Table 2.

Characteristics and design considerations of several delivery systems

Delivery systems	Formation	Functional characteristics	Design considerations	References
Polymer-based nanoparticles	Composed of natural/synthetic high polymer	The loaded active compounds can be embedded in the polymer core or adsorbed onto the polymer core surface	Controlled delivery of vaccines is achieved by controlling size, shape, and surface characteristics	[115, 116]
Liposome-based nanoparticles	The composition includes lipids that can be ionized, cholesterol, phospholipids, and lipids modified with polyethylene glycol	Can encapsulate water-soluble and fat-soluble drugs, has a high degree of targeting, can effectively protect the encapsulated drugs, and can control and slowly release drugs	Its formulation composition, size, and surface charge affect its efficacy and pharmacokinetics, and these factors can be controlled to optimize vaccine delivery	[117, 118]
Hydrogels	High hydrophilic polymers form three-dimensional network structures by cross-linking in water	High doses of non-specific drug distribution can be accurately delivered to specific sites while achieving continuous controlled release of drug molecules	Generally, drug-containing hydrogels are mainly diffused and released by controlling the degradation, swelling, and mechanical deformation of the hydrogel mesh, ultimately leading to controlled drug release	[119–121]
Microneedles	Consists of a plurality of micrometer small needle tips connected in an array on the base	Can be directed through the stratum corneum, avoiding blood vessels and nerves to deliver the drug precisely and effectively below the stratum corneum; allows the active ingredient to be released in a continuous or controlled manner	The length, size, and shape of the microneedles determine their mechanical properties, insertion capacity, drug-loading capacity, and drug-delivery efficiency and can be individually designed according to therapeutic needs	[122, 123]