Polymer-Based Nanosystems—A Versatile Delivery Approach

Polymer-based nanoparticles of tailored size, morphology, and surface properties have attracted increasing attention as carriers for drugs, biomolecules, and genes. By protecting the payload from degradation and maintaining sustained and controlled release of the drug, polymeric nanoparticles can reduce drug clearance, increase their cargo’s stability and solubility, prolong its half-life, and ensure optimal concentration at the target site. The inherent immunomodulatory properties of specific polymer nanoparticles, coupled with their drug encapsulation ability, have raised particular interest in vaccine delivery. This paper aims to review current and emerging drug delivery applications of both branched and linear, natural, and synthetic polymer nanostructures, focusing on their role in vaccine development.


Introduction
The variety and versatility of polymeric materials have drawn increasing scientific interest in their application in diversified fields [1][2][3][4]. In particular, polymer-based nanoparticles were noted to have advantageous properties for biomedical uses [5]. Features such as safety, stability, good solubility, tunable physicochemical characteristics, biocompatibility, and biodegradability have recommended polymeric nanomaterials for use as vehicles for a broad range of drugs, genes, vaccines, and biomolecules [6][7][8].
In this regard, the present paper aims to present the natural and synthetic polymers that are most relevant and most commonly used for delivery purposes, further reviewing the recent advances in the delivery of different cargos and focusing, in more detail, on the role of polymers in the development of vaccine formulations.

Polymers Used as Nanocarriers
Depending on their origin, two main categories of polymers can be distinguished: natural and synthetic polymers; a more detailed classification is provided in Figure 1. Natural polymers possess superior biocompatibility to synthetic-based materials, as they occur in nature and are fully renewable. In contrast, synthetic polymers are more appealing than natural macromolecular compounds from the reproducibility point of view. Specifically, Figure 1. Polymer classification. Created based on information from literature references [11,21,22].

Chitosan
Chitosan is a highly researched material for polymeric nanocarriers, being a nontoxic, biodegradable, hemocompatible, mucoadhesive polysaccharide generally recognized as safe by the Food and Drug Administration (FDA) [21,[23][24][25]. The abundance of hydroxyl and amino groups from its backbone renders this material suitable for chemical modifications and targeted delivery to particular organs or cells [6,26,27]. Moreover, various techniques can be employed for fabricating chitosan drug delivery nanosystems, including ionic gelation, emulsion crosslinking, spray-drying, nanoprecipitation, emulsion solvent diffusion, and reverse micellization method [28,29].
The intrinsic antitumor and antimicrobial properties of chitosan have attracted interest in enhancing the efficacy of corresponding loaded substances [23,30]. To put the antitumor potential of this material to use, particular attention has been drawn to the delivery of a plethora of anti-cancer drugs [28] such as doxorubicin [31,32], paclitaxel [33][34][35][36],

Chitosan
Chitosan is a highly researched material for polymeric nanocarriers, being a non-toxic, biodegradable, hemocompatible, mucoadhesive polysaccharide generally recognized as safe by the Food and Drug Administration (FDA) [21,[23][24][25]. The abundance of hydroxyl and amino groups from its backbone renders this material suitable for chemical modifications and targeted delivery to particular organs or cells [6,26,27]. Moreover, various techniques can be employed for fabricating chitosan drug delivery nanosystems, including ionic gelation, emulsion crosslinking, spray-drying, nanoprecipitation, emulsion solvent diffusion, and reverse micellization method [28,29].

Poly-ε-Caprolactone (PCL)
PCL is one more FDA-approved, biocompatible, and biodegradable synthetic polymer that has attracted attention for nanobiomedicine purposes [146]. Its inexpensiveness, hydrophobicity, stability, and slow degradation pattern are several important features that recommend PCL-based nanoparticles for mucosal antigen delivery and DNA delivery [6,147]. Compared to PLGA, PCL degrades very slowly and without subsequently producing an acidic environment; thus, it is considered a promising adjuvant and carrier candidate for different vaccines [147].

Polystyrene (PS)
Despite not being biodegradable, polystyrene nanoparticles (PSNPs) are also attractive for biomedical purposes. PSNPs are biocompatible, do not induce inflammation, bind to a range of antigens due to their easily modifiable surface, and generate CD8+T cell responses specific to the delivered peptides [75,148]. Moreover, PS can be associated with other polymers to create amphiphilic block copolymers that are stable in aqueous media, while also being able to encapsulate hydrophobic bioactive substances [149].

