Study of the heat - induced denaturation and water state of hybrid hydrogels based on collagen and poly (N - isopropyl acrylamide) in hydrated conditions

Accepted Manuscript

Title: Study of the heat - induced denaturation and water stateof hybrid hydrogels based on collagen and poly (N - isopropylacrylamide) in hydrated conditions

Author: M.T. Nistor D. Pamfil C. Schick C. Vasile

PII: S0040-6031(14)00225-1DOI: http://dx.doi.org/doi:10.1016/j.tca.2014.05.020Reference: TCA 76887

To appear in: Thermochimica Acta

Received date: 18-3-2014Revised date: 6-5-2014Accepted date: 16-5-2014

Please cite this article as: M.T. Nistor, D. Pamfil, C. Schick, C. Vasile, Study of theheat - induced denaturation and water state of hybrid hydrogels based on collagen andpoly (N - isopropyl acrylamide) in hydrated conditions, Thermochimica Acta (2014),http://dx.doi.org/10.1016/j.tca.2014.05.020

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Study of the heat - induced denaturation and water state of hybrid

hydrogels based on collagen and poly (N - isopropyl acrylamide) in

hydrated conditions

M.T. Nistor1, D. Pamfil1, C. Schick2, C. Vasile1*

1Romanian Academy, “Petru Poni” Institute of Macromolecular Chemistry, Department of

Physical Chemistry of Polymers, 41 A, Grigore Ghica Vodă Alley, 700487, Iaşi, Romania;2 Universität Rostock, Institut für Physik, Wismarsche 41-43, 18051 Rostock, Germany

* Corresponding author. Tel.: +40 232 217454; fax: +40 232 211299

E-mail address: [email protected] (C. Vasile)

ABSTRACT: Hybrid hydrogels of collagen and poly (N - isopropyl acrylamide)

(pNIPAM), various nanoclays and hydroxyapatite, were prepared by chemical cross-

linking. These bioartificial polymeric materials were studied to establish the influence of

collagen, pNIPAM, cross-linking agent and inorganic nanoparticles content on the thermal

properties and heat induced denaturation in hydrated state by differential scanning

calorimetry. It was established that the denaturation of collagen is an irreversible kinetic

process that is governed by the temperature history, nanoparticle type in hybrid hydrogels,

specimen hydration, and cross-linking agent amount, amongst other variables. The

interaction of water with hybrid collagen and water state in semi - interpenetrated polymer

network depends on hydrogel composition and thermal history. The hybrid hydrogels keep

the temperature responsiveness. Contact angle and swelling measurements

demonstrated the correlation between hydrophobicity / hydrophilicity balance and state of

water in the polymer network and the denaturation temperature values.

Keywords: DSC, collagen, hybrid hydrogels, denaturation, hydrated state, nanoclays

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1. Introduction

Collagen belongs to a protein class of the human body and is used in the

manufacture of biomaterials. The studies of physico - chemical and biological properties

are essential because the collagen - based materials are synthesized and destined mainly

for medical applications. Some of their properties are interesting also for other

applications, such as tanning treatments for animal skins, textiles, tailoring leathers and

parchments from heritage objects, etc. [1, 2]. The sensitivity of this protein in the

manufacturing step such as sensitivity to acidic or basic environments, changes in

temperature or presence of radiation, enzymes, and so on, has led to interest in chemical

modifications and thus to improve the physico - chemical and biological properties of

collagen [3]. The most common and effective treatment is chemical cross-linking of the

peptide chain using various cross-linking agents. The efficiency of cross-linking

procedures on the thermal behavior of collagen - based materials have been studied on

gels, sponges and collagen films using different types of cross-linkers, such as

glutaraldehyde, hexamethylene diisocyanate, cyanamide, 1-ethyl-3-(3-dimethyl

aminopropyl) carbodiimide, acyl azide, etc. [4, 5]. Polymeric hydrogels designed for

medical applications are sometimes prepared with toxic cross-linking agents, which can

not be easily and completely removed, thus leading to the occurrence of side effects at

contact with living tissue. As well, the inconvenience of using common cross-linking

agents for bioproducts synthesis is related to their low compatibility with human tissue,

which limits further applications of the cross-linked polymeric materials.

The compatibility assessment studies at cellular level, followed by in vitro and in

vivo compatibility evaluation revealed that a number of chemical cross-linking agents,

along with natural enzyme cross-linkers, are compatible with the human body, which

increase the applications of such collagen - based products [6]. In addition to improved

enzymatic resistance, swelling ability, rheological properties, cross-linking agents can

enhance the thermal stability of modified collagen [7]. In our previous studies, from

biocompatibility tests performed on rats, it has been shown that the diethylene glycol

diacrylate used as cross-linking agent is nontoxic even if the LD50 of hybrid hydrogels

exceeds 3200 mg/kg and the subcutaneous implanted biomaterials presents normal

response of rats’ immune parameters [6].

Under thermal stimulus, the transition of collagen from native to denatured state is

a cooperative phenomenon with significant uptake of heat, observable on DSC curves as

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an endothermic peak. The thermal transition is responsible for the rupture of

intermolecular hydrogen bonds and appearance of the unfolded triple helix. Transition to

the amorphous state of peptide chains occurs with changes in protein conformation and

certainly of other physical properties too. These processes are stamped on the

calorimetric behavior by variation of the enthalpy changes [8-12].

