# Synthesis of Hydrogel-Based Microgels and Nanogels Toward Therapeutic and Biomedical Applications
## 4 Paradigm Shift: Bulk Hydrogels to Hydrogel Micro/Nanoparticles
### Hydrogel Nanoparticle Synthesis
#### 4.3.4 Template-Assisted Synthesis  
Template-assisted synthesis is still at its early stage, but it has been increasingly gaining interest for its potential benefits as a straightforward, controllable strategy of nanogelscompatible with various conventional hydrogel chemistries [246; 247]. The most basic form of this approach is the previously discussed nano-emulsion method. Nanoscale emulsion could encapsulate the required precursors and crosslinkers for hydrogel formation, which serves basically as a nanoscale batch reactor (Figure 5). One major drawback is that the reliable formation of emulsion is often limited to hundreds of nanometers, and further miniaturization is accompanied by a substantial increase in the heterogeneity of emulsions.  
However, the concept of a nanoscale template that is compatible with conventional hydrogel synthesis methods offers an attractive strategy to realize sub-100 nm hydrogel nanoparticles. The template-assisted approach leverages pre-formed templates to dictate the geometry, size, and the characteristics of nanogels, where the templates such as nanoparticle, nanofibers, or nano-emulsions provide the physical framework for chemical synthesis (Figure 5a). The precursor of hydrogel is crosslinked within or around the template, which can be subsequently removed, leaving behind structured nanogels such as porous or hollow interiors. Most commonly used sacrificial colloidal templates are polymer nanoparticles made of polystyrene, poly(methyl methacrylate), or melamine, and inorganic nanoparticles like silica. However, these templates are hard solid particles, and the gelation chemistry is limited to the proximity of their surface and thus is not suitable to produce nanogels. Also, the selective removal of a polymeric template is generally achieved by adding or changing to a good solvent, which naturally undergoes swelling from solvent uptake before dissolution, leading to mechanical ruptures of the synthesized gel architecture [248; 249].  
Recently, another study utilized a liposome as a nanoscale aqueous reactor for photopolymerization of precursors (Figure 5b) [247]. The microfluidic methods could produce liposomes from below 30 nm to above 100 nm with exceptional stability [187], which provide an aqueous interior and selectively removable exterior with a small amount of surfactant. Both internal and external phases of a liposome are aqueous due to its vesicular  
Figure 5: Template-assisted synthesis of nanogels. (**a**) First demonstration of alginate nanogel prepared by using liposome as a temporary template. (**b**) Similar approach demonstrated using glycine diphenylalanine (GFF) as a precursor and liposome as a template. Both approaches showed successful synthesis of nanogels at the 100–200 nm scale but lacked controllability and uniformity as confirmed in the size distributions and transmittance electron microscopy. White arrows and a red arrow indicate unilamellar and multilamellar vesicles, respectively. Reproduced with permission from [246]. Licensed under CC BY 4.0, adapted from [247].  
nature, which makes it highly compatible with hydrogel and bioactive materials. Although it requires improvements and optimization in various aspects, this strategy shows great promise for the realization of nanogels that can incorporate large biomolecules or biological agents such as proteins, nucleic acids, or even cells.