**4. Conclusion**

90 Biomaterials – Physics and Chemistry

The uncomplete crosslinking of Ulvan macromers under UV exposure could be ascribed to both the nature and the morphology assumed by this polysaccharide in solution. Indeed the antioxidant activity of Ulvan could reduce the rate of radical polymerization of the macromers by quenching the radicals formed during the UV irradiation. Moreover the aggregative behaviour of Ulvan in aqueous solution reduces the amount of (meth)acryloyl

The property of retaining water represents a key parameter for evaluating the quality of a hydrogel and its potential use for biomedical applications because it usually affects its permeability, biocompatibility and rate of degradation. The swelling ability of hydrogels could also provide information about their mechanical stability and chemical and physical properties, since the degree of water uptake is related both to the chemical nature and to the physical structure of the polymeric network (Qi et al., 2004). It is known, for example, that gels exhibiting a larger pore structure – likely due to a lower degree of crosslinking – have

The swelling ability of hydrogels is usually quantified by measuring their Swelling Degree % (SD%) taken as the ratio (%) between the weight of the swollen hydrogel to that of the dried sample. The swelling degrees of the prepared Ulvan hydrogels have been carried out in phosphate buffer solutions (pH 7.4) and their behaviour was recorded during 7 days of

groups available for polymerization thus partially inhibiting the crosslinking.

poor mechanical strength and higher swelling ratios (Anseth et al., 1996).

Fig. 10. SD% in PBS buffer solution (0.1 M, pH 7.4) of UV crosslinked (365 nm, approximately 8 mW· cm-2) UMA and UGMA hydrogels as a function of time.

stable in terms of texture and mechanical properties (Figure. 11).

Pictures of the swollen scaffolds taken after 2 days of immersion in phosphate buffer saline (PBS) at pH 7.4, showed that the Ulvan methacrylate (UMA) hydrogels proved to be most

The swelling degree experiments of UGMA-based samples were stopped after 2 days of immersion since the hydrogels were no longer coherent and hence not easy to handle. This

immersion (Fig. 10).

Ulvan, a sulphated polysaccharide of algal origin, is worth of deeper attention for its potential use in technological and industrial-related applications. The exploitation of this abundant and renewable resource could represent an advantageous alternative approach to the use of fully synthetic materials based on fossil fuel feedstock.

In particular Ulvan represents an intriguing candidate material for biomedical applications due to its intrinsic beneficial biological activities and the possibility of easily modifying its structure according to the envisaged application. Its chemical structure similar to that of natural glycosaminoglycans such as chondroitin sulphate and hyaluronic acid make Ulvan an attractive candidate for their substitution or use in related applications.

Ulvan: A Versatile Platform of Biomaterials from Renewable Resources 93

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In order to be employed in biomedical applications such as tissue engineering, regenerative medicine and drug delivery, Ulvan needs to be converted into an insoluble material under physiological conditions and to have mechanical properties suitable for the end application. The preparation of physically crosslinked Ulvan hydrogels has been reported since long times, but their weak mechanical properties and uncontrolled dissolution in presence of physiological fluids make them unsuitable for biomedical uses, where a scaffolding role is required

A novel method for covalent crosslinking of Ulvan through the UV mediated radical polymerization of activated macromers by double bond conjugated moieties, revealed to be promising in the preparation of chemically crosslinked Ulvan hydrogels. The conjugation of methacryloyl group to Ulvan through the reaction with methacrylic anhydride under slightly basic conditions gave the best results in terms of product yield and substitution degree. The hydrogels obtained after their exposure to UV light seemed to be very stable in physiological conditions.

The crosslinking of the Ulvan macromer precursors is usually not complete because is hampered both by its aggregative behaviour in solution that limits the availability of the (meth)acryloyl groups and very presumably by the radical quenching activity of the polysaccharide during the UV exposure thus negatively affecting the mechanical properties of the final hydrogels. Nevertheless the antioxidant activity of Ulvan could make this material a good candidate as a matrix for cell encapsulation due to the possible protection against the radicals produced during UV crosslinking (Fedorovich et al., 2009). The use of these materials as a base for cytocompatible scaffolds is also promoted by the softness related to partial crosslinking of these macromers, since it is known that cell spreading within hydrogels is influenced by matrix stiffness and soft matrices interestingly are expected to promote cell spreading (Liu & Chan-Park, 2009).

Moreover the possibility of preparing Ulvan based hydrogels by a straightforward technique such as UV crosslinking makes the use of Ulvan in biomedical fields even more attractive. Indeed UV photopolymerization allows the spatial and temporal control over the crosslinking and the fabrication of hydrogels in situ with the possibility of forming complex architectures that adhere and conform to tissue structure.