Studies on the thermal behavior of collagen - based hydrogels showed their good

thermal stability compared with the native protein [13-15] or they demonstrated that the

functionalized collagen - based hydrogels preserved triple helix conformation because

covalent net-points were established during hydrogel formation, so that collagen triple

helices were successfully retained and stabilized. As mentioned above the collagen

denaturation is related to the unfolding of collagen triple helices into randomly - coiled

chains with appreciable heat absorption; it is therefore expected to be highly affected by

the formation of a covalent network [16].

Also, it has been reported that nanocomposites based on clay exhibited improved

thermal stability [17, 18] due to the interaction and surface activity of clay layers with

polymeric chains. This strongly depends on the degree of inorganic particles exfoliation /

intercalation through the polymeric mass, intrinsic thermal resistance of the polymer

chains, inorganic filler content and in case, by chemical constitution of organic modifier

inorganic particles and the chemical character of polar compatibilizers.

A better understanding of the thermal denaturation of collagen - based materials

included in polymeric systems is of a significant interest from the standpoint of medical

practical applications such as laser surgery, thermal therapy, tissue engineering, drug -

loading manufacturing process, etc.

For the practical application of such materials the interaction with water is of utmost

importance. The correlation between equilibrium water content and state of water of

hydrogels and their properties is well established, since the water has a key role on

mechanical properties and other characteristics of hydrogels [19]. Water is responsible for

some of peculiar characteristics of hydrogels, one of which is their injectability, which

makes these hydrogels suitable as matrices for mini - invasive surgery and localized

therapy. The water state in hydrogels is categorized into three different types [20], namely:

(1) free water - that is water not intimately bound to the polymer chain and behaves like

bulk water undergoing a thermal transition at temperature analogous to bulk water (at 0

ºC), (2) freezable bound water - water that is weakly bound to the polymer chain or

interacts weakly with nonfreezing water and undergoes a thermal phase transition at a

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temperature lower than 0 ºC. These two first types of water are so-called freezing or free

water and (3) bonded water (non-freezing water) which is tightly bound to the polymer and

does not exhibit a first order transition over the temperature range from –70 ºC to 0 ºC.

Differential scanning calorimetry (DSC) was widely used to characterize the water

state in hydrogel membranes being a convenient and informative method [21]. DSC was

used to assess the water state in polysaccharide hydrogels such as hyaluronic acid or

xanthan [22], chitosan [23, 24], chitosan / polyvinyl alcohol [25] chitosan / collagen [26]

poly (vinyl alcohol) hydrogels [27] acrylamide / acrylic acid copolymeric hydrogels [28]

salvia hydrogels [29] hydrogels based on poly (hydroxyethyl acrylate) [30]. According to

literature data [23, 31] the effect of hydrogel composition and morphology is discussed

mainly in respect with the bonded water content. The bonded water content in different

polymer / water systems depends on both chemical and higher order structure of the

polymer, the changes in hydrophilicity or hydrophobicity as well as crystallinity, degree of

cross-linking, a.s.o. By means of differential scanning calorimetry and cell proliferation

assay it was demonstrated that the state of water in the silk hydrogel influences the

cytotoxicity. Namely, the bonded water supports cell - adhesion proteins in the cellular

matrix to interact with the surface of the silk hydrogels [32].

Analysis of the thermodynamic properties of bulk and interfacial water of different

types of hydrogels containing collagen was also useful for their applications as soft tissue

scaffolds, artificial skin substitutes and other biomaterials [33].

The interest in hybrid systems originates from the widely debated thermo -

responsive hydrogels, especially based on poly(N-isopropylacrylamide) (pNIPAM) for

bioengineering applications, due to their capacity to respond to temperature changes as a

trigger for conformational modifications. Additionally, inorganic nanoparticles were found

to improve the physical and chemical properties of hybrid materials. Moreover by using

two sensitive polymers, respectively collagen with sensibility to pH and temperature and

pNIPAM sensitive to temperature, allows the preparation of a biomaterial with controlled

sensitivity suitable for tissue engineering or as drug-loaded polymeric system with

targeted release of therapeutic agents.

This paper deals with the study of the heat - induced denaturation and the water

state of some nanohybrid responsive hydrogels by means of differential scanning

calorimetry (DSC). The hybrid hydrogels consist in collagen and pNIPAM with various

kinds of nanoclays or hydroxyapatite nanoparticles embedded. This study focusses on the

influence of the hydrogel composition and of nanoparticles embedded in the matrix on

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denaturation temperature and bonded water content. The results have been correlated

with the swelling behavior and the variation of the contact angle. Studies were performed

in hydrated conditions taking into account the strong correlation between water content of

hydrogels and their properties.

2. Experimental part

2.1. Materials

Collagen matrix was purchased from Lohmann & Rauscher International GmbH &

Co KG - Germany. N - isopropyl acrylamide (NIPAM) purified in toluene/n-hexane mixture

and diethylene glycol diacrylate (DEGDA) were acquired from Sigma-Aldrich - Germany.

Ammonium persulfate and N, N, N, N - tetramethylethylene diamine supplied by Merck

were used as promoters of the crosslinking reaction. Dellite® 67G and Dellite® HPS

nanoparticles provided from Laviosa Chimica Mineraria S.r.l. - Italy, Cloisite® 93A

nanoparticles from Southern Clay Products – USA and hydroxyapatite nanoparticles

purchased from Sigma-Aldrich – Germany, represent the inorganic materials of hybrid

hydrogels.

