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Development of new shellac based pH responsive polymer for biomedical applications.

Introduction

Natural polymers carries importance in various biomedical/ biomaterial field of research (1). Shellac is a natural biocompatible polymer secreted by lac insects Lacciferlaca (Kerr) (2). Shellac is known for its usefulness in preparation of bio-packaging films, adhesives, thermoplastic materials, sealants and insulation material (3-9). It is a non-poisonous material which also finds a number of useful applications in food industries (3).

Chemical structure of shellac contains a number of carboxylic and hydroxyl side chain functional groups (1-3). Therefore, it is possible to chemically modify the functional groups of shellac to develop new materials with improved functional properties which can be useful for various biomedical applications (10). As an example, a new composite film prepared from chemical modification of shellac with hydroxylpropyl methylcellulose is widely used as food packaging material (11). Recently, shellac based carrier matrix have been developed for controlled release of metronidazole (MZ) drug (12). It is interesting since the release kinetics of MZ could be modulated with variation of shellac quantity or annealing temperature of the medium. The modulation resulted in shifting of controlled release kinetics of drug from relaxation-controlled to diffusion-controlled when either shellac quantity or the annealing temperature of medium varied (12). In a new application study, shellac-succinate derivative showed improved enteric properties with enhancement of solubility at pH of small intestine (13,14). Pearnchob et al. (15) studied the improvement in the disintegration of shellac-coated soft gelatin capsules in simulated intestinal fluid through the addition of pore-formers such as organic acids and hydrophilic polymers. It was observed that the addition of hydrophilic polymers resulted in improvement of slow disintegration of shellac-coated soft gelatin capsules (15).

Literature survey reveals that use of shellac based materials in biomaterial research and pharmaceutical applications are yet to be exploited and lot of opportunity exists in this direction (15). However, one of the limitation in using shellac in biomedical field research is attributed to its poor mechanical strength (8,9,15). Hydrogel materials, which are pH responsive in characteristics, represents a new kind of polymeric material used since decade for various biomedical applications (16-23). Therefore, development, characterizing of shellac based hydrogel material for biomedical use could be a new attempt in this direction. Apart from this, use of shellac in biomedical field of research is expected to improve local economy and acceptance of material in research/industry.

Therefore, the present research work aims at development of a new shellac based pH responsive material which can be used as carrier matrix for controlled drug release applications. The material was prepared using polymerization technique (24).Further characterization of prepared materials was done using various instrumentation techniques such as elemental analysis, FTIR, SEM, DSC and TGA. The kinetics of thermal decomposition of material was also studied using various mathematical models. In addition, various physical and chemical properties (i.e., equilibrium swelling, kinetics of controlled release of 5-asa in simulated environment and biodegradation of the material) were also evaluated using standard protocols.

Materials and methods

Shellac was obtained from Indian Institute of Natural Resin and Gums (IINRG), Ranchi, India. Acrylic acid (AA, Fluka A.G.) was vacuum distilled at 63[degrees]C/12 mmHg prior to use. 2,2 dimethoxy-2-phenyl acetophenone was procured from Acros Organics. 5-aminosalicylic acid was obtained from Sun Pharmaceuticals, India. All other chemicals were AR grade and obtained from sources such as Fluka, CDH, Merck, Acros, and Bengal Chemicals.

Artificial gastric juice was prepared by mixing 7.0 mL of HCl (37 wt%) and 2.0 g NaCl dissolved in 1 L of de-ionized water (25). Similarly, artificial intestinal fluid was prepared by dissolving 6.8 g potassium dihydrogen phosphate in 500 mL of de-ionized water (25). Further, the pH was adjusted to 6.8 followed by dilution to 1 L using de-ionized water.

