‘loops’ giving rise to the creation of more expanded
structures and to a denser packing [7]. Ofori-Kwakye et al related the change in the
intrinsic viscosity of different MW chitosans to the change in particle shape and size [29].
Aggarwal et al highlighted the dependence of chitosan molecular architecture and MW
on the mechanical strength of branched chitosans synthesized by grafting low MW
chitosan chains [30]. Furthermore, bulk density and flow of powders were found to be
dependent on the MW of chitosan when the later is spray-dried with lactose [31]. In this
regard, it was noted that the lower the MW of chitosan the higher the bulk density and
better flow of their processed mixture. However, the poor powder flow and thereby
compressibility have never been investigated as a function of chitosan MW.
Consequently, an insight into the behavior of different MW chitosan powders upon
compression is of significant interest.
In light of the foregoing remarks which are important parameters in characterizing
powder compression properties for direct compression pharmaceutical applications, the
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work reported herein addresses the mechanistic densification behavior of depolymerized
chitosan powders. The properties of the compressed tablets were assessed in the context
of testing tablet mechanical properties as a function of MW.
2. MATERIALS AND METHODS
2.1. Materials
Chitosan (Hong Ju Ginseng Co., Ltd., Dalian Liaoning, China), particle size <150 μm,
MW 100 kDa, (93% degree of deacetylation) of 100 kDa MW was supplied as
pharmaceutical grade. The 8, 13, 18, and 30 MW chitosan powders were prepared by the
Jordanian Pharmaceutical Manufacturing Company (JPM, Naor, Jordan) using the acid
hydrolysis of the 100 kDa chitosan. This technique was described in details by Qandil et
al [32], who illustrated the de-polymerization of chitosan by acid hydrolysis, the
determination of viscosity average molecular weight using the relative and reduced
viscosities of the Mark-Houwink technique, and finally the determination of the degree of
deacetylation which was approximately 100% for all chitosan samples according to the
first derivative UV spectroscopic method [33]. Diclofenac sodium was supplied by
CHEM. Co., Ltd (CCS. Xiaoshan Hangzhou, China).
2.2. METHODS
2.2.1. Compression of chitosan samples
Prior to compression the chitosan samples were grinded and sieved using an oscillating
granulator with a mesh size 250 µm and collected on a 90 µm mesh. 100 mg of each MW
(8, 13, 18, 30, 100 kDa) chitosan sample was compressed using a Gamlen Tablet Press
(Gamlen Tableting Ltd. Biocity Nottingham, UK). Initially, the powders were
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compressed at a punch size of 6.0 mm diameter, enough to accommodate the chitosan
samples. The die was manually filled and the machine operated under an automatic
compression force or displacement control. Ejection was carried out in the same direction
as compression i.e., the tablets were pushed through the bottom of the die into a tablet
holder which was removed from the machine. The test speed was set at 60 mm/min. The
applied loads used were; 100, 200, 300, 400, and 500 kg equivalent to 098-4.9 kN
compression force or to 34.6-173.2 MPa compression pressure (compression force/punch
surface area) applied on a 6 mm diameter circular punch. The Gamlen Tablet Press
(GTP) software displayed a full data capture of the work of compression and work elastic
recovery on a PC screen. After ejection the thickness of the tablets were immediately
measured using a caliber. Other tablets produced in the same manner were directly
subjected to tensile strength tests using a hardness tester (Copley, Nottm Ltd.,
Therwil,Switzerland). The average tensile strength of 10 tablets was recorded at each
compression force for each MW chitosan. From the measured tablet hardness, the tensile
strength was calculated according to the equation [34]:
σ = 2F/(πDh) (1)
where σ is the tensile strength (Pa), F is the tablet hardness (N), D the tablet diameter
(m), and h the tablet thickness (m).
2.2.2. Work of compression, elastic recovery and tablet ejection, and extent of
powder packing
A typical illustration of the force displacement curve is presented in Fig. 1 for the 100
kDa chitosan (100 mg) whereby the input load was set at 100 kg (4.91 kN). The upper
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punch, at a distance of 27.379 cm from the base, displays an automatic displacement
starting from the zero position (point A), followed by point B where the upper punch
makes the first contact with the powder. The powder then undergoes compression
whereby the recorded curve represents an increase in the amount of force as a function of
the upper punch displacement. The maximum recorded force terminates the punch
movement in the downward direction and is presented by point C of the displacement
value D. However, the powder’s elasticity exerts an upward force to the punch in the
form of a decreasing force profile until it reaches zero at point E.
From Fig. 1, the area under the curve BCD is the work of compression in J (N.m). On the
other hand, the area under the curve ECD represents the work of elastic recovery. In both
cases, the data are tabulated by the software of the GTP into two columns, one for the
position of the upper punch (mm) and the other for the load applied (kg). The total area
was numerically calculated by approximation using areas of rectangles. The width and
height of such rectangles were taken as the difference between two successive
displacement points and their corresponding ‘load value’ length respectively. The
summation of all rectangles areas represents the area under the curve (AUC).