The hybrid hydrogels have been prepared using a two steps procedure described

in a previous paper [3]. In brief the procedure occurs by the dispersion of montmorillonite

nanoparticles in water which preceded the hydrogels synthesis step. Insertion of

nanoparticles was possible by immersing the collagen matrices in the nanoparticles

dispersion. The hybrid hydrogels have been prepared by crosslinking the pNIPAM with

diethylene glycol diacrylate in the collagen membrane loaded with inorganic nanoparticles.

Semi-interpenetrated polymeric support fabricated by insertion of poly (N-isopropyl

acrylamide-co-diethylene glycol diacrylate) onto collagen (ND) was used as reference for

all nanocomposites hydrogels.

The study underlines the effect of natural nanoclay - Dellite® HPS (purified

montmorillonite) (HPS) compared with chemically modified montmorillonite nanoparticles,

respectively chemically modified with dimethyl dihydrogenated tallow ammonium - Dellite®

67G (G) or methyl dehydrogenated tallow ammonium - Cloisite® 93A (C) and

hydroxyapatite (HA) nanoparticles on the thermal behaviour of hybrid hydrogels. In the

synthesis of hybrid hydrogels 1 wt.% montmorillonite nanoparticles were used [6]. In the

synthesis of nanohybrid hydrogels with two inorganic nanoparticles Dellite® G67 and

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hydroxyapatite (HA) a ratio of 20 wt.% or 80 wt.% was used reported to collagen content

of polymeric matrices. Table 1 presents the compositions of nanohybrid hydrogels used in

the present study.

Table 1

The composition of the nanocomposite hydrogels

Sample Montmorillonite nanoparticles

type

Collagenwt %

pNIPAMwt %

DEGDAwt %

APSwt %

TEMEDwt %

Montmorillonite or/and HA, reported to collagen, %

1 HPS Dellite® HPS 38.4 52.9 5.4 1,5 0.8 12 HPS Dellite® HPS 30.4 59.9 6.1 1.7 0.9 13 HPS Dellite® HPS 29.8 58.6 8.1 1.7 0.8 1

1 G Dellite® G67 38.4 52.9 5.4 1,5 0.8 12 G Dellite® G67 30.4 59.9 6.1 1.7 0.9 13 G Dellite® G67 29.8 58.6 8.1 1.7 0.8 1C Cloisite® 93A 30.4 59.9 6.1 1.7 0.9 1

1G/20HA Dellite® G67 38.4 52.9 5.4 1,5 0.8 1/7.72 G/20HA Dellite® G67 30.4 59.9 6.1 1.7 0.9 1/63 G/20HA Dellite® G67 29.8 58.6 8.1 1.7 0.8 1/5.91 G/80HA Dellite® G67 38.4 52.9 5.4 1,5 0.8 1/30.72 G/80HA Dellite® G67 30.4 59.9 6.1 1.7 0.9 1/24.43 G/80HA Dellite® G67 29.8 58.6 8.1 1.7 0.8 1/23.8

2.2. Investigation methods

2.2.1. Differential scanning calorimetry

Differential scanning calorimetry (DSC) analyses were carried out with a heat-flux

Mettler Toledo instrument (model DSC 822e, Switzerland) and the thermograms were

recorded at heating and cooling rates of 5 oC / min using a two stage mechanical cooling

system. Indium (m.p. = 156.6 oC; ΔHm = 28.45 J/g) and double distilled water (TmaxW = 4.0oC; heat of ice melting ΔHw = 333.9 J/g water) were used as reference material to calibrate

the temperature and energy scales of the instrument. 5-10 mg of hydrogels was

accurately weighed (± 0.01 mg) and encapsulated in 40 μL flat - bottomed aluminum pans.

The sealed sample pans were used to prevent water loss in the DSC measurements.

Empty pans were used as references.

Investigations in the hydrated state of the hydrogels were performed in the

temperature interval starting from -50 °C to 110 °C and in the dehydrated state between 0

°C and 150 °C. The weight of all samples was checked after the DSC measurements to

ensure that no water loss had occurred during the heating runs. Data processing has been

carried with STARe and OriginLabTM software.

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Hydrated samples were prepared after direct contact of 6 - 8 mg of sample with 5 µl

of distillated water. Before performing DSC measurements, the specimens have been

previously dried and leaved in water overnight to let the water to be absorbed in the

sample.

Enthalpy changes for a given phase transition (∆H) were estimated by integrating

the DSC peak over the temperature range of the transition, Eq.(1).

dtCΔHf

i

T

T

P (1)

where the limits of integration Ti and Tf are the initial and final temperatures over which the

process occurs and Cp is the apparent specific heat at constant pressure [34].

The weight fraction of free water, nW (g) was evaluated by the method proposed by

Ross (1978) [35] according Eq.(2).

W

mW

ΔH

ΔHn (2)

where ΔHm is the melting enthalpy of free water in the sample (in J) and ΔHw is the latent

heat of ice melting (333.9 J/g water). By using Eq. (2), the bonded water content (nUW),

was determined as the difference between the total water content and the free water

content (nW) [36].

The parameters determined for collagen - based materials include: melting

temperature of ice read as onset temperature and peak temperatures, phase transition

temperature (Td) which corresponds to the maximum temperature of the denaturation

peak of collagen, enthalpy of denaturation (ΔHd, Eq.(1)) which define the amount of heat

required for denaturation [37], and the glass transition temperature (Tg), which refers to

the midpoint of the curve section in which the heat flow show a step like change [38].