Dendrimers
Dendrimers' compact, well-defined, highly branched, and radial chemical structure makes this class of synthetic polymers suitable for encapsulating various drugs [6,9]. Bearing multiple surface-accessible functional groups, dendrimers can be employed in coupling with biologically relevant molecules. Moreover, their characteristic three-dimensional structure, size, and surface charge enable them to interact with, and pass through, cell membranes, making them better delivery vehicles than classical polymeric materials [6,130,150]. Nonetheless, the use of dendrimers in biological systems is hindered by their inherent toxicity, mostly attributed to the interaction of surface cationic charge of dendrimers with negatively charged biological membranes [151]. In particular, higher cytotoxicity has been observed for higher-generation dendrimers and for cationic dendrimers, such as poly(amido amine) (PAMAM) and poly(propylene imine) (PPI) [152]. To minimize their toxicity, different chemical modifications can be performed on dendrimers' surface (e.g., PE-Gylation, acetylation) [151] or biocompatible molecules (e.g., maltose, maltotriose) can be used to decorate the nanosystem's outer shell [153].

Other Synthetic Polymers
Phosphazenes are attractive polymers for vaccine formulations. They can induce strong and sustained antigen-specific humoral and cell-mediated immune responses, which are considered better and safer options than conventional adjuvants [75].
Polyanhydrides represent another polymer class of interest for controlled release products. These materials are biodegradable, biocompatible, safe, and approved for human use. Specifically, polyanhydrides degrade through surface erosion, releasing non-toxic and easily metabolized carboxylic by-products. Furthermore, this process of erosion that takes place only at the surface of nanoparticles contributes to the tailored and sustained release of encapsulated cargos [6,65]. Moreover, the surface of polyanhydride particles can be easily functionalized [65].
Polymersomes have attracted increasing research interest as versatile carriers due to their colloidal stability, tunable membrane properties, and capacity of encapsulating various drugs and biomolecules. These vesicles made of self-assembling synthetic block copolymers have tunable stability, degradation, and functionalization. They can deliver hydrophilic compounds by incorporating them inside the vesicle or hydrophobic cargos by membrane delivery [130].

Polymeric Nanoparticles Synthesis
Polymer-based NPs are one of the most commonly used forms of soft materials for nanomedicine applications not only due to their versatility and the broad spectrum of applications but also due to their facile synthesis [88]. Recent polymer chemistry progress has allowed the preparation of tailored NPs with well-controlled structures (e.g., finetuned size, shape, morphology) and compositions, which are essential factors in obtaining vehicles for targeted delivery and controlled cargo release [171].
In drug delivery applications, two main categories of nanoparticles can be distinguished, namely nanocapsules (reservoir systems) and nanospheres (matrix systems) ( Figure 2). Nanocapsules present an inner core in which the freight is usually incorporated, surrounded by a polymeric shell, whereas nanospheres are composed of a continuous polymeric network that can entrap the drug or absorb it onto the nanoparticle's surface [172].
lease of biomolecules/drugs [157], and nanocontainers for antibiotic therapy [ Polymersomes have attracted increasing research interest as versatile car their colloidal stability, tunable membrane properties, and capacity of encaps ious drugs and biomolecules. These vesicles made of self-assembling synthet polymers have tunable stability, degradation, and functionalization. They can drophilic compounds by incorporating them inside the vesicle or hydrophob membrane delivery [130].