The packing extent can be estimated from the final volume of the powder after being
compressed compared to its initial bed volume before compression. Empirically, the
packing extent can be estimated from Fig. 1 using the following equation;
Packing extent = %100B27.379
BD
(2)
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where (D-B) and (27.329-B) represent the final (after compression) and initial (before
compression) heights of the powder bed respectively, mm.
Generally, the die-wall friction can be usually estimated by either of the following
methods; by calculating the difference in the work of compression of unlubricated and
lubricated granules [45], or by calculating the difference in the work of compression for
the upper punch forces and that for the lower punch [46]. However, both methods were
not adopted in this work as lubricants, in the first case, may disrupt the arrangement of
particles through agglomeration [47]. The extent of such disruption may vary between
different MW of chitosans. In the second case, the GTP does not provide a lower punch
for compression, it can, however, measure the work for tablet ejection by the upper punch
from within the die. A typical force-displacement profile for the ejection of the 100 kDa
chitosan tablets compressed at the input load of 500 kg (4.9 kN) is presented in Fig. 2.
The AUC, which represents the work for tablet ejection, was calculated for all force-
displacement curves for each MW of chitosan and then plotted at each compression force.
Accordingly, after the powder was being compressed into a tablet, the GTP setting was
turned into the ejection mode using the same upper punch and inverting the closed die
base position into an open position.
2.2.3. Analysis of compression data
Heckel, Kawakita, and Leuenberger analysis were used to characterize the compression
properties of different MW chitosan samples. The Heckel equation (Eq. 3) describes the
change of powder porosity as a function of the applied pressure upon powder
densification [38,39].
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AKPD
)1(
1ln (3)
where D (compact density/true density) is the relative density of a powder compact at
pressure P (MPa), whereby compact density is the tablet weight (100 mg)/volume of 6
mm diameter tablet. The true densities, measured using a helium pycnometer
(Ultrapycnometer 1000 v. 2.2, Quatachrome Co., Odelzhausen, Germany) of the 100, 30,
18, 13, and 8 kDa chitosans were 1.351, 1.549, 1.592, 1.758, and 1.853 g/cm3
respectively. K (MPa-1
) is the slope and it is a measure of the plasticity of a material. The
inverse of the slope is the yield pressure (Py) of the materials (MPa) [40], which describes
the tendency of the material to deform either by plastic flow or fragmentation. Py is
inversely related to the ability of the material to deform plastically under pressure. The
constant A is related to the die filling and particle rearrangement before deformation and
bonding of the discrete particles.
For comparison purpose, the compression behavior of the samples can be also evaluated
using Kawakita analysis of powder compression data. Such a model is usually used to
linearize the non-linear pressure-porosity relationship provided by the Heckel model into
a linear pressure-volume reduction relationship. Therefore such a model is dependent
upon the initial powder bulk density instead of the true density of the powder, as in the
case of the Heckel model. The Kawakita equation (Eq. 4) is used to study powder
compression via the degree of volume reduction, C (Eq. 5) under an applied pressure, P
(MPa).
aba
P
C
P 1 (4)
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(5)
where V0 is the initial bulk volume and V is the volume at pressure of P. a and b are
constants which are obtained from the slope and intercept of the P/C versus P plots.
The constant a is the initial porosity of the material before compression, while the
constant b relates to the degree of plasticity of the material. The reciprocal of b or Pk
defines the pressure required to reduce the powder bed by 50% [41,41].
In addition to the previous particle size of chitosan adopted for compression analysis, i.e.
between 250 µm and 90 µm, all of the aforementioned compression tests were repeated
for the 100 kDa and the 8 kDa chitosans at two additional particle sizes. The first particle
size represents the coarse grade whereby the chitosan powder was passed over a mesh
size 500 µm and collected on a 250 µm mesh. The second particle size represents the fine
particles that pass through the 90 µm mesh.
The last compression analysis carried out in this work is the Leuenberger analysis. In this
model, the correlation of the tablets tensile strength with the compression pressure is
found to be further dependant on the tablets density. Blattner and Leuenberger analysis
describe such correlation whereby there is a non linear relationship between the tensile
strength (σ) and the product of applied pressure (P) with the tablets relative density (ρR)
as Eq. 6 implies [43].
)(1σσ maxRP
e
(6)
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where σmax is the maximum tensile strength when P is infinite and ρR (the relative density
of the tablet) = 1, γ is the compression susceptibility which represents the decrease in
porosity under pressure. γ is related to the how fast (high γ ) or how slow (low γ) is the
transition state between a powder and tablet when compacts are formed under
compression pressure [44]. The obtained data were analyzed by using GraphPad Prism
(GraphPad Software Inc., La Jolla, CA).