2.2.2. Contact angle measurements

The static contact angles for polymer films were determined by the sessile drop

method with twice distilled water, at room temperature and controlled humidity, within 30

s, after placing 1 μL drop of liquid on the film surface, using a CAM-200 instrument from

KSV - Finland. Contact angle measurements were taken at least fifteen times at different

locations on each surface, and the average values were further used.

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2.2.3.Swelling behaviour

The swelling behavior of the hybrid hydrogels was determined by immersing dried

samples to swell in double distilled water. At specific time intervals, the samples were

removed from the swelling medium, the excess droplets were gentle and quickly wiped

with filter paper and the progress of the swelling process was monitored gravimetrically.

The swelling degree was evaluated with formulae

SD = (W-Wo)/Wo (3)

where W is weight of hydrogel in hydrated state and Wo is weight of dry hydrogel. The

determination of the swelling behaviour was described in a previous paper [3]. Some

results are used here to explain the thermal behaviour of the hydrated hybrid hydrogels.

2. Results and Discussion

Hydrogels are absorbent or superabsorbent materials. The swelling behaviour gives us

an idea about how much solvent can gels take up. The water content is related to swelling

degree (SD) defined by eq.3. Equilibrium swelling degree (ESD – the maximum swelling

degree attained, swelling profiles showing a plateau) varies in very large limits depending

on hydrogel composition, crosslinking degree, etc. reaching even order of thousands

percentages. High water content confers to hydrogels a degree of flexibility very similar to

natural tissue.

The state of water and the denaturation process of collagen, collagen / pNIPAM

hydrogel, and their hybrid hydrogels containing nanoclays and hydroxyapatite were

studied using differential scanning calorimetry (DSC) and correlated with swelling behavior

and contact angle measurements.

The most dominant feature of hydrogels over other polymers is inclusion of a high

water content within the network. The water - absorption characteristic of gels affect the

diffusive behavior of small molecules through gels, interfacial energetic of gels, and bulk

properties, etc. It is also supposed that the sorption of water decrease the glass transition

temperature of polymers because of the plasticization effect of water [24]. Therefore, a

study on the physical state of water in the hydrogels might provide useful information on

their microstructure and behavior.

Thermal behavior of collagen - based materials is dictated both by the effect of

temperature upon collagen fibers and presence of water molecules attached as free and

bonded water to polymeric chains. The temperature effect on collagen fibers is manifested

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as a thermal transition with splitting the hydrogen bonds between the polypeptide chains.

This effect is observed as an irreversible / reversible denaturation process. The

denaturation temperature is influenced by the hydration degree of the materials containing

collagen [7]. Correlation between the collagen denaturation temperature and the moisture

content was also demonstrated [39].

An important role in denaturation has the water content, respectively the hydrogen

bonds between collagen molecules and water. Also, the stability of collagen is conferred,

in part, by the hydrogen bonds between the adjacent polypeptide chains of collagen. The

water affinity to functional groups of the polymeric network or to inorganic nanoparticles is

reflected in the thermal behavior of the hydrogels. Thus, by heating collagen containing

materials the hydrogen bonds unfold more easily and the hydrothermal stability will

decrease with the increase of moisture [40, 41].

The study of specific interactions in collagen based hydrogels during the heating

program because of free and bound water in the polymeric structures was the aim of the

DSC experiments. The samples have been placed in tightly closed pans too.

DSC curves of hydrated collagen sponge presents, on the first heating run, three

endothermic processes, Fig. 1. The water melting process observed at -8 oC H = 39.5

J/g was followed by a relaxation process of collagen fibers accompanied by the absorption

of heat at 18 oC and H = 1.3 J/g and the collagen denaturation process at 73 oC and a

transition heat of 48 J/g. The denaturation process, distinct from thermal degradation,

implies that the rupture of interchain hydrogen bonds leads to the formation of an

amorphous polymer, called gelatin. The high enthalpy of unfolding the collagen immersed

in water is thought to derive mainly from the breaking of hydrogen bonds forming the

hydration network around the collagen molecules. The hydrogen bonding may be

dominated by the number and layout of the fixed hydrogen bonding sites on the collagen

itself, e.g., C=O, N–H, and hydroxyl groups on hydroxyl - proline [42].

On the second DSC heating curve appears a strong endothermic process at -6 oC

with H = 52.9 J/g. Also, the second DSC run is characterized by the disappearance of

the calorimetric peak at 74 oC visible in the first run, which underlines the irreversibility

character of collagen denaturation. The irreversible character of collagen denaturation is

described in more detail in the literature by Vyazovkin et al. [43] and other authors [44,

45].

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10

-40 -20 0 20 40 60 80-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

Hea

t Flo

w /

Mas

s (m

W/m

g)

Temperature (oC)

Endo

Exo

Collagen (first run)Collagen (second run)ND (first run)ND (second run)

Fig. 1. Calorimetric curves of collagen sponge and ND - hydrogel without nanoclay in the

hydrated state

Information about the state of water in the samples can be collected from the shape

of the corresponding melting peaks. In the case of the collagen sample, the melting

process of water takes place over a large temperature range, which informs that the

melting of water is a slow process and also it could be due to a distribution of melting

temperatures of water molecules bonded by different functional groups in the hybrid

hydrogels, requiring the capture of significant amounts of energy during the heating scan.