Instrumentations

FT-IR spectra of sample materials were recorded in IR-Prestige 21 (Shimadzu) instrument. The SEM of sample materials were done using JSM 6360LV instrument (accelerated at 20 kV in vacuum). TGA of sample materials were conducted using DTG 60 (Shimadzu) in inert environment (nitrogen flow rate: 20 mL [min.sup.-1]) and @10[degrees]C min-1 heating rate. The DSC study was conducted on a TA Instrument (Model Q 10) in nitrogen environment. The concentration of drug sample was measured at wavelength of 214 nm using UV-Vis spectro-photometer (Perkin Elmer, Lambda-25). pH of sample solutions were measured using Systronic digital pH meter (model 335) equipped with calomel glass electrode.

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Preparation of Sh-AA-MBA

A solution of shellac (Sh) sample was prepared by dissolving fixed amount (1g) of shellac flakes in dried ethanol. Acrylic acid (AA) was added drop-wise to the solution. The solution mixture was placed in a photochemical reactor (Philips Comptalux)fitted with incandescent broad-spectrum lamp positioned 25cm above the reaction mixture. Polymerization reaction was initiated by adding 2,2 dimethoxy-2-phenyl acetophenone (1.0 mol%) to the reaction mixture and subsequently methylene-bis-acrylamide (MBA) was added to the mixture. The composition (mol%) of final solution mixture was: Sh:1::MBA:0.3::AA:0.4. The entire solution mixture was kept under irradiation for 2.5 h till gelation. The material (Sh-AA-MBA) was collected and thoroughly washed with ethanol. Samples were then immersed in distilled water for 24 h and subsequently dried at (50[+ or -]2)[degrees]C till constant weight. The proposed mechanism of formation of chemically modified shellac (Sh-AA-MBA) is illustrated in Scheme-1 (26-29).

[FIGURE 1 OMITTED]

Swelling study

The swelling study of polymer Sh-AA-MBA was carried out at constant temperature (37[degrees]C) using buffer solutions of pH 2.1, 4.2, 7.1, 9.0 and 11.0. All pH values were precisely checked to the accuracy of [+ or -]0.1. The swollen weights of sample material was measured after carefully removing the surface liquid. Thus, the percent swelling ratio was calculated using following equation (30):

%Swelling ratio =100 x [[W.sub.t] - [W.sub.o]]/[W.sub.o] [1]

Where,'[W.sub.o]' is the initial weight and '[W.sub.t]' the final weight of the material at time 't'. All data points were average means of three determinations and less than 3% variation from the mean was noted in all cases.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Controlled release of drug (5-asa)

The controlled release of 5-asa was studied using following protocol Zhang et al. (30). A known amount of 5-asa was incorporated in situ reaction mixture while monitoring the reaction parameters. The polymeric material containing the drug was washed with solvent and stored under freeze-dried condition until further use.

Further, in vitro cumulative release studies of 5-asa was measured by placing fixed weight of material loaded with 5-asa in different buffer solutions (simulating gastric fluid and simulating intestinal fluid at 37[degrees]C). At different time intervals, aliquot absorbance was checked. It is to be noted that plotted data points were the average value of three determinations and in all cases less than 5% variation from the mean was observed.

Study of biodegradation of polymeric material

The biodegradation of polymeric material Sh-AA-MBA was carried out by following protocol: The polymeric material was immersed in phosphate buffer solution of pH 7.4 along with enzyme lipase (1 mg [mL.sup.-1]). The entire mixture was incubated at 37[degrees]C with constant shaking at 100 r.p.m. The immersed polymeric materials were taken out at different time interval from buffer solution; washed thoroughly with de-ionized water and lyophilized. The solution was replaced after every 8 h in order to maintain the enzymatic activities (30). The dry weight of polymeric material was calculated. The study was continued for 48 h. The residual weight (%) of the polymeric material was calculated by following equation (30):

[FIGURE 6 OMITTED]

Polymer residual weight (%) = 100 x (weight of dried sample after degradation) / (weight of sample before degradation)

Results and Discussion

Elemental analysis

Shellac contains polyester unit consisting of aleuritic acid and terpenic acid parts (14).The elemental analysis of C, H and N content of shellac sample shows % of C and H as 28.3, 16.5, respectively. Further, elemental analysis (C, H and N) of Sh-AA-MBA shows % of C, % of H and % of N as 45.2, 21.9 and N 4.2, respectively.