2.2.4. Molecular modeling
Computations were performed with Hyperchem® (6.03 professional, Hyperchem Inc.,
Waterloo, Canada), using the Amber Force field implemented in Hyperchem. All
molecular calculations were carried out using MM+ force fields (bond dipoles). The
partial atomic charges were calculated using AM1 semi-empirical methods. Energy
minimizations were performed using the Polak-Ribiere algorithm (0.01 and 0.1
kcal/mol.Å for D-glucosamine and chitosan, respectively) in order to obtain the stable
conformations of chitosan. For simplicity, chitosan polymers were built up from amine
groups (100% deacetylated). Chitosan monomer (D-glucosamine) was also built up from
natural bond angles, as defined in the Hyperchem software and then optimized at the HF-
ab initio level uisng the 3-21G* basis set (structures I). A set of named selections has
been made to define the HEAD and TAIL for building the polymer. An additional set of
named selections was made to define the atoms involved in the torsion angles used to
assemble the monomers. The HEAD is assumed to be connected to x which in turn is
connected to x´. The TAIL is assumed to be connected to y which in turn is connected to
y´. The torsional angles connecting the monomers are therefore x´–x–y–y´, with retention
of the internal structure of the monomer. The torsion angle used to bring the monomers
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together may be specified to have a random value rather than the constant value described
earlier. The chitosan polymer was constructed with a suitable length from the monomer
unit [45].
2.2.5. Dissolution of diclofenac sodium
Three preparations of diclofenac sodium tablets were formulated so that each tablet (150
mg) contained 50 mg diclofenac sodium and 100 mg of the 100, or 8, or the 30 kDa
chitosan powders. The foregoing MWs represent the insoluble, the lowest MW soluble
and the highest MW soluble chitosans respectively. All components were physically
mixed for each preparation and then directly compressed at pressures of 30-35 kN using a
single punch tabletting machine (Manesty, Merseyside, UK) in which a 10-mm flat
circular punch was fitted. Compression was carried out at different forces (30-35 kN) to
maintain similar tablet tensile strength at 3.0 (±0.1) MPa for the purpose of having a fair
comparison in drug release data. The tablets had an average tablet density of 6.25 (±
0.01) g/cm3. For drug dissolution studies, eight tablets were used according to USP
specifications (USP30-NF25, 2007) for the dissolution media and apparatus.
3. RESULTS
3.1. Extent of powder packing and compression, elastic recovery, and tablet ejection
works
For each MW, the extent of powder packing, the work of compression, elastic recovery,
and tablet ejection were calculated for each chitosan powder from the force-displacement
curves measured using the Gamlen tablet press. Figures 3, 4, 5, and 6 illustrate the plots
of powder packing extent and the works for powder compression, elastic recovery, and
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tablet ejection, respectively as a function of the five applied forces; 1-5 kN which are
equivalent to the GTP compression loads from 100-500 kg on100 mg of the chitosan
samples.
In Figures 3 and 4 the increase in powder packing extent and compression work as the
compression force increases is clearly evident for all the different MW chitosan samples
(8 kDa-100 kDa). Moreover, decreasing the MW of chitosan from 100 kDa to 8 kDa
shows an increase in both the powder packing extent (at compression forces > 2.94 kN)
and the work for compression.
The work for elastic recovery calculated from the force-displacement curves is illustrated
in Fig. 5 for the same chitosan samples. It is clear from the data in Fig. 5 that increasing
the compression force for all samples results in an increase in the elastic recovery work.
Such behavior manifests the high amount of elastic energy stored at higher compression
forces which is then recovered upon termination of the applied force. In addition, the data
in Fig. 5 shows that all of the low MW chitosans (8, 13, 30 kDa) show lower elastic
recovery work than the 100 kDa chitosan for all compression forces below 2.94 kN.
Higher than 2.94 kN, the elastic work of all the low MW chitosans approach that of the
100 kDa. The foregoing suggests that although higher compression work is displayed for
chitosan samples of lower MW at all compression forces (Fig. 4), there appear to be
force-related factors that contribute to the observed elastic recovery trend when compared
with 100 kDa chitosan.
Since tablet ejection depends on the frictional forces between the tablet and the die, the
work of tablet ejection measured using the GTP was used to indicate variations with
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compression forces and chitosan MW. Fig. 6 illustrates the foregoing criteria. In this
regard, with the exception of the 100 kDa chitosan, the work increases with increasing
the compression force and decreasing the MW. In contrast, for the 100 kDa chitosan,
ejection work decreases with increasing the compression force.
3.2. Heckel, Kawakita, and Leuenberger compression analysis
The properties and behavior, e.g. plastic/elastic deformation, brittle fracture, and extent of
particles arrangement, of powder compacts upon compression may justify the changes
obtained in the work for compression and elastic recovery when increasing the applied
force on one hand and decreasing the MW of chitosan on the other. Heckel and Kawakita
analysis of the compression data are considered herein in an attempt to evaluate the
behavior of chitosan powders .when subjected to different compression forces.
Fig. 7 illustrates the Heckel plots of the samples whereby non-linear trends are obtained
over all the compression pressure data range (i.e. 34.6-173.2 MPa). It is clear from the
Figure that powder porosity decreases when the compression pressure increases and
when the MW of chitosan deceases. The lowest porosity is thus obtained for chitosan of 8
kDa MW. From the same Figure, linear portions can be drawn through three successive
points (porosity-pressure points). For example, for all the samples, except for chitosan of
8 kDa, the first three points were considered whereas the 3rd
, 4th
, and the 5th
points were
considered for chitosan of 8 kDa. The slopes and the intercepts of the straight line
portions are tabulated in Table 1.
Kawakita analysis was carried out using the same compression data obtained by the GTP.