Ice melting, in the case of hydrogel, is a fast process marked on the DSC curves by a

sharp peak in Fig. 1. The peak temperature TmaxW was used in the discussion of the

results. Knowing that this value could depend on mass of sample also starting

temperature of endothermic peak of ice, as onset temperature Tonset was read and its

variation among samples is discussed. The denaturation temperature of the polypeptide

chains of the semi-interpenetrated network (ND) occurs at 53 oC and H = 6.1 J/g. A sub-

melting shoulder appears also at 49 oC because of different contribution of the netwok

components. The melting enthalpy of the frozen water in the polymeric network is different

to normal water. The melting temperature of frozen water appears at higher temperatures

of 0.6 oC, H = 8.3 J/g for the hydrogel compared with -8 oC - -6 oC, H = 39.5 J/g for pure

collagen. This temperature difference is due to the polymeric network formation and its

ability to keep water molecules inside by interaction with functional groups.

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The DSC curves corresponding to the first (Fig. 2a) and the second run (Fig. 2b) of

hydrated hydrogels present a dominant peak for the melting process of ice which suggests

the increase of free water after the first heating – Fig. 2 and Table 2.

The effect of collagen encapsulation in a cross-linked polymer matrix was observed

on the second heating scan as an endothermic process, attributed to the denaturation of

polypeptide chains. The maximum temperature of the second endothermic process

decreases for the hydrogel to 53 oC compared to the pure collagen matrix of 73 oC. An

important effect on the denaturation temperature has the way of dispersion of the collagen

in the synthetic matrix, the ratio between the natural and synthetic polymers, degree of

cross-linking of the synthetic polymer but not at least the presence of water molecules in

the hydrogel structure, which leads to a destabilization of the polypeptide chains. Inside

the collagen and hydrogels structure, the water molecules exist more in free form as

sorbed water, observable in the DSC curve as a broad peak. The thermograms of semi -

interpenetrated structures show a sharp water melting peak with a maximum temperature

at 0.1 oC – Fig. 2. The cross-linked structures have the ability to bind the water molecules

by hydrogen bonds to the functional groups of the pNIPAM and DEGDA agents [46, 47].

Also the amount of bonded water is higher for the cross-linked networks in

detriment of free water (Table 2). This indicates that more bonded water is present in the

hydrogel structure, but with selective bonding to functional groups of the semi -

interpenetrated polymeric chains.

-40 -20 0 20 40 60 80

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

40 45 50 55 60 65 70-0,70

-0,65

-0,60

-0,55

-0,50

-0,45

-0,40

-0,35

-0,30

Temperature (oC)

Hea

t Flo

w /

Ma

ss (

mW

/mg)

Temperature (oC)

Hea

t Flo

w /

Ma

ss (

mW

/mg)

C ND 2 HPS 2 G 2 G 20 HA 2 G 80 HAEndo

Exo

(a)-40 -20 0 20 40 60 80

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

40 45 50 55 60 65 70-0,70

-0,65

-0,60

-0,55

-0,50

-0,45

-0,40

-0,35

-0,30

Temperature (oC)

He

at

Flo

w /

Ma

ss (

mW

/mg

)

Temperature (oC)

He

at F

low

/ M

ass

(mW

/mg

)

C ND 2 HPS 2 G 2 G 20 HA 2 G 80 HA

Endo

Exo

(b)

Fig. 2. DSC curves of the hybrid hydrogel nanocomposites in hydrated state recorded in -

50 oC - 100 oC temperature range: (a) the first run, (b) the second run

The endothermic peaks in the first and the second heating scans of hydrated

nanohybrid hydrogels are attributed to the melting process of free and bonded water,

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characteristic to hydrophilic materials. The second endothermic peak in the first DSC

heating scan corresponds to denaturation of collagen – Table 3.

Table 2

Parameters of water state and content from DSC curves of nanohybrid polymer hydrogels

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Sample Sample mass[mg]

Water added

[µl]

TonsetW

(first run)[oC]

TmaxW peak(first run)

[oC]

ΔHW

first run[J/g]

TonsetW

(second run)[oC]

TmaxW peak(second

run)[oC]

ΔHW

second run [J/g]

nUW

(first run)*[g/g]

nUW

(secondrun) *[g/g]

nW (firstrun)* [g/g]

nW

(second run)* [g/g]

Collagen 6.92 4.08 -39.4 -8 39.5 -27.3 -6 52.9 0.47 0.43 0.11 0.16ND 8.35 4.63 -15.5 0.6 8.3 -10.5 0.8 12.9 0.53 0.52 0.02 0.04

1 HPS 6.99 4.87 -22.5 1.08 106.9 -17 1.1 and 2.1 128.7 0.37 0.31 0.32 0.382 HPS 7.15 4.19 -23.8 0.7 88.7 -18.8 1.1 103.6 0.50 0.41 0.29 0.333 HPS 6.91 5 -25.2 1.2 84.4 -21.1 1.9 100.7 0.47 0.42 0.25 0.30

1 G 7.48 5 -18 -1.3 and 0.7 64.6 -17.4 1.3 73.2 0.47 0.45 0.19 0.222 G 7.27 5 -14 -0.7 and 0.9 58.6 -14 1.5 65.6 0.51 0.49 0.17 0.193 G 6.99 4.69 -18 -0.7 and 1 59.8 -18 1.2 71.8 0.49 0.46 0.18 0.21C 6.78 4.63 -28.6 -3.9 and 0.5 58.3 -26.7 -3.7 48.1 0.51 0.54 0.17 0.14

