Synthesis and Characterization of Novel Bio-Elastomers for Tissue Engineering

 

Indira R., Harihara Priya G., Tamizharuvi T. and Jaisankar V.*

PG and Research Department of Chemistry, Presidency College, Chennai – 600 005

*Corresponding Author E-mail: vjaisankar@gmail.com

 

Biodegradable elastomers have recently found widespread application in many areas of biomedical engineering such as tissue engineering, therapeutic delivery and bioimaging. Recent developments led to development of elastomers involving citric acid based polyesters are being explored that are endogenous to the human metabolism. In terms of mechanical stability, crystallinity, hydrophobicity and biocompatibility, polyesters synthesised from these monomers can display a wide range of applications. In this investigation, a series of novel bio-elastomers based on citric acid were synthesised by catalyst free polycondensation reaction. These polymers were characterised by solubility, IR, 1H NMR spectral analysis, thermal and mechanical studies. The physicochemical properties of the synthesised polymers are controlled by the variation of monomers in polycondensation reaction forms a pivotal role in the synthesis.

 

 

1. INTRODUCTION:

Biodegradable polymers have made a considerable impact in various fields of biomedical engineering including tissue engineering and drug delivery, where cell-seeded constructs are designed to replace damaged or diseased tissues^{[1, 2]}. Biodegradable elastomers are advantageous in that they can sustain and recover from multiple deformations without causing irritation to the surrounding tissue in a mechanically demanding environment^[3-5]. In recent years, catalyst-free synthesis has emerged as a potential route to synthesize elastic polyesters with appropriate mechanical integrity, suitable surface characteristics and compatibility for fabrication of tissue engineering scaffolds^[6]. Various polyester elastomers, synthesized so far from low-cost and nontoxic precursors such as 1,8 Octanediol (OD), citric acid (CA), glycerol and sebacic acid (SA), represent a new generation of advanced biocompatible and biodegradable synthetic materials with potential biomedical applications^[7,8].

 

It has been recognized that cross-linking confers elasticity to the polymers as similar to those naturally occurred cross-linked polymers such as collagen and elastin.

 

PGS and poly(diol citrates) are soft and elastomeric cross-linked polyester networks and the mechanical properties of these polymers have been shown to match those of the soft tissues such as cardiac tissues and blood vessels in the body, thus considered as suitable candidate materials for soft tissue engineering^[9,10]. Specifically, poly(diol citrates) hold significant promise for use in vascular tissue engineering due to their anticoagulant properties, ability to support the adhesion, proliferation, and differentiation of endothelial cells and reduced platelet adsorption and activation^[11]. 

 

Citric acid is a versatile monomer that participates in pre-polymer formation through a simple polycondensation reaction while preserving pendat functionality for post polymerisation to produce a crosslinked polyester network with degradable ester bonds. Crosslinking confers elasticity to the polymers similar to the extracellular matrix, in which collagen and elastin are all crosslinked polymers^[12]. Citric-acid-derived biodegradable elastomers (CABEs) such as poly(diol citrate)s, cross-linked urethane-doped polyesters (CUPE)s, elastomeric cross-linked biodegradable photoluminescent polymers (CBPLPs), poly(xylitol-co-citrate)s, etc., have recently received significant attention in various biomedical applications, including tissue-engineering orthopedic devices, bioimaging and implant coatings^[13-19].  Maleic acid, an important component of the citric acid cycle, has been used in many synthetic biomaterial designs, and was chosen as a difunctional acid bringing vinyl functionality to the polymer ^[20-23].  Yang et al.^[24] have reported the citric acid based biodegradable copolymers for Tissue engineering application. Their study indicated that the material selected, which has multifunctional monomer to provide valuable pendant functionality^[25]. Herein we report the synthesis and characterisation of two polyesters: Poly (1,6 Hexane diol citrate-co-1,6 Hexane diol Maleate) (P1) and Poly(1,6 Hexane diol citrate-co-1,6 Hexane diol Suberate) (P2).

 

2. EXPERIMENTAL:

2.1 Materials

Citric acid CA (Merck AR grade) Maleic acid (Lancaster AR grade) and Suberic acid SA (Lancaster AR grade) were recrystallised from deionised water and used. 1, 6 Hexane diol HD (Lancastaer AR grade) were dried with CaO overnight and then distilled under reduced pressure.  All the other materials and solvents used were of analytical grade.

 

2.2. Synthesis

The polyesters were synthesized by the catalyst-free melt condensation technique by the following procedure. The amount of diacid (SA/MA), Hexane diol (HD ) and citric acid in a molar ratio of 1:2:1 were placed into a 250ml, three-necked round bottom flask and melted at 160°C – 165°C under the flow of nitrogen gas; this was followed by mixing at 140°C – 145°C for 2h, which produced the P1 and P2 prepolymers. The pre-polymer thus obtained were dissolved in 1,4-dioxane(20% w/w solution) and the resulting pre-polymer solution was used for film preparation without further purification^[26]. Films for mechanical and structural analysis were cast into Teflon petri dishes and placed in an air oven maintained at 80°C for 24h for post polymerisation of the pre-polymers.