In this model, bulk densities were considered for calculations of the powder volume
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reduction upon compression instead of the true densities used in Heckel analysis when
calculating porosity changes upon compression. Kawakita parameters; a and 1/b (PK)
were calculated from the slopes and intercepts of Kawakita equation (Eq. 4). Results are
tabulated in Table 2.
From the compression analysis using Leuenberger equation, the radial tensile strength at
each compression force, calculated using Eq.1, was plotted against the product of the
pressure of compression and the relative density of the different MW chitosans as Figure
8 illustrates. By choosing a logarithmic regression for curve fitting of the data, the
unknown parameters representing the compression susceptibility and the maximum
tensile strength can be displayed. Values of these parameters are presented in Table 3.
The first two high (100 and 30 kDa) and the lowest (8 kDa) MW chitosans had low
compression susceptibility, whilst values of the 18 and the 13 kDa chitosans were the
highest. Thus the maximum tensile strengths (Table 3) of the 18 and the 13 kDa are
reached faster than all the other MW chitosans. However, the σmax of the 18 amd 13 kDa
chitosans were the lowest indicating poor bonding at higher compression pressures.
The tensile strength of chitosan compacts are rearranged in Table 4 at each compression
force for the whole MW range of the samples. Generally, increasing the compression
force, allows closer packing of the powder bed, thus minimizing the porosity and forming
a more coherent compact. The tighter less porous structure of the compacts, the higher
their tensile strengths when compressed at increasing force, as the data in Table 4
illustrates. In addition, the highest tensile strengths were obtained for the highest MW
chitosan, i.e. 100 kDa at the 4.9 kN compression force. In contrast, the same MW
manifested the lowest compacts tensile strengths when compressed at 0.98 and 1.96 kN
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compression forces. It can be further noticed that, the tensile strengths of the 18 and the
13 kDa chitosans started to form stronger compacts than the other MW chitosans at the
compression pressure of 2.94 MPa. Further increase in the compression force (>2.94
MPa), however, resulted in weaker compacts for the same foregoing MWs of chitosan.
On the other hand, all the other MWs encountered a tensile strength increase with
pressures higher than 2.94 MPa.
3.3. Particle size effect on compression properties
All compression properties were found to be affected by chitosan particle size (Table 5)
using the highest (100 kDa) and lowest (8 kDa) MWs. Although the packing extent
decreases as the particle size is decreased, the work of compression is higher for smaller
particle size chitosans. Such behavior is contradictory to Figures 3 and 4 which indicate
that the recorded high compression work belongs to high chitosan packing when low
MW chitosans are compressed at 4.9 kN. On the other hand, the work of elastic recovery
and tablet ejection are reduced when lower chitosan particle size is used. Heckel analysis
of the three particle size chitosans indicates the decrease in the yield pressure (PY) and
rearrangement values when the particle size is reduced. Finally, high tensile strength of
the compacts is achieved for the compression of low particle size chitosans.
3.4. Molecular modeling
In molecular modeling using Hyperchem, the chemical structure of D-glucosamine
(chitosan monomer) built up from natural default bond angles is shown in Fig. 9(A).
Building chitosan polymers (high MW chitosan and low MW chitosan) were constructed
with a suitable length from the monomer unit. The high MW chitosan (Fig. 9(B)) was
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constructed using 365 monomer units (MW is 66 kDa), while the low MW (Fig. 9(C))
chitosan from 45 D-glucosamine units (MW is 8 kDa). Accordingly, both high and low
MW chitosans exist in two different orientations owing to the chain length. Compared to
the low MW chitosan, the high MW chitosan, Fig. 9(B), shows a longer and more
extended structure (more extensive H-bonding). The interior space interactions within
polymeric species were studied manually by allowing two chitosan polymers of the same
MW to approach each other, and then system optimization was carried out under the
conditions described in the experimental section.
It is noteworthy that the high MW chitosan polymer units approach more closely to each
other than the low MW chitosan, allowing more extensive intermolecular H-bonds when
compared to the less extended low MW chitosan polymers (Figures 10(A) and (B)). The
calculated binding energy per monomer unit, Ecal, is determined from the following
equation:
Ecal = {Esystem –(Ep1 + Ep2)}/n (7)
Where Esystem is the energy of the optimized system, Ep1 and Ep2 are the energies of
the isolated gas phase polymers in similar conformations, with N being the number of D-
glucosamine units in the polymer.
Dissolution data for diclofenac sodium tablets containing different MW chitosan powders
is presented in Fig. 11. The 100 kDa chitosan tablets showed fast drug release whereas
slow drug release was the case for the 30 kDa and the 13 kDa chitosan tablets. The extent
of powder packing was tested for the three chitosan MWs used in the diclofenac product
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preparation (Fig. 12). There is clearly an increasing extent of packing the higher the MW
of chitosan.