1 G 20HA 7.17 5 -16.3 -1 and 0.84 80.5 -16.4 -0.6 and 1.4 87.6 0.45 0.43 0.24 0.262 G 20HA 7.1 5 -16.5 -0.9 and 1 67.7 -16.5 -1 and 1.4 76.6 0.52 0.49 0.20 0.233 G 20HA 7.45 4.79 -16.5 -1 and 0.84 61.8 -16.2 -0.8 and 1.4 67.7 0.46 0.44 0.18 0.201 G 80HA 7.38 4.96 -14.5 1.2 100.2 -14 2 109.2 0.37 0.34 0.30 0.322 G 80HA 7.65 4.49 -13.5 1.2 95.7 -13.5 1.8 101.9 0.30 0.28 0.28 0.303 G 80HA 7.18 5 -14.5 1.2 98.5 -14.5 1.8 112.2 0.39 0.35 0.29 0.33

T onsetW = onset temperature of the ice melting process; TmaxW = the peak temperature of the ice melting;ΔHW = melting enthalpy of water; nW = free water content, nUW = bonded water content, *reported to dry hydrogel sample.

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Because of interaction of water with hydrogel components the values of Tonset W = -

14 oC and Tmax w = 4.0 oC are strongly modified in all hybrid hydrogels.

The characteristic temperatures TonsetW and TmaxW of the ice melting are the lowest

for collagen (of -39 °C and -8 oC, respectively). In the second run the values found are of

-27.3 oC and -6 oC respectively and also a new peak appears at low temperatures of about

- 44 oC. According to literature data [48], the last peak can be associated with a β-

relaxation process.

These characteristic temperatures significantly increase for hydrogels to -15 oC and 0.6 oC, respectively, in the case of hydrogels without nanoparticles (ND) and they vary from -

13 to -29 and -3.9 -0.5 oC in the case of hybrid hydrogels. The lowest values have been

recorded for hydrogels containing Cloisite® 93A and Dellite® HPS. A significant reduction

of the melting enthalpy of ice was also observed for hydrogels and hybrid hydrogels -

Table 2. Each group of hybrid hydrogels exhibits particular values for these thermal

characteristics namely: TonsetW = -25 - -22oC; HW = 84 - 100 J/g dry sample for HPS

hydrogels; TonsetW = -14 - -18 oC; HW = 58 - 65 J/g dry sample for hydrogels loaded with

Dellite® 67G nanoparticles; TonsetW = -16 oC; Hw = 62 - 81 J/g for hydrogels with Dellite®

67G and hydroxyapatite nanoparticles (20 wt. % HA reported to collagen content); TonsetW

= -13 - -14.5 oC; HW = 96 - 100 J/g dry sample for hydrogels with Dellite® 67G and

hydroxyapatite nanoparticles (80 wt. % HA reported to collagen content) corresponding to

the first run. This particularity is maintained also for ΔHW values determined in the second

run, all values being increased in respect with those found for the same process in the first

run as follows: of TonsetW = -18 - -21 oC; HW = 100 - 129 J/g; TonsetW = -14 - -18 oC; HW =

66 - 73 J/g; TonsetW = -16 oC; HW = 68 - 88 J/g and TonsetW = -13 - -14.5 oC; HW = 109 -

112 J/g dry sample for hybrid hydrogels containing Dellite® HPS, Dellite® 67G, Dellite®

67G / 20 wt. % HA and Dellite® 67 G / 80 wt. % HA nanoparticles, respectively. The

hydrogel with Cloisite® 93A nanoparticles has also specific values for the thermal

characteristics of these processes. This is probably due to the specific bonds of each type

of hydrogel / nanoclay with water molecules. Also, the hybrid hydrogels present

considerable free water molecules. The melting enthalpy of water has been significantly

increased for the hybrid hydrogels, which highlights the effect of inorganic nanoparticles.

Because the pans were hermetically closed and the total water amount was constant as it

was checked after each experiment, only the ratio between bonded and free water was

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changed. The bonded water percentage is lower in the second run increasing

correspondingly the amount of free water.

Hybrid hydrogels uptake a high water quantity, see Table 3 and Table 4. The

curves for G and Dellite® 67 G / 20HA hydrogels present a splitting of the ice melting

peak. The two peaks are a proof for differences between the two kinds of water. The

higher the swelling capacity (for HPS and C- containing hybrid hydrogels) – Table 4 - the

more pronounced is the splitting. The splitting was not found for hydrogels containing

Dellite® 67G and hydroxyapatite nanoparticles in a ratio of 80 wt. % (reported to collagen

content) which exhibit a low swelling capacity - Table 4. Also, Higuchi et al. [20] found that

the absence of peaks below a certain water content in DSC curves indicated that this

water is of a non-freezing bound type, which has no detectable phase transition over the

temperature range usually associated with the water freezing / melting transition from 200

- 300 K. It is supposed that in the initial swelling process, water molecules first disrupt the

intermolecular hydrogen bonds that are not strong and then bond to the hydrophilic sites.

