DC conductivity and dielectric permittivity of collagen–chitosan films

Materials Chemistry and Physics 99 (2006) 284–288
DC conductivity and dielectric permittivity of collagen–chitosan films
C.G.A. Lima a , R.S. de Oliveira b , S.D. Figueiró c ,
C.F. Wehmann d , J.C. Góes d , A.S.B. Sombra d,∗
a Departamento de Engenharia Mecânica, Universidade Federal do Ceará, Caixa Postal 6030, CEP 60455-760, Fortaleza, Ceará, Brazil
b Departamento de Física, Universidade Estadual do Ceará, Caixa Postal 6030, CEP 60455-760, Fortaleza, Ceará, Brazil
c Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, Caixa Postal 6030, CEP 60455-760, Fortaleza, Ceará, Brazil
d Laboratório de Telecomunicações e Ciencia e Engenharia dos Materiais (LOCEM), Departamento de Física, Universidade Federal do Ceará,
Caixa Postal 6030, CEP 60455-760, Fortaleza, Ceará, Brazil
Received 23 February 2005; received in revised form 14 September 2005; accepted 24 October 2005
Abstract
In this paper we studied the physicochemical and dielectric properties of collagen–chitosan films, considering the development of new biomaterials
which have potential applications in coating of cardiovascular prostheses, support for cellular growth and in systems for controlled drug delivery.
The dielectric permittivity ε 11 were obtained for the collagen and collagen–chitosan films. Our results also show that the presence of chitosan
contributes to an increase of the thermal stability of the collagen films, which is associated with the increase of the denaturation temperature of
the collagen–chitosan samples compared with the collagen sample. We believe that the increase of the organization of the microscopic structure of
the sample, results in an increase of the thermal stability. For the samples under study, the highest conductivity was obtained for the chitosan film
compared to the pure collagen film. Around room temperature it is around 10 times bigger than pure collagen.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Collagen; Chitosan; Dielectric properties; Thermal decomposition
1. Introduction
Collagen, as well as chitosan, has great potential, in the
field of biomaterials. The study of the interactions which can
occur between these two biopolymers bears a great importance.
Mainly, to consider the processing of new materials from these
two macromolecules. Collagen is the major structural component
of connective tissues. It is an important biomaterial finding
several applications as prosthesis, artificial tissue, drug carrier
and cosmetics [1]. Collagen has a unique structure, size and
amino acid sequence [2]. The collagen molecule consists of
three polypeptide chains twined around one another as in a threestranded
rope. Each chain has an individual twist in the opposite
directions. The principal feature that affects a helix formation is
a high content of glycine and amino acid residues. The strands
are held together primarily by hydrogen bonds between adjacent
–CO and –NH groups, but also by covalent bonds. The basic col-
∗ Corresponding author. Tel.: +55 8540089909; fax: +55 8540089450.
E-mail address: sombra@fisica.ufc.br (A.S.B. Sombra).
URL: www.locem.ufc.br.
lagen molecule is rod-shaped with a length and a width of about
3000 and 15 Å, respectively, and has an approximate molecular
weight of 300 kDa. Monomeric collagen molecules form stable
solutions in acids, pH 3.5 and low ionic strength at temperatures
in the range from freezing to denaturation. In this work
collagen was prepared from bovine intestinal submucosa, with
deamidation by alkaline hydrolysis of carboxyamide side chains
of asparagine (Asn) and glutamine (Gln) residues present in collagen
chains, in order to develop polyanionic collagen materials
[3].
On the other hand, chitosan is a cationic polysaccharide,
which contains -1-4-linked 2-amino-2-deoxy-d-glucopyranose
repeat units and is readily obtained by alkaline N-deacetylation
of chitin. Chitin is the second-most abundant biopolymer
in nature, widely distributed in the shell of crustacean, the
cuticles of insects and the cell walls of the fungi. Chitosan
has many useful biological properties such as biocompatibility,
biodegradability and bioactivity [4,5]. This polysaccharide,
having structural characteristics similar to glycosaminoglycans,
seems to mimic their functional behavior. The inductive and
stimulatory activity of chitosan on connective tissue-rebuilding
is clearly demonstrated, and it is suggested that chitosan could
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2005.10.027

C.G.A. Lima et al. / Materials Chemistry and Physics 99 (2006) 284–288 285
be considered a primer on which a normal tissue architecture is
organized.