 

2.3    Characterizations:

2.3.1. Solubility test

Solubility of the copolymer samples was examined in chloroform and Dimethyl sulphoxide (DMSO). Essentially, 0.5 g of sample was dissolved in 10 ml of solvent, and vigorously stirred for 1 hour.

 

2.3.2. Fourier- Transform Infrared (FTIR) Spectroscopy

IR Spectra of the copolyesters were recorded using a perkin Elmer IR spectrometer in the range of 700cm^-1 to 4500 cm^-1. The samples were embedded in KBr pellets.

 

2.3.3. Nuclear Magnetic Resonance (NMR)

¹H NMR spectra were recorded on AV 500 MHz Spectrometer by using 7% wt of C₂D₆O solvent.   Differential Scanning Calorimetry (DSC).The DSC scans were recorded at a heating rate of 10°C/min using a Perkin-Elmer Pyris I analyser. Indium was used as the calibration standard.

 

2.3.4. Mechanical Properties

The stress-strain curve and young’s modulus were achieved. Samples were measured and their values were averaged.

 

3. RESULTS AND DISCUSSION:

3.1. Solubility studies

The synthesised copolyesters maintain a good solubility in acetone, CHCl3, DMSO, Methanol, Ethanol, THF, DMF and insoluble in water

 

3.2.Fourier- Transform Infrared (FTIR) Spectroscopy

 

Fig 1.P1

 

Fig 1.P2

Fig 1. IR spectra of copolyesters P1 and P2

 

TABLE -1 Solubility of Copolyesters  P1 and P2

S.No	Polyester	Acetone	CHCl3	DMSO		Methanol	Ethanol	THF	DMF	Water
1	P1	++	+ + +		+ ++	+ + +	+ + +	+ +	+ +	- - -
2	P2	++	+ + +		+ +	+ + +	+ + +	+ +	+ +	- - -

+++ - Freely Soluble, ++ - Soluble,  --  Insoluble

Solubility of the copolyesters are presented in table

 

In FTIR spectra the pronounced peaks at 1690–1750 cm⁻¹ suggest the presence of carbonyl (C=O) groups from the ester bond and pendent carboxylic acid from the citric acid. The shoulder peak of a lower wavelength at 1639 cm⁻¹ proves the presence of the olefin moiety^[27] from maleic acid. The bands centered at around 2944 and 2938 were assigned to methylene(-CH2-) groups for diacids/diols and observed in all the spectra of the polyesters. Hydrogen-bonded hydroxyl functional groups showed absorbance as a broad peak centered at 3570 cm⁻¹ and 3469cm⁻¹

 

3.3.¹HNMR spectroscopy:

The peaks observed were attributed as follows; Peak  located at 2.8-2.6 ppm, was assigned to –CH₂– from citric acid.The multiplet peak at 2.9-2.7ppm were attributed to the protons in –CH₂- from SA. The multiplet peak at 1.55-1.25 ppm  were attributed to the protons in –CH₂- from HD. The peaks located between 6 and 7 ppm were assigned to the protons of –CH=CH– incorporated into the polymer chain^[28].

 

Fig.2 P1(a)

 

Fig.2 P2(b)

Fig.2 ¹H NMR spectra of copolyesters p1 (a) and p2(b)

 

3.4. Thermal Analysis

The DSC heating thermograms of P1 and P2  are depicted in Fig.3.  The thermal studies revealed that the elastomers were thermally stable. The DSC analysis of both the polyesters showed Tg below room temperature, a characteristic feature that determined their elastomer (11). The glass transition temperature (Tg) of P1 matrix              (-20.77°C) was higher than P2 matrix (-40.74°C). This revealed P1 elastomer had better cross-linking than that of P2.

 

3.5. Mechanical Properties

TABLE 2  Mechanical properties of the copolyesters P1 and P2.     

Polyester	Tensile	Young’s	Elongation
	Strength (MPa)	Modulus (MPa)	at break(%)
P1(CMH)	0.037	0.19	21.06
P2(CSH)	0.02	0.14	24.55

 

The mechanical properties of the synthesized copolyesters are tabulated in Table 2. Tensile strength, Young's modulus and elongation at break are calculated from mechanical study graphs. As shown in the table tensile strength and the modulus of the polyester P1 was relatively higher than polyester P2 due to increase in the cross-link density of P1.

 

4. CONCLUSION:

A new series of citric acid based polyester elastomers have been synthesized by a simple catalyst-free polyesterification process with hexane diol (HD), citric acid (CA) and suberic/maleic acid (SA/MA) as monomers. The synthesized materials were elastomeric in nature.  The thermal and mechanical properties of the polyesters showed that polyester P1 had better cross-inking than that of polyester P2.We believe that this new material will significantly improve the ease of fabrication and performance of biocompatible elastomers for future tissue engineering applications.

 

Fig 3.P1

 

Fig 3.P2

 

Fig 3. DSC thermogram of copolyesters  P1 and P2

 

 

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Received on 12.07.2013       Modified on 25.07.2013

Accepted on 28.07.2013      © AJRC All right reserved