FTIR Analysis

FTIR is one of the useful tools for elucidating the basic structural characteristics of materials. The spectrum of shellac and Sh-AAMBA is illustrated in Figure 1. Shellac shows characteristics hydroxyl groups stretching vibration (i(O-[H.sub.str])) in the range 3500-3400 [cm.sup.-1] (2). The peaks observed at 1720[cm.sup.-1] and 1250[cm.sup.-1] were attributed to i (C=O) and a[(C-O).sub.def], respectively, indicating presence of carboxylic functional groups (12,31). Peaks observed at 2843 [cm.sup.-1] and 2905 [cm.sup.-1] were assigned to i [(CH).sub.str] of methylene (-C[H.sub.2]) and methyl (-C[H.sub.3-]) groups, respectively. The bending/deformation mode of methylene and methyl groups (a(C[H.sub.2]/C[H.sub.3])) were observed within the range 1500-1400[cm.sup.-1]. The peak observed at 1100[cm.sup.-1] indicated presence of ester linkage in shellac moiety (32).

In FTIR spectrum of Sh-AA-MBA, peak observed at 3600[cm.sup.-1] was attributed to the i(N-[H.sub.str]) vibrational frequency (18). No other significant changes in other peak positions were noted in modified shellac with reference to shellac. However, overlapping and broadening of peaks of methylene and methyl group of protons (i [(C-H).sub.str]) and a(C[H.sub.2]/C[H.sub.3])) were observed in the region 1300-1500[cm.sup.-1] Hence, analysis of FTIR spectrum of materials indicated the chemical modification in shellac material (13,33).

Thermo-gravimetric analysis

Thermo gravimetric analysis (TGA) is helpful in determination of relative thermal stability of the materials. The thermograms of shellac and Sh-AA-MBA is illustrated in Figure 2. Shellac shows negligible weight loss within 250[degrees]C. On the other hand, nearly 10% weight loss was observed in case of Sh-AA-MBA within 200[degrees]C temperature. The corresponding derivative curve of Sh-AA-MBA, in Figure 3, shows inflection at 141[degrees]C (onset at 122.2[degrees]C and end-set at 160.5[degrees]C) associated with 865.1mJ of energy absorption in the process. The weight loss within 200[degrees]C incase of Sh-AA-MBA could be attributed to evaporation/ elimination of solvent molecules present in the core of the matrix and/or elimination of small molecules by breaking of hydrogen bonds (14). Sh-AA-MBA shows further 20% weight loss in the temperature range of 200-310[degrees]C. Thus, an inflection at 270.3[degrees]C (onset at 254.7[degrees]C and end-set at 308[degrees]C)was noted in derivative curve of Sh-AA-MBA which could be attributed to the breaking of various cross-linked chains present the core of the polymeric matrix and the process involves 9.5J of absorption of energy (34).It is to be noted that the decomposition of Sh-MBA-AA occurs in stages. It was observed that on increasing the temperature beyond 300[degrees]C, the material rapidly degrade. It could be noted that almost 60-65% material loss occurred within 500[degrees]C temperature which could be attributed to the complete breaking of molecules leading toproduction of small fragmented species.

[FIGURE 7 OMITTED]

Analysis of TGA data could be helpful in evaluation of various kinetics parameters involving the decomposition process which also carries immense importance in studying material behavior for various biomedical applications (35). Hence, two different mathematical models i.e., Coats-Redfern (CR) (a model-fitting method) and Kissinger-Akahira-Sonuse (KAS) (a model-free method)were utilized to evaluate the kinetic parameters (36,37).

Both CR and KAS mathematical models involves basic Arrhenius equation which can be expressed as:

k = A x [e.sup.(-E/RT)] (2)

where, 'k' is the reaction rate constant, 'A' is the pre-exponential factor, 'R' is the molar gas constant and 'E' denotes the activation energy.