4. DISCUSSION
The increase in compression work as the compression force increases is clearly evident
for all the chitosan samples. Generally, upon powder compression, increasing the applied
force by the GTP allows either more folding for plastically deforming materials or high
extent of particle fragmentation for brittle materials. Thus, in the course of compression
by the upper punch there will either be a change in the fracture behavior of the materials,
or extra displacement to be undertaken when achieving a complete input load, or the
establishment of both of the foregoing changes. In the current investigation, the extent of
powder packing using force-displacement curve has been used to give measurable
estimates of the amount of these changes taking place as a function of compression force
and chitosan MW. In this regard, the effect of compression force on the powder packing
was found to be more significant than chitosan MW whereby, as expected, higher
compression forces induce extensive packing (Fig. 3). Examining the effect of chitosan
MW, the extent of powder packing was found to increase with increasing the MW and
decrease with increasing the MW for the lowest and highest tested compression forces
respectively. In other words, decreasing the chitosan MW implies more packing extent
attained with increasing the compression force. Such finding explains the obtained data in
Fig. 4 whereby there is more displayed work of compression when lower MW chitosans
are used. Since the work required for compression is the sum of work necessary to
rearrange the particles, deform, and finally to fragment them [46], it is suggested that
decreasing the MW of chitosan implies that the compacts undergo excessive folding
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and/or fragmentation at the micro level upon compression. Furthermore, such particles
behavior will have an impact on the resulted profiles of elastic recovery work for each
MW (Fig. 5). In this regard, the profiles of the elastic recovery work of all MW chitosans
except for the 100 kDa are similar, with values lower than the 100 kDa chitosan for
compression forces less than 2.94 kN. Above 2.94 kN, the aforementioned profiles start
to approach that of the 100 kDa chitosan. Such observation is in harmony with the
packing extent as a function of MW (Fig. 3) whereby above 2.94 kN the packing extent is
clearly in increasing order with chitosan MW. This indicates that higher powder packing
induced by higher compression forces manifests higher elastic recovery which increases
with MW. Since elastic recovery is related to the amount of stored energy during
compression [47] and is released during decompression, then the apparent high
packing/elastic recovery extents are thought to be correlated to higher ability of the lower
MW to store more energy than the higher MW chitosans. The foregoing will be given a
further attention in the molecular modeling work. In the meanwhile, compression data
analysis using Heckel and Kawakita equations may assist to elucidate the impact of the
MW of chitosan on powder properties upon compression.
Heckel and Kawakita analysis determine optimum tabletting properties usually obtained
with plastically deforming materials. Generally, plastic materials exhibit greater degree of
folding resulting at higher compression pressures thus allowing the appearance of new
surfaces for binding to take place. With regard to Heckel analysis, the lower the yield
pressure of a material is an indication of higher extent of plastic deformation. Thus the
data in Table 1 shows that the lower the MW of chitosan (e.g. 8 kDa), the lower the
extent of plastic deformation. On the contrary, the 100 kDa chitosan displays the highest
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plastically deforming sample. In fact, there is a tendency for the lower MW chitosan
samples to undergo brittle fracture upon compression. Such a conclusion is supported by
the fact that lower MW chitosan samples showed higher intercept values (A) of the
Heckel analysis implying that there is greater tendency of the lower MW chitosan to
undergo fragmentation and to fill the die.
The results for the Kawakita analysis of the experimental data are shown in Table 2. It is
clear from the data in Table 2 that the highest porosity (highest ‘a’ Kawakita parameter)
was obtained for the highest MW chitosan, i.e. 100 kDa, whereas the lowest porosity
(lowest ‘a’ Kawakita parameter) was obtained for the 8 kDa chitosan. The foregoing is
agreement with the results of Fig. 3 whereby the extent of packing increases, and
subsequently porosity is decreased, when lower MW chitosans are used. On the other
hand On the other hand, the PK values are lower for the higher MW chitosan samples
indicating that they readily deform plastically under pressure. This MW-plasticity
relationship is in agreement with the yield pressure obtained by Heckel analysis of
compression data.
Optimal tabletting properties are generally attained for powders with maximum amount
of plastic deformation and/or with minimum amount of elastic recovery [48,49]. The
highest MW chitosan (100 kDa) showed the highest compacts tensile strength (Table 4)
as its total amount of plastic deformation was the highest revealed by its lowest yield
pressure (Table 1). However, despite their higher work of compression, the low MW
chitosan samples exhibited less plastic behavior and therefore slightly lower tensile
strength of the compacts than the 100 kDa chitosan. This result contradicts the findings
by Picker-Freyer et al who investigated the slope of the Heckel plot as a function of
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different MW chitosans [50]. They found that chitosan with a MW of 87.2 kDa behaves
in a similar way to microcrystalline cellulose as a highly plastically deforming excipient.