These water molecules are isolated and uniformly distributed throughout the polymer and

exhibit greatly restricted mobility, so the temperature at which the ice melts is higher for

hybrid hydrogels. In other studies [49], it was demonstrated that above a characteristic

level of water for each polymer system, only one phase in the free water portion called

freezing intermediate water is detectable calorimetrically. The additional water is

preferentially oriented around the bound water and the polymer network structure as a

secondary or tertiary hydration shell, which is in a form generally called clusters in which

appears a tendency of water molecules to form the maximum amount of hydrogen bonds

[50]. As the water content further increases, the splitting of the melting peak becomes

more apparent in the DSC curves, suggesting the existence of two states of free water in

the hydrogels. It is already known that the intermediate water in the free part exhibits

lower melting temperature than the free water, as stated in the three - state water model

[51] and the free water portion becomes more distinct thermodynamically as the hydrogels

gradually approach the equilibrium state with bulk water.

The values of the bonded water quantity does not varies very much between the

first and the second run although at a careful examination a slight decrease can be seen,

while free water quantity values increased in the second run in respect with values

recorded in the first run because of decreasing bonded water.

These findings are in accordance with those found by other authors. Ping et al.

found that the bonded water molecules depend on the chemical nature of the polar site of

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collagen, more available for amide and carboxylic group then for hydroxyl groups [52].

Tseretely et al. show that thermal characteristics of gelatins depend on the type of water

contents and the free water molecules, in distinction to the bound water forms a rigid

matrix inhibiting the glass transition, similarly to several denatured biopolymers [53].

Usually, the hydrogels properties were attributed to the number of molecules of freezing

bound water in the polymeric network and the chemical modification of collagen and its

cross-linking degree [40].

As concerns the variation of the characteristic values with the cross-linking agent

content of the samples, all characteristic temperatures increase; ΔHW decrease for most

samples in both runs except Dellite 67® G / 80HA hydrogel, where also the bonded water

increases while free water quantity decreases.

In the hydrated state, the transition temperatures decrease with increasing water

amount present in the samples and in the environment. The first transition temperature in

the DSC curves recorded at RH 23 % lies between 34 - 39 oC, has smaller values in the

second run but they remain close to those found in the first run. This could be explained

by a less ordered structure of collagen chain after the first heating. This transition seems

to be reversible as it appears also in the second run and it can be assigned to pNIPAM

chains collapse by heating [54]. The low values of this transition in the second run in

respect with those found in the first run are explained by changing of the interactions from

collagen / pNIPAM to gelatin / pNIPAM after denaturation of collagen during the first

heating run.

The thermal behavior of the nanohybrid hydrogels is dependent on the composition

of the polymeric matrix, respectively on the ratio between natural and synthetic polymers,

crosslinking degree, type of inorganic nanoparticles, nanoclays or content of

hydroxyapatite nanoparticles, as can been seen in Table 3 and Fig. 2.

The transition temperature was not detected for the hydrogel without nanoclay and

also not for the hybrid hydrogel containing HPS, but this transition was detectable

between 40 - 62 oC for the other hybrid hydrogels.

The denaturation temperature varies from 58 - 64 oC for all hydrogels and is not

reversible, it does not appear in the second run (Fig. 2b).

As concerns the effect of the type of nanoparticles on the denaturation process it

can be recognized from Table 3, that both the Td values and the corresponding H take

particular values for each type of nanoclay, namely of 63 - 64 oC for hybrid hydrogel

containing HPS, 58 - 60 oC for hybrid hydrogel containing G, 40 oC for C, 59 - 60 oC for

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hybrid hydrogel containing G / 20HA and 59 - 64 for hybrid hydrogel containing G / 80HA.

The transition heat of the processes has low values but we observe a decrease with

increasing quantity of cross-linker.

Table 3

Transition temperatures of hydrogels in hydrated state

Endothermic deviation (first run)

Endothermic deviation

(second run)

Endothermic (first run)

Endothermic (first run)

Sample

T [oC] T [oC] T [oC] H [J/g] Td [oC] RH 23%

Collagen NO NO 73 48 75ND NO 30 53 6.1 79

1 HPS NO NO 63.8 6.1 702 HPS 39.4 37 62.5 5.3 783 HPS 35.5 30 63 4.9 77

1 G 35.5 30 58 6.2 982 G 36.5 31 59.5 5.3 823 G 36 30 60 7.3 76C NO NO 40 9.3 81

1 G 20HA 34.8 30 60.5 6.1 822 G 20HA 35.3 34 58.5 4.4 853 G 20HA 35.7 31 61 3.6 851 G 80HA 34.4 33.2 57.8 1.2 782 G 80HA 34.1 33.2 61.6 0.8 803 G 80HA 34.6 33.2 64.5 1.8 80

Denaturation temperatures of the hybrid hydrogels only conditioned at a relative

humidity of 23 % show higher values than native collagen. These last results are in

accordance with those found by other authors. Tronci et al. also found for the cross-linked

samples a denaturation temperature in the range of 68 – 88 °C, higher than the

denaturation temperature of native collagen (Td ~ 67 °C) [16]. The effect of crosslinking

agents was highlighted by Usha and co-workers, which used covalent and coordinate

covalent cross-links, respectively formaldehyde and basic chromium sulfate to treat

collagen fibers. The collagen treated with basic chromium sulfate exhibits a marked

increase in the peak temperature and enthalpy due to a net increase in the number of

intermolecular cross-links bonds [55]. The highest value of 98 oC was found for hybrid

hydrogel containing Dellite® 67G nanoparticles (G) which is higher than that of collagen

therefore by using this nanoclay the thermal stability of the materials is improved.