Polymeric composite materials, both of natural and synthetic
origin, constitute by far the broadest and most diverse class of
biomaterials. Charge and polarization storage via the electret
state has been found in many biomaterials. As biomaterials,
electrets have found interesting applications as antithrombogenic
surfaces, stimulation of tissue growth in bone and special
artificial membranes [6,7]. Fundamental macromolecules
of biology, such as collagen and chitin, exhibit the effect.
Studies on the collagen and chitosan interactions showed that
two kinds of interactions can give rise between the two polymers
when they are in contact with water: an electrostatic
complex and a hydrogen bonding type complex, in the presence
of a great excess of chitosan [8]. The objective of this
research is the study of physicochemical, dielectric and electrical
properties of collagen–chitosan films, considering the development
of new biomaterials, since that piezoelectric polymer
films have been tested as implants to stimulate bone growth
[9].
2. Experimental methods
2.1. Materials
The soluble anionic collagen was prepared by alkaline treatment of bovine
intestinal submucosa, at 20 ◦ C from period of 72 h [10], followed by homogenization
in 0.5 mol L −1 acetic acid solution and brought to a final collagen
concentration of 10 g L −1 . The fully deacetylated chitosan was purchased from
SIGMA and was solubilized in 0.5 mol L −1 acetic acid solution to obtain a concentration
of 10 g L −1 .
2.2. Preparation of the films
The membranes, casted in acrylic molds, from a collagen solution containing
chitosan in various proportions ranging from 0 to 100%. The collagen–chitosan
membranes were prepared with 0, 10, 50, 90 and 100% of collagen (w/w) (Samples
CH100, CH90CO10, CH50CO50, CH10CO90, CO100, respectively).
2.3. Differential scanning calorimetry
Table 1
Density (ρ), thickness (e), dielectric permittivity (ε), dielectric loss (diameter
for all samples: 1 cm)
Samples ρ (Kg m −3 ) e (m) 1 MHz (ε/ε 0 ) 1 GHz (ε/ε 0 )
CH100 1078.45 40.8 3.94 2.71
CH90CO10 1192.4 68 3.35 2.29
CH50CO50 977.71 83.4 2.68 2.05
CH10CO90 936.53 86.4 2.96 2.41
CO100 670.76 55.9 2.60 2,3
2.6. DC conductivity
Samples used for electrical measurements were plates in the circular geometry,
using silver electrodes, with the same surface area (1 cm of diameter and
thickness ranging from 40 to 86 m, see (Table 1)). The DC electrical conductivity
was obtained by measuring the resistance of the samples with an electrometer
type 617 programmable electrometer (Keithly 617 Programmable Electrometer),
using the constant DC voltage (50 V) and measuring V/I when the sample
is heated from low to high temperature (6 K min −1 ). Where V is the voltage
applied to the sample and I is the measured current intensity. The relative error
of the electrical conductivity was less than ±5%.
3. Results and discussion
The IR spectra of the collagen, chitosan and collagen–
chitosan blend films are shown in Fig. 1, for samples CH100,
CH50CO50 and CO100, respectively. The primary bands of
CO100 arise from the peptide bond vibrations: amides I, II and
III, centered at 1660, 1550, and 1238 cm −1 , respectively [11,12].
The major feature of the IR spectrum of collagen film is the
Amide I band between 1640 and 1660 cm −1 , which arises from
the stretching vibration of C O groups of amide groups in protein.
The intense absorption between 1500 and 1600 cm −1 is
due to the Amide II mode, observed at 1550 cm −1 in the spectrum
for collagen, which arises from N H stretching vibration
strongly coupled to the C N stretching vibration of collagen
amide groups. Signals in the spectral region of 1200–1400 cm −1
absorption are generally attributed to the Amide III, arising due
to the C N stretching and N H in plane bending from amide
linkages. The C N stretching vibration of the cyclic proline
This was carried out using a Shimadzu DSC-50 equipment. Accurately
weighed (5–8 mg) dry material was placed in an aluminium cup and hermatically
sealed. Empty cup was used as reference. Samples were analyzed under
continuous flow of dry nitrogen gas at a heating rate of 5 ◦ C min −1 from 0 to
200 ◦ C.
2.4. X-ray diffraction
The X-ray diffraction (XRD) patterns were obtained at room temperature
(300 K) by step scanning using powdered samples. We used 5 s for each step
of counting time, with a Cu K tube at 40 kV and 25 mA using the geometry of
Bragg-Brentano.
2.5. Dielectric function measurements
The complex dielectric function measurements were obtained from a HP
4291A Material Impedance Analyzer in conjunction with to HP 4194 Impedance
Analyzer, which jointly cover the region of 100 Hz to 1.8 GHz.