The other higher MW chitosans ((173.3 and 210.5 kDa) showed a lower Heckel slope and
thus higher resistance against deformation. They suggested that the higher Heckel slope -
thus a higher and easier deformation- of the 87.2 kDa chitosan are correlated to its low
MW. In re-evaluating such behavior, the aforementioned judgment did not consider
differences in the physical properties of the chitosans used. For example all their
investigated MW chitosans differ in the degree of N-deacetylation, bulk density, and
particle size. All these parameters affect the packing and extent of deformation upon
compression. In addition, the determination of the Heckel slopes includes plastic and
elastic deformation and therefore the contribution of the undetermined elastic recovery
might have added up to the Heckel slope estimation [51-55]. The foregoing is justified in
the current work as the high plastically deforming character of the 100 kDa chitosan
presented the highest work of elastic recovery (Fig. 5). All the other lower MW chitosans
(8, 13, 18, 30 kDa) had lower elastic recovery due to their lower extent of plastic
deforming or to their brittle-fracture character. Such behavior is, however, valid up to a
compression force of 2.94 kN (Fig. 5) which is almost equivalent to the linear portion
range considered for the Heckel plots (Fig. 7). Higher than that, the elastic recovery work
seems to be almost the same for all MW chitosans. It is suggested that, under high
compression forces (>2.94 kN), the low MW chitosans deviate away from brittle-fracture
deformation and start to exhibit plastically deforming character as their elastic recovery
work becomes equivalent to the 100 kDa chitosan. Patel et al highlighted such behavior
for dicalcium phosphate where they noted that as the pressure is increased, more and
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more primary particles fragment to smaller particles, which have higher resistance against
deformation due to the appearance of elastic character [56].
In summary, the recorded increase in the work for compression is due to appearance of
brittle-fracture characteristics. The foregoing implies the appearance of new smaller
granules that require extra work to compress and exhibit less elastic recovery work. The
increased (A) values of the Heckel analysis for chitosan MW<100 kDa implies more
dense packing due to high fragmentation which again justifies the brittle-fracture
behavior upon compression.
Changes in the compacts tensile strength with increasing the compression pressure (Table
4) are interestingly in agreement with the deduced parameters using Leuenberger analysis
of the compression data (Fig. 8/Table 3). For example, the high compression
susceptibility parameters (γ) for the 18 and 13 kDa chitosans (Table 3) indicate that high
tensile strengths are attained at lower pressure of compression compared to the other MW
chitosans. The foregoing is in agreement with Table 4 which indicates that the 18 and the
13 kDa reach a higher tensile strength values at 2.94 MPa than the other MW chitosans.
The 100, 30, and 8 kDa chitosans showed low γ values, thus higher compression
pressures are required to produce stronger tablets. However, their σmax values (Table 3)
are higher than that for the 13 and 8 kDa chitosans indicating stronger compacts at higher
pressures. Table 4 confirms the foregoing finding as the highest tensile strengths are
achieved by the 100, 30, and 8 kDa chitosans.
In order to rationalize differences in tablets strength with pressure for different MW
chitosans, the powder elastic recovery and compacts ejection work have to be considered
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hereafter. Initially, when the tensile strength of the compacts in the whole applied force
range (Table 4) is examined for chitosan lower than 100 kDa, up to an applied force of
2.94 kN, almost all samples exhibited higher tensile strength than the 100 kDa chitosan.
Increasing the applied force above 2.94 kN the tensile strengths for the same samples
were lower than that for the 100 kDa. Examining the profiles of the elastic recovery work
in Fig. 5 for forces lower and higher than 2.94 kN, shows that for chitosan < 100 kDa, the
elastic recovery was much lower than the 100 kDa chitosan up to 2.94 kN. Above 2.94
kN, the elastic recovery work of the low MW chitosans started to approach the elastic
recovery work value of the 100 kDa. This trend correlates well with the tensile strength
trend obtained below and above 2.94 kN. In other words, compacts with lower tendency
for elastic recovery work have higher tensile strength values. On the other hand, when the
tablets ejection work is considered (Fig. 6), at the lowest compression force (0.98 kN),
the compact of the 100 kDa chitosan displayed the highest ejection work indicating the
highest frictional forces with the die wall. Such high forces may have further contributed
to the lowest tensile strength of the 100 kDa at 0.98 kN. Higher compression forces
render smoother ejection of the 100 kDa compacts and thus higher tensile strengths
compared to the other MW chitosans. In contrast, lower MW chitosan manifest higher
ejection work when the compression pressure is increased thus lower compacts tensile
strength.
The high mechanical strength of the high MW chitosan has been previously reported and
attributed to the entanglement network of the high MW chitosan [57]. The 100 kDa
sample shows a β conformation whereby the individual chains are arranged in a parallel
manner. Hence giving rise to the appearance of ordered structure and thereby to a semi-
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crystalline lattice [58]. Acharya et al reported that lowering the MW of chitosan results in
a decrease in crystallinity due to a larger d-spacing as a result of an increase in the
dimensions of the unit cell. Such a change results in a weaker network of hydrogen-bonds
between chitosan chains as evidenced by FT-IR experiments. The later technique
indicated a change in the environment of primary -OH groups and thus in the overall
hydrogen bonding network [59].
When lower chitosan particle size is used (Table 5), the reduced packing extent indicates
that the powder bed is of less porous structure resulting in lower volume reduction upon
compression of both MWs at 4.9 kN. Such reduced powder porous structure takes place
concurrently with enhanced powder deformation resulting in higher work of compression
for lower particle size of chitosan. The powders are found to undergo increased plastic
deformation (Lower PY) when smaller particle size chitosan is used. Such increased
plasticity, and thereby extent of particle deformation, results in the appearance of fresh
surfaces for binding. Therefore more coherent and stronger compacts are produced when
lower chitosan particle size is compressed. The lower A Heckel parameter for lower
particle size indicates the lower particle rearrangement and thereby explains the lower
volume reduction obtained. Consequently, having less porous powders, more coherent
compacts are produced with less frictional forces with the die wall as the work of tablet
ejection is lower for lower particle size.
