Table 3.

Syntheses, uses, advantages and disadvantages of various polymeric structures in vaccine development.

Structure	Synthesis methods	Uses	Advantages	Disadvantages	Ref.
Solid particles	•Homogenization of oil-in-water emulsion (single emulsion) or water-in-oil-in-water emulsion (double emulsion), and followed by solvent evaporation •Ionic complexation between polyelectrolytes and nucleic acid encoding the antigens	•As antigen carriers against many emerging diseases (see Table 2) •PLGA microparticles with adsorbed DNA encoding HIV antigens used as prophylactic vaccines reached Phase I clinical trial •PEI complexed with DNA encoding HIV antigens used as therapeutic vaccines is directed towards Phase III clinical trial •As adjuvant by loading with immunostimulant molecules	•Tunable physicochemical properties (e.g. hydrophobic, size and charge) simply by tuning synthesis parameters (e.g. composition, molecular weight of precursors, energy shearing, etc.) •Wide range of size and aspect ratio •Tunable surface characteristics for antigen loading and targeting moieties •Enable high antigen loading and sustained antigen release	•Acidic degradation of PLGA may pose adverse effects on protein antigen integrity •Pre-loading of antigens into the particles may lead to degradation of the antigens that are sensitive to organic solvents, temperature or shearing forces •Higher levels of both molecular weight and charge density of polycationic are better for transfection efficiency, but may lead to in vivo cytotoxicity	[42,127,251]
Nanogels	•Physical self-assembly of interactive polymer chains •Emulsion polymerization •Crosslinking of self-assembled polymer aggregates or preformed polymer chains •Template-assisted nanofabrication with PRINT technology	•As antigen carriers against respiratory-based infections caused by viruses (e.g. H1N1) and bacteria (e.g. Streptococcus pneumoniae) •As cancer vaccines, in which cholesteryl-group-bearing pullulan nanogels have already reached clinical trials •As adjuvants by loading with immunostimulant molecules	•Controllable swelling for cargo loading and release depending on chemical structure of polymers, crosslinking density, charge density, or environmental parameters like pH, temperature, ionic strength •Tailorable deformability for efficient tissue penetration by engineering the crosslinking density and/or moiety	•The cargo-loading and stimuli-responsive release are often based on the charged groups incorporated into polymeric network, in which the global charges (especially positive) could induce cytotoxicity •Physically crosslinked nanogels pose stability problems during storage or in vivo delivery	[191,252,253]
Micelles	Self-assembly of amphiphilic block copolymers (with >50% of block copolymers are hydrophilic block)	•As antigen carriers against bacterial pathogens and cancers •As adjuvant by loading with immunostimulant molecules	•The core can be loaded with a poorly water-soluble cargo, while the shell is for water-soluble ones •Small size for facilitating efficient tissue penetration or lymph node retention •Stimuli-responsive cargo release	•Limited loading capacity •Limited translation of micelle platforms between protein antigen cargoes	[34,254]
Polymersomes	Self-assembly of amphiphilic block copolymers (with 35% of block copolymers are hydrophilic block) through thin film rehydration or flash nanoprecipitation	•As antigen carriers against cancers, lassa virus, influenza virus, and Mycobacterium tuberculosis	•The core domain allows loading of hydrophilic antigens, while the membrane shells can entrap poorly water-soluble immunostimulants	•Novel polymers are developed to make polymersomes along with organic solvents and/or additives, which require rigorous evaluation on cytotoxicity	[39,218,255,256]
		•As adjuvants by loading with immunostimulant molecules	•Tunable membrane-shell permeability or thickness for better retention of loaded cargo	•Incorporation of highly hydrophobic copolymers makes the polymer aggregation and hydration challenging
			•Tunable crosslinking and targeting moieties for enhanced stability and targeting ability	•Limited experience for scale up
			•Stimuli-responsive cargo release
Lipid coated particles	Homogenization of oil-in-water emulsion (single emulsion) or water-in-oil-in-water emulsion (double emulsion), and followed by solvent evaporation	•As antigen carriers against cancers and viral infections such as MERS-CoV	The lipid shells facilitate: •Loading of antigens and/or immunostimulant molecules •Affinity of particles towards cells and their cell membrane penetration, which can lead to enhanced cell uptake	•The manufacture involves multiples steps which would pose scalability and reproducibility problems	[96,228,230]
Cell membrane coated particles	Extrusion or sonication of a mixture of preformed solid polymeric nanoparticles with cell membranes purified from red blood cells, macrophages, or cancer cells	•As antigen carriers against bacterial pathogens •As personalized cancer vaccines	•Enable to display wide repertoire of antigens, replicating the properties of the sourced cells	•Large scale manufacture is limited for personalized vaccine use	[234,235]
Protein coated particles	•Chemically synthesized particles are based on a layer-by-layer functionalization of preformed solid polymeric nanoparticles •Biologically synthesized particles are based on in vivo self-assembly of PHB driven by polymer synthase protein in engineered bacteria	•Chemically synthesized particles: antigen carriers against cancers, influenza, and malaria •Biologically synthesized particles: antigen carriers against bacterial and viral infections (Fig. 9)	•Controlled orientation and repetitive structure of protein antigen display on chemically synthesized particles •Antigen loading and particle formation of biologically synthesized particles occur in one step in vivo, scalable and reproducible through high-cell-density fermentation, and the degraded PHB is not cytotoxic	•Manufacture of chemically synthesized particles involves multiple steps which would pose problems in large scale production and reproducibility •It is difficult to control size and predict yields of biologically synthesized particles with potentials of difficult-to-remove host-cell-protein impurities	[42,104,188]

Abbreviation: HIV, human immunodeficiency virus; MERS-CoV, Middle East Respiratory Syndrome Coronavirus; MPLA, monophosphoryl lipid A; PEI, poly(ethyleneimine); PHB, poly(3-hydroxybutyric acid) PLGA, poly(lactic-co-glycolic acid), PRINT, particle replication in nonwetting templates.