Biohybrids for Environmental Remediation and Biosensing 269

In 1968, a pioneering work was reported by Stober et al. (1968) for controlled synthesis of

monodispersed silica and spherical particles. This method offers advantages over acid-catalyzed

systems as using this monodispersed spherical silica particles can be prepared. However, in the

sol-gel process, production of alcohol as a by-product is one of the limitations when biological

components are going to be immobilized. Thus, there is a need to find suitable methods for

associating bio-components with silica nanoparticles to develop biohybrids and to improve the

practical applicability of associated bio-components.

15.5.2 Synthesis of Biohybrids using Spray drying Technique

Spray drying is a widely used technique for the preparation of powdered formulation using aqueous

feed samples by atomization in hot drying air. It has emerged as one of the suitable techniques for

synthesizing biohybrids of various morphologies like microspheres and microcapsules and holds

application in food, remediation and drug delivery. Various aspects of spray drying have been

explored and used as immobilizing techniques for the immobilization of proteins, enzymes and

microbial whole cells (Mishra et al. 2014, Mishra et al. 2017, Mishra et al. 2021a, Mukundan et al.

2020, Shukla et al. 2020). The characteristics of the spray-dried product is regulated by various factors

such as spray dryer design, feed characteristics, drying temperature and processing parameters. By

using spray drying, it is even possible to prepare the product of desired characteristics by adjusting

the spray drying parameters.

15.6 Different Morphologies of Biohybrids

A wide range of biohybrid materials can be synthesized depending on the applications. It could be

in particles form, sheet, scaffold, fibrous, etc.

15.6.1 Biohybrid in Microparticles form

Biohybrids synthesized using spray drying techniques are mostly in the form of microparticles.

Biohybrids in a microparticle shape find applications in remediation and biosensing. There are also

reports when single cells were encapsulated to develop biohybrids as encapsulated single-cell offers

a number of advantages (Mao et al. 2017, Kamperman et al. 2017, Lienemann et al. 2017). Different

strategies are employed for encapsulation of single cells. Enzymatically crosslinked microgels were

used for encapsulation of single cells (Kamperman et al. 2017). Microfluidic droplets have been

widely explored to synthesize biohybrid microparticles. It also helps to retain the viability of cells

(Choi et al. 2016). The flow-focusing device in combination with covalent crosslinking process

was used by Headen et al. (2014) to encapsulate the cells in droplets. Water-in-oil-in-water (w/o/w)

emulsion droplets with an ultrathin oil layer were also reported for immobilization of cells in the gel

matrix which leads to the development of biohybrid microparticles.

15.6.2 Biohybrids in Fibrous form

Fibre-like structures have their own advantages over microparticles as these offer flexible structure,

large aspect ratio and could be explored for developing complex large scale biohybrids for various

applications in the biomedical engineering as these resemble nerves, blood vessels and muscle

tissues. Several fibre-shaped biohybrids have been prepared (Letnik et al. 2015, Neal et al. 2014,

Onoe et al. 2013). Using a sacrificial outer moulding approach, an elongated fascicle-inspired

biohybrid material was prepared by Neal et al. 2014 wherein polydimethylsiloxane (PDMS) was

used for moulding. A fiber-like cavity tube was constructed using a cylindrical steel pin and gelatin.

Cells were seeded in the cavity tube and an increase in the temperature caused the gelatin to melt

followed by solidification of cells and led to the formation of fibre-like structures.

Microfluidic spinning was also explored by Onoe et al. 2013 to develop fibre-shaped biohybrids.

Using microfluidic spinning various types of cells have been applied to develop biohybrids.