Table 4

Swelling capacity of the collagen based materials in DSC conditions (SDDSC) and

equilibrium swelling degree (ESD)*

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Sample name SDDSC, % ESD, % SDDSC/SDEQSD

Collagen 58.9 272 0.21ND 55.5 340 0.16

1 HPS 69.7 2533 0.032 HPS 58.6 2248 0.033 HPS 72.4 217 0.33

1 G 66.8 170 0.392 G 68.8 1066 0.063 G 67.1 195 0.34C 68.3 948 0.07

1 G 20HA 69.7 100 0.692 G 20HA 70.4 83 0.853 G 20HA 64.3 54 1.191 G 80HA 67.2 75 0.892 G 80HA 58.7 49 1.193 G 80HA 69.6 45 1.54

*swelling degree was evaluated with formulae SD = (W-Wo)/Wo where W is weight of hydrogel in hydrated

state and Wo is weight of dry hydrogel. ESD was determined and discussed in previous paper [6]

The effect of inorganic nanoparticles is also evident in the contact angle variation

which strongly depends on the particle type – Fig. 3. Thus, for Dellite® HPS and Cloisite®

93A the contact angle decreased in respect with that of the hydrogel without nanoparticles

while Dellite® 67G nanoparticles incorporation led to an increased contact angle. This

variation depends on hydrophylicity of the clay nanoparticles. The classification was based

on the modifier type of the clays which determines the hydrophobicity of the clay given by

the producer Cloisite® Additives Nano-Scale Additives for Reinforced Plastics [56].

The wettability (hydrophilicity / hydrophobicity balance) of a biomaterial surface is

considered as one of the critical factors determining its biological performance [57, 58].

The Dellite® 67G is a dimethyl dihydrogenated tallow alkyl ammonium cation with

high hydrophobicity, while Dellite® HPS (hydrophilic smectite clay) and Cloisite® 93A

(methyl dehydrogenated - tallow quaternary ammonium) are hydrophilic nanoclays and

they will decrease as expected the contact angle. In the second series of hybrid

hydrogels, the HA content determine the contact angle this being decreased at high

content of HA.

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ND

1 HPS

2 HPS

3 HPS

1 G

2 G

3 G C

0

20

40

60

80

100

Co

nta

ct a

ng

le, d

egre

es

a)

ND

1G 2

0HA

1G 8

0HA

2G 2

0HA

2G 8

0HA

3G 2

0 HA

3G 8

0HA

0

20

40

60

80

100

Co

nta

ct a

ng

le, d

egre

es

b)

Fig. 3. Contact angle with water of hydrogel containing nanoclays (a) and nanoclays and

HA (b)

Shu-Hua Teng et al. [59] studied collagen / hydroxyapatite nanocomposite thin films

containing 10, 20, and 30 wt. % HA. A minimum contact angle of 36.5° was obtained at 20

wt. % HA, suggesting that these coatings would exhibit the best hydrophilicity. Collagen

itself is known to have good hydrophilic properties, showing a contact angle of 42.2°. It

was reported that pure HA disks have a contact angle of about 67°, reflecting their more

hydrophobic nature as compared to that of collagen. Therefore, when the HA content in

the composite was further increased up to 30 wt. %, their contact angle showed a

tendency to increase again toward the value of pure HA. It should be noted that,

compared with the pure collagen, the composite based on collagen with 30 wt. %

hydroxyapatite still exhibited a relatively hydrophilic surface. The variation in the contact

angles of these composites with their composition is likely to result in different biological

performances.

The characteristic contact angle for each type of hybrid hydrogel indicate that their

interactions with a surrounding medium will be different. This also explaines the

dependence of the thermal characteristics and the water state in the hydrogels on the

nanoclay type.

4. Conclusions

Understanding the thermal behavior and the denaturation of collagen especially in

fluid medium is very useful taking into consideration widespread use of heating therapies

in cardiology, dermatology, gynecology, neurosurgery, oncology, ophthalmology,

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orthopedics, and urology or tissue engineering. It was established that the denaturation of

collagen is an irreversible kinetic process that is governed by the temperature history,

nanoparticle type in hybrid hydrogels, specimen hydration, and cross-linking, amongst

other variables. The denaturation of the hybrid hydrogels is not reversible. As well, the

transition temperature was not detected for hydrogels without nanoclay and hybrid

hydrogel containing Dellite® HPS nanoparticles, but detectable around 40 - 62 oC for most

hybrid hydrogels. The interaction of water with hybrid collagen and the water state in semi

- interpenetrated polymer networks depends on hydrogel composition and thermal history.

After the first heating scan, the ratio between bonded and free water is changed.

We observe that, the hybrid hydrogels keep the temperature responsiveness even

after several heating scans.

Contact angle and swelling measurements demonstrated a correlation between

hydrophobicity / hydrophilicity balance and the state of water in the polymer networks and

the denaturation temperature values.

Acknowledgement

The authors acknowledge the financial supported by COST FA0904 action through

a STSM at Universität Rostock, Institut für Physik, Rostock, Germany.

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Highlights

Denaturation temperature and water state of hybrid hydrogels were determined by

DSC;

Denaturation temperature is irreversible process

Hydrogels kept responsiveness no matter their crosslinking degree and hydrated

state

Relationship between water state - swelling degree and contact angle was

established.