[FIGURE 8 OMITTED]

Coats-Redfern (CR) model is an integral method which assumes various order of reaction and compares the linearity in each case to select the correct order. Asymptotic series expansion is used for approximating the exponential integral. Hence, logarithmic form of CR model can be represented as [38]:

ln[ln(1-x)/[T.sup.2]] = ln[AR/aE] E/RT (3)

where, a denotes the heating rate. The slope of the curve ln[ln(1-x)/[T.sup.2]] vi. 1/T should yield the value (-E/R) from which activation energy E can be computed (39).

Kissinger-Akahira-Sonuse (KAS) model, on the other hand, represents integral iso-conversional technique which can be helpful in calculation of activation energy (39). KAS model, in logarithmic form, is represented as:

ln(a/[T.sup.2]) = ln[AR/Eg(x)] E/RT (4)

From equation4 it is clear that for constant 'x', the plot of ln(a/ [T.sup.2]) vs. 1/T should yield a straight line and subsequently activation energy (E) could be evaluated from the slope of the curve. It is to be noted that CR model assumes1st order decomposition kinetics. Hence, computational results obtained for CR and KAS model, in case of Sh-AA-MBA, is illustrated graphically in Figure 4 and Figure 5, respectively. Further, various kinetic parameters are also furnished in Table-1. Hence, Arrhenius equations for the thermal decomposition reactions were computed ask = (3.4 x [10.sup.21]) [e.sup.-1599/RT][sec.sup.-1] and k = (122.0) [e.sup.-1844/RT][sec.sup.-1], respectively, for shellac and Sh-AA-MBA. It was observed that the activation energies were positive in all cases indicating that no phase transition took place over the selected temperature range (39). Moreover the best fitting model is decided from the correlation coefficient (R2) whose value should approach 1 (39). The magnitude of the activation energies obtained by both the models were equal indicating that the correct approach was adopted for calculation purpose in case of both models (39). The thermodynamic parameters were also calculated using following equations:

A = (kT/h) [e.sup.(AS/R)] (5)

[FIGURE 9 OMITTED]

Where 'k' is the Boltzmann's constant and 'h' is the Planck's constant.

The enthalpy (AH) and Gibb's free energy (AG) factors were computed from following equations (39):

AH = E RT (6)

and AG = AH TAS (7)

Hence, the values of AG, in Table-1, indicated that the decomposition reaction of both shellac and Sh-AA-MBA were not spontaneous in nature. Further, values of AS, AH and AG, computed from CR model for both shellac and Sh-AA-MBA almost remained in close range indicating similarity in thermodynamic characteristics of materials (39). Study of these properties of materials shall be helpful in appropriately designing the material for various biomedical and pharmaceutical purposes involving material--drug interactions (5).

Differential Scanning Calorimetric (DSC) analysis

The DSC of both shellac and Sh-AA-MBA is illustrated in Figure 6. Dried shellac sample were properly grinded with mortar and pestle and sieved through 120 mesh. The grinded sample material was used for the study. All the experimental measurements were carried out in nitrogen environment (20ml/min flow rate) and heating rate of 5[degrees]C/min. Shellac shows one sharp endotherm at 70.2[degrees]C (onset at 60.7[degrees]C and end-set at 88.4[degrees]C) attributed to the melting of the material (13). The energy involved in the process was computed to be 42.36J [g.sup.-1]. The broad endotherms observed in the range 150-289[degrees]C, (onset at 175.1[degrees]C and end-set at 345.2[degrees]C) could be attributed to energy absorbed due to self-polymerization of carboxylic acid and hydroxyl functional groups present in shellac structure (12). The energy absorbed in the process was computed to be 162.6J [g.sup.-1] Exothermic peak observed at 376.8[degrees]C could be attributed to the decomposition of material at higher temperature.

