Morphology and gelation of thermosensitive xyloglucan hydrogels
Introduction
There has been growing interest in the use of thermally sensitive in situ forming, physical hydrogels for biomedical applications [1], [2] as minimally invasive scaffolds for tissue engineering. Because of their low interfacial tension and high molecular and oxygen permeability, hydrogels are ideal tissue engineering constructs [3]. Consequently, hydrogels have been investigated for cell gene therapy [4], enzyme and cell encapsulation [5], [6], [7], drug delivery [8], [9], [10], [11], joint cushioning and lubrication [8], [12] and as environmental shape memory materials [4]. The utility of hydrogels as tissue engineering scaffolds appears to be related to microscopic [13] and macroscopic [14] porosity within the structure of hydrogels. Microscopic pore interconnectivity and the volume of water phase present within the hydrogel ensures cellular viability, permits cell migration, increases transportation of nutrients, oxygen and metabolites [15], and influences drug release profiles and enzymatic degradation [8]. The shape and size distribution of the pores also influence cellular function [8]. By exploiting these features it may be possible to mimic features of the natural extracellular matrix, and control tissue structure and cellular functions [16].
Xyloglucan is a neutral, non-toxic polysaccharide [17], whose degradation products consist of naturally occurring saccharides and are assumed non-toxic, although the experimental evidence for this conclusion is limited. Xyloglucan is extracted from the tamarind seed and is a major component of higher plant cell walls [18]. It is composed of a β-1,4 linked D-glucan backbone where the O-6 positions of the glucopyranosyl residue are partially substituted with α-D-xylopyranose residue [11], [19]. Fig. 1 shows the backbone structure of xyloglucan and Fig. 2 presents the structure of the galactopyranose, glucopyranose and xylopyranose which make up the polysaccharide.
Thermally responsive xyloglucan is formed using fungal β-galactosidase to remove more than 35% of the galactose residue [17]. Shirakawa et al. [17] found that the optimum removal ratio was between 35% and 50%. Using DSC and rheology they related gelation temperature to the concentration of xyloglucan in the pre-gel solution. Miyazaki et al. [11] found that increasing the concentration of the pre-gel solution from 1% to 2% decreased gel temperature from 27 to 22 °C as well as the gel time. A lower and upper transition from sol–gel and gel–sol, respectively, were found, and the gel was shown to be thermo-reversible upon cooling. It was also determined that with a higher galactose removal ratio there was an increase in temperature range of the gelation peaks, hence a broader gelation range [17].
While tissue engineering applications for xyloglucan are not extensive, it has been used as skin patches [20], oral and rectal delivery of drugs [11], and for intraperitoneal injections [21]. It was drug loaded in the latter two applications and was found to provide stronger bioavailability of the relevant drug and longer residence times than previous commercial suppositories [22]. Importantly there was no apparent tissue damage [11] implying that xyloglucan hydrogel is a biocompatible material that can be implanted non-invasively via injection in tissue engineering applications.
X-ray scattering studies of the gel nano-structure of galactose-modified xyloglucan, showed that flat structures were formed from lateral stacking of rod-like chains [23] whereas the addition of ethanol to the solution created a random structure consisting of condensed phase aggregates amongst dilute, single chain areas [24].
This study aims to investigate the thermal, rheological properties of thermally gelling xyloglucan with a galactose removal ratio of 48% with consideration of its use as an injectable tissue engineering scaffold. The effect of changing the ionic strength of solution media on the properties of the hydrogel were determined by comparing deionised water and phosphate buffered saline (PBS) as solution media. This study also presents for the first time, the morphology of the gel networks as determined using electron microscopy.
Section snippets
Materials
Xyloglucan with a 48% galactose removal ratio was prepared by enzymatic modification from tamarind seed xyloglucan, according to previous method [17]. It was purified by dissolving 1 wt.% xyloglucan in deionized water with a magnetic stirrer at a temperature between 0 and 5 °C. The solution was then precipitated out in 60% ethanol at room temperature and washed with 60% ethanol through a sintered glass filter and flask with attached vacuum pump. An additional wash was conducted using acetone.
Thermal and rheological characterisation
The gelation characteristics of the xyloglucan hydrogels at concentrations ranging from 0.5 to 3 wt.% were studied using DSμC and rheology in both water and PBS to determine if there was a change in the gelation behavior due to changes in ionic strength. At concentrations less than 2.0 wt.%, rheological characterisations showed that the physical gels were weak and their elastic modulus, (E′), did not increase as temperature increased. Consequently, DSμC was used to study the gelation behavior
Conclusion
Xyloglucan is a thermally reversible hydrogel that has a significantly higher modulus than most other hydrogels. In this study the microstructural and rheological results indicated that the gelation of xyloglucan occurs as a 2 stage process. The first step, which is not concentration dependent, is the initial formation of large membrane structures in the pre gel. This accounts for the initially high modulus and, as the sheets refract light, also explains why the pre-gel solution is opaque. The
Acknowledgements
I would like to thank W.D. Cook for use of the rheometer, and I. Harper for use of the SEM. This project was funded by the CRC for Polymers and the ARC project DP0450618.
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2020, International Journal of Biological MacromoleculesCitation Excerpt :Nisbet et al. [20] have reported the formation of thermoreversible gels from xyloglucan. At higher concentrations of xyloglucan (3% w/w), an interconnected cellular three dimensional network was observed in the hydrogel matrix, which helped in the gelation [20]. A careful observation of the microarchitectures over here showed that the colored globular aggregates housed several dark-colored globules.