Fig. 1. Infrared spectra of the samples CH100, CH50CO50 and CO100.

286 C.G.A. Lima et al. / Materials Chemistry and Physics 99 (2006) 284–288
may also contribute absorption at 1452 cm −1 . In addition, significant
absorption due to CH 2 wagging vibrations from the
glycine backbone and proline side chains are also seen in the
region between 1200 and 1400 cm −1 . The absorption seen at
1340 cm −1 is attributed to CH 2 wagging vibration of the proline
side chain.
Strong absorption bands have been detected in CH100 film
spectra, namely: the complex band in the range 1690–1300 cm −1
corresponding to absorption of carbonyl and amide groups
(Amide I band at 1665 cm −1 and Amide II band at 1565 cm −1 )
and the strong absorption band between 800 and 1200 cm −1 ,
which is characteristic of pyranose rings of chitosan [13,14].
Moreover, these spectra can be influenced by parameters such as
the deacetylation percentage or crystallinity. Chitosan is essentially
produced from chitin by a deacetylation reaction. The IR
spectra of chitosan correspond to a convolution of specific signals
specific to both carbohydrates and absorption due to amine
and amide functions. A shoulder is first observed at 1637 cm −1 ,
which corresponds to the overlap of an Amide I band due to
the residual acetylated residues and –OH bands corresponding
to the pyranose structure. A transition corresponding to the
Amide II band can be detected by the presence of a strong band
at 1560 cm −1 . In the IR spectra of the CH50CO50, one can
observed a decrease of the band around 1657 cm −1 observed
in the pure chitosan, which can be attributed to local modifications
leading to small variations of the rotation and vibration
frequencies.
The DSC technique has been used by different authors to
elucidate the native structure of collagen, and how the molecule
associates to a cross-link agent or other macromolecules [15].
Denaturation is defined as a transition from the triple helix to
a randomly coiled form, taking place in the domains between
the cross-links. The bonds which stabilize the superhelix are
hydrogen, hydrophobic, Van der Waal’s bonds and interactions
between oppositely charged residues on side chains. The nonrandom
distribution of ions and hydrophobic side chains along
the repeating unit results in the occurrence of charged, positive
and negative, and hydrophobic patches that contribute to stabilization
of the structures through electrostatic and hydrophobic
interactions. On the other hand, hydrogen bonded water plays
a big part in the stabilization of the molecule. All these noncovalent
bondings break down on heating. The breakdown starting
at the weakest points of the helix, between the stabilizing
clusters. A small region containing a few linkages of low energy
will act as a favorable site to initiate denaturation [16].
The DSC of chitosan (CH100), collagen (CO100) and the
blends films are shown in Fig. 2. The results of the thermogram
analysis indicate that heating temperature influences the membranes
structure significantly. Both samples show an endothermic
transition with denaturation temperatures of 60.75 ◦ C for
CH100 and 73.75 ◦ C for CO100, (see Table 2). This is an indication
that the denaturation energy of collagen film was higher
than chitosan film. One can see the DSC spectra for samples
CH90CO10, CH50CO50, CH10CO90 which show denaturation
temperatures of 50.31, 75.59 and 58.73 ◦ C, respectively. Close
examination of thermograms in Fig. 2 reveals that there are differences
in the endotherm peak area, showing that CH50CO50
Fig. 2. DSC curves of the samples CH100, CO100 CH90CO10, CH50CO50
and CH10CO90.
Table 2
E DC activation energy from the TSPC, σ DC is the DC conductivity at room
temperature (300 K)
Samples E DC (eV) σ DC (m) −1 (300 K) a T D ( ◦ C) b
CH100 0.31 3.4 × 10 −17 60.75
CH90CO10 0.39 1.3 × 10 −17 50.31
CH50CO50 0.38 9.2 × 10 −18 75.59
CH10CO90 0.77 5.2 × 10 −18 58.73
CO100 0.50 2.4 × 10 −19 73.75
Denaturation temperature from the DSC measurements (T D ).
a The relative error of the electrical conductivity was less than ±5%.
b The relative error of the DSC was less than ±1%.
film has the higher denaturation energy. These results suggest
that collagen and chitosan may interact, in this concentrations,
to form more stable films, which may be due to formation of
a polyanion–polycation complex between the anionic collagen
and chitosan [8] that improve the water uptake in the blend
matrix [17].
The X-ray diffraction analyses are shown in Fig. 3. Ascan
be seen from the data, the collagen membrane (CO100) shows
one reflection around 7.9 ◦ and a very broad band around 21 ◦
Fig. 3. X-ray diffraction analysis of the sample CH100, CH90CO10,
CH50CO50, CH10CO90 and CO100.