The molecular structure of the high MW chitosan has been previously shown to be of
open helical orientation whereas the low MW chitosan has been shown to be of closed
and highly packed helical complex [45]. Such finding was established when chitosan
molecular modeling was carried out in water. In contrast, when chitosan molecular
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modeling was carried out in vacuum, as in the current work, the high MW chitosan
showed high extended and closely packed molecular structure. This result is reasonable
with regard to the fact that high MW chitosan has more hydrogen bonds across parallel
polymeric chains, and consequently, more stable as compared to low MW chitosan.
Therefore, it can be postulated that low MW chitosan requires higher energy to yield
compacted helical structures where molecules can be intermingled. For high MW
chitosan of highly packed structure and, accordingly low d-spacing, requires lower
energy to bind. This was evident in the current work (Fig. 9) using molecular modeling
when calculating the binding energies between two polymeric species of two different
MW chitosans (88 and 8 kDa). The binding energy of high MW chitosan (-2.12 kCal/mol
per unit) is higher than that for the low MW chitosan (-1.56 kCal/mol per unit). Such high
binding energy leads to a more rigid structure of the high MW chitosan. Consequently
high MW chitosan requires less applied compression forces to bind and allows less
energy (as measured herein from the force displacement curve) to get the two polymeric
species closer. On the other hand, a greater compression force and energy are required to
get the small helical strands closer for the low MW chitosan.
The powder compression and tablet properties dependence on chitosan MW will clearly
impart variations in drug dissolution profiles when the MW is changed. For diclofenac
sodium (Fig. 11) the relatively fast drug release for the 100 kDa chitosan tablets reflects
appropriate application of such chitosan immediate release tablet preparations. On the
contrary, the slow drug release for the 30 kDa and the 13 kDa chitosan tablets reflects
suitability to be used in controlled release drug applications.
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The high MW chitosan in its own is not generally used for sustained release applications;
in contrast, it is used as an immediate disintegrating additive [60]. The reason behind that
is due to the fact that high MW chitosan, if not modified, does not undergo gelling which
represents a perquisite for sustained action to take place [61]. On the contrary, the high
water uptake of chitosan makes it a good candidate for immediate release applications
[60]. Thus, in order to prolong the release, chitosan is generally combined with anionic
polymers which form swellable non-erodible gels that are capable of prolonging the drug
release [62-65]. On the other hand, low MW chitosan, in its own, has not been
investigated thoroughly and compared with higher MW chitosans in regards to drug
release behavior. However, the low MW chitosan was noticed to cause prolonged release
action on some drugs [66]. In the current investigation, tablet swelling, as a factor in
hindering the drug release, is excluded in having a contribution since the drug dissolution
media was not acidic (phosphate buffer pH 6.8 according to USP specifications) [67].
Thus the apparent slow drug release using low MW chitosans is suggested to be
attributed to other factors such as the slow erosion rate. On the other hand, both modes of
drug release, i.e. immediate and sustained, are preliminary attributed to the behavior of
packing extent in response to increasing the MW. Such behavior is valid irrespective of
the absence (Fig. 3) or the presence (Fig. 12) of diclofenac Na with chitosan whereby
lower MW chitosans induce higher packing extents. The net result, therefore,
conclusively demonstrates the suitability of chitosan to be used in immediate and
sustained release applications for the high and low MW chitosan powders respectively.
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5. CONCLUSIONS
The change from high to low MW chitosan brings about changes in the mechanical
properties. Above specific compression force limits, low MW (≤30 kDa) chitosans
manifest higher levels of; fragmentation and brittle-fracture tendency, compression work,
packing, and, to some extent, compacts ejection work, relative to the 100 kDa chitosan.
All of the aforementioned changes depend on two main factors; i.e. the extent of
compression force applied and the chitosan particle size. In the first factor, Leuenberger
analysis revealed that stronger compacts of high MW chitosans (≥30 kDa) are reached
slower, upon increasing the compression forces, than the other lower MW chitosans. The
foregoing has been attributed to the increasing elastic recovery and ejection work of the
low MW chitosans upon increasing the compression pressure thus contributing to weaker
compacts. In the second factor, lowering the chitosan particle size, for the highest (100
kDa) and the lowest (8 kDa) MWs, reduces the particle rearrangement and volume
reduction upon compression resulting in less porous powders and more coherent
compacts. Modeling high MW chitosan interacting polymeric pairs appears more
extended and shows more extensive hydrogen bonding parallel chains leading to more
compact and stable structure than the low MW chitosan. The aforementioned results
explain the low energy required to compress the high MW chitosan as indicated by the
force displacement curves. Differences in compression properties may further correlate to
the immediate release profile of diclofenac with the 100 kDa chitosan and the sustained
release profile with the 30 and 8 kDa chitosans upon dissolution of the drug.
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CONFLICT OF INTEREST
The authors have declared that there is no conflict of interest.
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Captions:
Tables
Table 1: Heckel parameters; yield pressure (PY) and the intercept (A), of different MW
chitosan powders upon compression using the GTP.