Sh-AA-MBA recorded an endotherm at 136[degrees]C indicating that the structural characteristics of the material was changed after modification. The endotherm could be attributed to the elimination of solvent molecules or phase transition (28,40). Endothermic peak observed at 289.9[degrees]C indicate absorption of energy due to elimination of hydrogen bonded molecules present in the core of the matrix (40). It is to be noted that the chemical modification of shellac could result in formation of more crosslinked structure which can hold molecules/formed hydrogen bonded structures in the inner core of the matrices. A large exothermic peak observed at 327.6[degrees]C was attributed to the decomposition of material at higher temperature due to the breaking of cross-linked chain structures followed by elimination of molecules (12-14). The amount of energy involved in the process was calculated to be 397.9 J [g.sup.-1].

Morphological analysis

The scanning electron micrograph (SEM) image of both shellac and Sh-AA-MBA is illustrated in Figure 7. Prior to the experiment, the sample materials were kept in buffer solution of pH 8.0 for 24 h and subsequently vacuum dried. Shellac shows a uniform fibrous surface structure. On the other hand, a porous surface characteristics was observed in case of modified shellac (Sh-AA-MBA). The new structural characteristics of Sh-AA-MBA could be resulted due to functional group interactions leading to flexibility in the molecular structure (41). Few other studies based on shellac based materials has also reported similar kind of results (13).

Equilibrium swelling studies

The percentage of equilibrium swelling of Sh-AA-MBA in buffer medium is shown in Figure 8. The dried sample materials were allowed to remain immersed in buffer solution for 48 hours at 37[degrees]C temperature. Materials were removed from solution and adhered surface solutions were removed carefully using tissue papers. The average value of three different weight measurements for each sample material was noted. A negligible swelling was observed in case of shellac; whereas, Sh-AA-MBA showed good percentage of swelling ratio with increase in pH of the medium. A maximum swelling ratio of 1417% (at pH 11.0) was noted for Sh-AA-MBA. In general, the swelling behavior indicates the attractive interactions between polymeric chain and solvent molecules. Sh-AA-MBA contains ionizable carboxylic functional groups. Hence, at higher pH of the medium, the repulsion between the charged groups (carboxylate ions) could resulted in enhancing the swelling of the material (22). A swollen polymer also contains large amount of unbound water which contribute to the greater solute release (22,25).

Controlled release of 5-asa

Sh-AA-MBA was tested for the sustained release of a model drug 5-asa in buffer solution (pH = 1.5 [+ or -] 0.5) (similar to artificial gastric juice) for first 3 hours and then at pH 6.8 [+ or -] 0.5, similar to artificial intestinal liquid, for next 8 hour duration at 37[degrees]C. It was observed that within first 20 minutes time durational most 21% of drug could be released from Sh-AA-MBA in acidic pH of the medium; whereas, almost 45% drug release could be possible in alkaline pH of the medium. The release of 5-asa at initial stage could be attributed to dissolution of the drug from the outer surface of polymer matrix (42,43).

The amount of release of drug gradually increases with time at higher pH of the medium. The reason could be attributed to the fact that the polymer could remained in swollen state and associated with larger equilibrium water content at higher pH (43). Therefore, the drug molecule embedded within the polymeric matrix and associated with water molecules can easily diffuse out from the network structure due to the availability of more water as a diffusion medium (43). This facilitated greater amount of drug release at higher pH of the medium. It was observed that almost 6 hour was needed for at least 80% release of loaded drug in buffer solution. The release of drug molecules from the polymeric material could also be possible due to the degradation of the polymeric chain due to breaking of amidic linkage in simulated body fluid. The diffusion process in different buffered solutions using Fick's model (44) equation was also evaluated in case of Sh-AA-MBA. The Fick's equation can be written as:

[M.sub.t] / [M.sub.e] = k [t.sup.a] (8)

Where, [M.sub.t] and [M.sub.e] are the amount of drug absorbed by the polymeric material at time 't' and in the equilibrium, respectively. 'k' is a characteristic constant of the system, and 'n' an exponent related to the kind of transport of the buffer solutions. The value of n = 0.5 indicates a Fickian diffusion process; where as, a value 0.5 < n < 1 indicates a non-Fickian or anomalous diffusion (44). In the special case in which n = 1,the transport mechanism is named Type II diffusion. Hence, in present case, plot of ln ([M.sub.t]/ [M.sub.e]) vi. ln t, shows a linearity and the swelling fraction[M.sub.t]/ [M.sub.e] was found to be less than or equal to 0.60. The value Of constant 'n' computed from the slope was 0.48 indicating a Fickian behavior (45-47).