C.G.A. Lima et al. / Materials Chemistry and Physics 99 (2006) 284–288 287
varies from 0.38 to 0.77 eV. For the samples under study, the
highest conductivity was obtained for CH100. Around room
temperature it is around ten times bigger than pure collagen
(CO100). We believe that the polycationic characteristic of the
chitosan molecule is the responsible for the higher electric conductivity
compared to the collagen. For the collagen film one
has a mixing of positive and negative charges which lead to a
decrease of the conductivity. With the addition of chitosan in the
blends film, the cations are the dominant charges, and may be
responsible to the increase of the DC conductivity.
4. Conclusions
Fig. 4. DC conductivity of samples CH100, CH90CO10, CH50CO50,
CH10CO90 and CO100 as a function of temperature.
which is quite characteristic of an amorphous phase. If one start
the increase of the chitosan concentration the peak at 7.9 ◦ start
decreasing and new peaks are present in the diffraction pattern.
In the case of the chitosan membrane (CH100), two peaks were
observed. One is at 11.5 ◦ and the other at 18.2 ◦ . These two
peaks are related to two different types of crystals: crystal (1)
and crystal (2) [18]. The first peak at 11.5 ◦ is related to crystal
(1) having unit cell of a = 7.76, b = 10.91, c = 10.30 (Å), β =90 ◦ .
The unit cell size of crystal (1) is larger than that of crystal (2).
The unit cell of crystal (1) consists of two monomer units along
the main chain axis. The peak at 18.2 ◦ is related to crystal (2) in
the chitosan membrane. The unit cell of crystal (2) is of a = 4.4,
b = 10.0, c = 10.30 (Å), and β =90 ◦ [18].
In Table 1 one has the dielectric constant (ε) at 1 MHz and
1 GHz for all the samples. Table 1 also show that chitosan has
the highest value of the dielectric constant for the frequencies
under study. The chitosan film (CH100) shows the highest value
for the dielectric permittivity, 3.94 at 1 MHz, while collagen
film (CO100) presents the lowest density, 670.76 kg m −2 , and
dielectric permittivity, 2, 6 at 1 MHz.
On the basis of electrical conductivity studies, the activation
energy E A of the charge conduction process for collagen,
chitosan and blends films can be determined according to the
following Arrhenius formula:
( )
EA
σ(T ) = σ 0 exp
(1)
kT
where stands for electrical conductivity, T, the temperature, and
k is the Boltzmann’s constant. Calculations were performed in
the intervals of linearity of the following relationship:
( )
EA
log σ = log σ 0 +
(2)
kT
The Arrehenius plot of the electrical conductivity and the temperature
is shown in Fig. 4.InTable 2 we have the DC conductivity
at room temperature and the activation energy of all samples.
At room temperature (T = 300 K) the conductivity increases from
2.4 × 10 −19 to 3.4 × 10 −17 (m) −1 . The activation energy also
We did a study of the physicochemical and dielectric properties
of collagen–chitosan films.
The IR spectra of the collagen, chitosan and collagen–
chitosan blend films are compatible with the main results
reported in the literature. The DSC of chitosan and collagen indicate
that heating temperature influences the membranes structure
significantly. Both samples show an endothermic transition with
denaturation temperatures of 60.75 and 73.75 ◦ C, respectively.
This is an indication that the denaturation energy of collagen film
was higher than chitosan film. For the blends, it was observed
that the collagen and chitosan may interact to form more stable
films, which may be due to formation of a polyanion–polycation
complex between the anionic collagen and chitosan that improve
the water uptake in the blend matrix. The chitosan film presents
the highest value of the dielectric constant for the frequencies
under study. At room temperature (T = 300 K) the conductivity
increases from 2.4 × 10 −19 to 3.4 × 10 −17 (m) −1 . The activation
energy also varies from 0.38 to 0.77 eV. For the samples under
study, the highest conductivity was obtained for CH100. Around
room temperature it is around ten times bigger than pure collagen
(CO100). We believe that the polycationic characteristic of
the chitosan molecule is the responsible for the higher electric
conductivity compared to the collagen. For the collagen film one
has a mixing of positive and negative charges which lead to a
decrease of the conductivity. With the addition of chitosan in the
collagen film, the cations are the dominant charges, responsible
to the increase of the DC conductivity.
Acknowledgements
This work was partly sponsored by FINEP, FUNCAP, CNPq,
CAPES (Brazilian agencies).
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