**4. Methods of composite preparation**

There are broad means of preparing alginate composites – both physical or chemical preparations. Before gel formation, the latter entails a chemical reaction between alginate and other materials, such as monomers, polymers, and nanoparticles. For example, alginate-pyrrole [47] and alginate-biotin [48] composites have been prepared by covalent conjugation to obtain electrically conductive and affinity-labelled hydrogels. When the composite is prepared from two polymers, the polymers are mixed and then crosslinked sequentially via crosslinkers or other treatments (**Figure 6**). *Alginate-Based Applications in Biotechnology with a Special Mention to Biosensors DOI: http://dx.doi.org/10.5772/intechopen.110737*

**Figure 6.** *Methods of composite preparation.*

In an event where the composite is between alginate and nanomaterials, the nanomaterials are incorporated within the bulk hydrogel framework by any of three broad methods, blending, *in situ* precipitation, or grafting-onto [49]. The blending method involves mixing the nanomaterial with a hydrogel precursor solution at an optimised molar ratio, followed by a crosslink of the hydrogels to entrap the nanomaterial. In contrast, in the case of *in situ* precipitation, the hydrogel network is prepared via crosslinking, after which the nanomaterial is synthesised by precipitation into the polymer hydrogels after the crosslinking reaction. Grafting several functional groups onto the surface of the nanomaterials as nano-crosslinkers eventually leads to the crosslinking reaction [49].

The preparation of alginate composites has, over the years, evolved, taking different physical formats starting with beads, and microbeads formation [50], to nanoscale preparations such as nanoparticles [51], nanogel [52], thin film [53], and nanofibers [54], variations which relate to different biotechnological applications (**Figure 7**) such as in diagnostics and therapeutics. Beads of alginate are formed by extruding alginate droplets into a crosslinking bath and are found in medical applications in cell immobilisation and scaffolding for tissue regeneration.

Alginate nanoparticles (NPs) (typically between 150 and 250 nm) [55] were anticipated to gain popularity in biomedical applications due to the significant reduction in particle size that has been achieved, thanks to rapid technological advancement.


#### **Figure 7.**

*Applications of different formats of alginate composites.*

However, unlike other mucoadhesive polysaccharides such as chitosan, which has been chiefly employed for the development of NPs, the alginate-based nanoparticles are only sporadically reported because of the difficulty of achieving nanoscale size ranges using only alginate [56]. The conventional synthetic approach entails the modification [57] of Ca2+-mediated ionotropic crosslinking and/or replacement of the crosslinking with water-in-oil nano-emulsions [58, 59] and, in some cases, using polyelectrolytes such as chitosan [60, 61]. An advancement of the conventional method was the incorporation of nanocarriers such as liposomes into the alginate, exhibiting a mucoadhesive property. Haidar and colleagues exemplified this approach in encapsulating and delivering growth factors using core-shell hybrid NPs formed by the layer-by-layer assembly of alginate and chitosan on liposomes [62]. Furthermore, alginate NPs were prepared using liposomal cores with a high melting point (MP) as reaction vessels to template the alginate NPs assembly. The alginate was encapsulated within the liposome core and then exposed to Ca2+ at a temperature higher than the MP of the bilayer such that the Ca2+ could diffuse into the core and initiate a gradual

**Figure 8.**

*Preparation of alginate nanoparticles using liposomal templates.*