Study of biodegradation of Sh-AA-MBA

Degradation of material in biological environment represents one of the important aspect for various biomedical applications. Thus, degradation study of Sh-AA-MBA in presence of lipase enzyme is illustrated in Figure 9 for three different compositions of materials i.e. Sh:1::MBA:0.3::AA:0.4 (Pol-1), Sh:0.7::MBA:0.3::AA:0.4 (Pol-2), and Sh:0.5::MBA:0.3::AA:0.4 (Pol-3). From the graphical illustration in Figure 9, it was observed that the polymer with higher shellac content was able to degrade at faster rate. The reason could be attributed to the breaking of bonds due to enzymatic action (30). Shellac contains a number of hydroxyl and carboxylic acid groups which could form ester bonds due to self-polymerization reaction. Therefore, cleavage of ester bonds could led to material degradation (13). The preliminary study in this direction shall be helpful to carry out more elaborative investigation for commercial utility of the material in future.

Conclusions

A new pH sensitive hydrogel material (Sh-AA-MBA) was prepared by polymerization technique. The material was characterized using FTIR and the peak at 3600cm-1 attributed to i(N-Hstr) vibration indicated the requisite chemical modification of shellac. Chemical modification of shellac changed the surface characteristics. From thermal studies, the parameters AS and AH were evaluated using different mathematical models and the results can be helpful in computing drug-material interaction. Sh-AA-MBA shows 1417% swelling in buffer medium. The enzymatic degradation of materials with more shellac content was relatively faster. The efficacy of releasing the drug in a sustained manner from the newly developed material indicate its future potential application as new carrier material in pharmaceutics and biomedical applications.

Acknowledgements

The work is funded by Department of Science and Technology (DST), New Delhi; Grant No. SR/S1/PC-24/2009. The valuable suggestions of Prof. Alok R. Ray, IIT, Delhi and Dr.GS.Tiwary, Ranchi University, is gratefully acknowledged.

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Arundhati Barik (a), P. Parhi (a), R. K. Dey (b) *

(a) Post-Graduate Department of Chemistry, Ravenshaw University, Cuttack, Odisha, India

(b) Centre for Applied Chemistry, Central University of Jharkhand, Ranchi, India

Received 18 August 2015; Accepted 5 October 2015; Available online 5 October 2015

(#) Coresponding author: Dr. R.K. Dey; E-mail: rkdey@rediffmail.com, ratan.dey@cuj.ac.in
Table 1: Computations for kinetics and thermodynamic parameters for
Shellac and Sh-AA-MBA from thermo-gravimetric analysis

                                           E(kJ
Materials        Model    Temp.(K)     [mol.sup.-1])   A([sec.sup.-1])

Shellac           CR       287-573         14.16            153.4
                  CR       573-673        159.90       3.4x[10.sup.21]
                  CR       287-773         20.11             23.0
                  KAS      287-773          8.04

Shellac-AA-MBA    CR     298.4-884.9       18.44            122.0
                  KAS    298.4-884.9        8.39

                                      [DELTA]S(J
                                      [K.sup.-l]
Materials        Model   [r.sup.2]   [mol.sup.-l])

Shellac           CR      0.9351
                  CR      0.9646
                  CR      0.9037         3.67
                  KAS

Shellac-AA-MBA    CR      0.9898         2.0
                  KAS

                         [DELTA]H(kJ      [DELTA]G(kJ
Materials        Model   [mol.sup.-1])   [mol.sup.-1])

Shellac           CR
                  CR
                  CR         15.74           13.82
                  KAS

Shellac-AA-MBA    CR         14.07           13.05
                  KAS
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Title Annotation:Original Article
Author:Barik, Arundhati; Parhi, P.; Dey, R.K.
Publication:Trends in Biomaterials and Artificial Organs
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2015
Words:4814
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