Table 2: Kawakita parameters; compressibility (a) and PK, of different MW of chitosan
powder upon compression using the GTP.
Table 3: Compression analysis using Leuenberger equation of different MW chitosans
compressed using the GTP. The tensile strength and density of the 6 mm tablets were
calculated from their hardness and tablet thickness values.
Table 4: Tensile strengths (MPa) of the different MW chitosan samples compressed at a
force range 0.98-4.91 kN (equivalent to 100-500 kg load applied by the GTP).
Table 5: Compression and tablet properties of different particle size chitosans for the 100
kDa and the 8 kDa MWs.
Figures
Fig. 1.: A typical force-displacement curve displayed by the GTP for the compression of
100 kDa chitosan (100 mg sample) at 100 kg (4.91 kN) compression force.
Fig. 2: A typical force-displacement curve displayed by the GTP for the tablet ejection
compressed at 100 kg (4.91 kN) compression force for the 100 kDa chitosan (100 mg
sample).
Fig. 3.: Extent of powder packing after compression relative to the initial bed volume for
each chitosan MW at different compression forces.
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Fig. 4.: Compression work of different MW chitosan powders as a function of
compression force
Fig. 5.: Elastic recovery work of different MW chitosan powders as a function of
compression force.
Fig. 6.: Tablet ejection work of different MW chitosan powders as a function of
compression force.
Fig. 7.: Heckel plots of different MW chitosan samples compressed at a pressure range
34.6-173.2 MPa.
Fig. 8.: The tensile strength versus the product of the pressure of compression and the
relative density of different MW chitosan powders. The powders were compressed using
the GTP.
Fig. 9.: Chemical structure of D-Glucosamine (A), high MW (B) and low MW (C)
chitosans.
Fig. 10.: A section of the high MW/more extended (A) and low MW/less extended (B)
chitosans showing extensive and less extensive intermolecular hydrogen bonds.
Fig. 11.: Dissolution of diclofenac sodium tablets containing different MW chitosan
powders; 100 kDa, 30 kDa and 13 kDa chitosans.
Fig. 12.: Extent of powder packing of diclofenac/chitosan preparations after compression
relative to the initial bed volume for each chitosan MW at different compression forces.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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Figure 11
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Figure 12
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Table 1
MW
(kDa)
Yield pressure (1/K), PY (MPa) A
100 72.46 0.41
30 98.03 0.45
18 106.38 0.46
13 120.48 0.52
8 153.84 0.59
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Table 2
Chitosan
MW (kDa)
Kawakita
slope
Kawakita
intercept a 1/b (PK)
100 1.32 10.83 0.75 8.15
30 1.86 15.13 0.54 8.15
18 1.58 11.87 0.63 7.50
13 1.45 55.67 0.69 38.32
8 1.91 41.16 0.52 21.55
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Table 3
Chitosan MW (kDa) Compression susceptibility
(MPa-1)
Maximum tensile strength,
σmax (MPa)
100 0.003751±0.001 14.23±1.1
30 0.003009±0.001 14.98±0.9
18 0.04288±0.005 3.853±0.5
13 0.02535±0.003 4.149±0.8
8 0.006552±0.001 9.446±1.0
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Table 4
MW of Chitosan (kDa)
100 30 18 13 8
Force (kN) Tensile strength (MPa)
0.98 0.80±0.11 2.26±0.33 2.22±0.24 1.41±0.18 2.78±0.14
1.96 1.53±0.20 2.60±0.17 3.59±0.31 2.37±0.24 2.50±0.30
2.94 3.42±0.29 2.88±0.18 3.63±0.28 4.27±0.21 3.19±0.19
3.92 4.90±0.12 4.73±0.17 3.44±0.38 3.78±0.26 4.85±0.14
4.91 6.53±0.31 6.11±0.24 4.20±0.14 3.75±0.23 5.76±0.19
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Table 5
Test MW (kDa)
Test values
at chitosan particle size
500-250 µm 250-90 µm <90 µm
Packing extent (%) 100 83.46 81.21 79.27
8 84.37 82.82 80.94
AUC*/compression 100 341.23 379.13 417.19
8 495.68 501.98 557.25
AUC/Elastic
recovery
100 59.45 57.14 56.79
8 57.65 55.51 53.06
AUC/Tablet
ejection
100 48.72 37.56 29.63
8 69.43 59.19 48.05
Heckel parameter,
PY (MPa)
100 91.65 72.46 60.71
8 790.74 153.84 130.76
Heckel parameter,
A
100 0.58 0.41 0.38
8 0.67 0.59 0.54
Tablet tensile
strength (MPa)
100 3.71 6.53 7.14
8 3.29 5.76 6.42
*AUC: area under the curve which is equal to work (x 10-2
J)
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Graphical abstract
Chemical structure of D-Glucosamine (A), high MW (B) and low MW (C) chitosans.
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Highlights
Tensile strength of chitosan compacts is associated with chitosan molecular
weight.
Low chitosan compacts tensile strength is attributed to high elastic recovery.
High MW chitosan has a more stable structure than the low MW chitosan.
High MW chitosan provides immediate release properties in solid form
preparations.
Low MW chitosan provides sustained release properties