Polymeric Nanoparticles Synthesis
Polymer-based NPs are one of the most commonly used forms of soft m nanomedicine applications not only due to their versatility and the broad s applications but also due to their facile synthesis [88]. Recent polymer chemis has allowed the preparation of tailored NPs with well-controlled structure tuned size, shape, morphology) and compositions, which are essential factors vehicles for targeted delivery and controlled cargo release [171].
In drug delivery applications, two main categories of nanoparticles ca guished, namely nanocapsules (reservoir systems) and nanospheres (matr ( Figure 2). Nanocapsules present an inner core in which the freight is usua rated, surrounded by a polymeric shell, whereas nanospheres are composed uous polymeric network that can entrap the drug or absorb it onto the nanopa face [172].  Created based on information from literature references [172][173][174].
Depending on the type of cargo to be delivered by the polymeric NPs and their proposed administration route, different methods can be employed in the production of nanospheres and nanocapsules [172]. The standard synthesis methods involve one of two fundamental mechanisms: kinetically driven encapsulation, during nucleation and particle growth, and thermodynamically self-assembly. Out of these possibilities, the first one has shown particular promise as it allows the encapsulation of large amounts of hydrophobic drugs while preserving a narrow size distribution [175].
The first strategy used for manufacturing polymeric NPs from a preformed polymer was the solvent evaporation method (Figure 3a), which leads to the formation of nanospheres. It assumes the preparation of an oil-in-water emulsion, starting from an organic phase (consisting of polar organic solvent, polymer, and drug) and an aqueous phase (consisting of surfactant and water). Initially, dichloromethane and chloroform have been most widely used as organic solvents, but due to toxicity considerations, they have been replaced by ethyl acetate [172,176]. For obtaining small particle size, ultrasonication or high-speed homogenization stages can be employed. This method is suitable for the encapsulation of hydrophobic drugs [177]. A similar synthesis route for nanospheres production is the emulsion/reverse salting method (Figure 3b), which mainly differs from the previous method by the emulsion composition. Specifically, the organic phase is formulated from a polymer, drug, and solvent miscible in water (e.g., acetone, ethanol), and the aqueous phase contains salting-out agents and a stabilizer [176,177]. A derived synthesis method, the emulsification/solvent diffusion technique (Figure 3c), can be used for producing both nanocapsules and nanospheres [177]. This method assumes the formation of an oil-in-water emulsion between a partially water-miscible solvent (e.g., benzyl alcohol, ethyl acetate), containing the polymer and the desired cargo, and an aqueous solution with a surfactant [172]. This method may yield particles with a high encapsulation efficiency of lipophilic and hydrophilic active substances, batch-to-batch reproducibility, narrow size distribution, and ease of scale-up production [176,177]. In contrast to the above-described methods, nanoprecipitation (also known as solvent displacement method or interfacial deposition) ( Figure 3d) requires two miscible solvents. The polymer and drug are dissolved in a water-miscible solvent and further injected into an aqueous solution, resulting in a colloidal suspension. The as-such-obtained nanospheres and nanocapsules have a better-defined size, and a narrower size distribution, than the emulsification processes [172,176,177].  Reprinted from an open-access source [172].
Other chemical methods for polymeric nanoparticles manufacturing involve the polymerization of monomers, instead of nanoparticles construction, from preformed polymers. In this category, the most used techniques are emulsion polymerization and inter- Reprinted from an open-access source [172].
Other chemical methods for polymeric nanoparticles manufacturing involve the polymerization of monomers, instead of nanoparticles construction, from preformed polymers. In this category, the most used techniques are emulsion polymerization and interfacial polymerization, allowing simultaneous polymer synthesis and drug encapsulation [177].
Alternatively, physical methods can be used for polymer NPs manufacturing. One such method is laser ablation, which uses a high-power laser beam to evaporate particles from a solid material source. Similarly, pulse laser deposition (PLD) can be employed; this method assumes that the target material is hit by high-power laser pulses, leading to its melting, evaporation, and ionization. Another technique that provides flexibility and control over surface parameters of the synthesized nanoparticles is electrospraying. The synthesis process starts with a solution of polymer and solvent, placed in a syringe, and the application of a high voltage to its capillary tip. The solvent is evaporated while the particles or fibers are pushed to a collector [178].
More recently, polymer-based nanoparticles started being synthesized with the aid of microfluidic devices. The small channel dimensions and the special geometry of these devices allow the synthesis of high-quality nanocarriers in shorter times and with lower consumption of reagents. Moreover, microfluidics technology brings better control over the size, size distribution, morphology, and composition of the final products. Specifically, the size, polydispersity, and drug encapsulation can be simply tailored by varying experimental parameters such as flow rates, polymer composition, and polymer concentration [179].

Applications of Polymer-Based Delivery Nanosystems
Either alone, in blends, or in combination with other nanomaterials, polymer-based nanoparticles can deliver a variety of cargos, including active pharmaceutical ingredients, nucleic acids, imaging agents, antigens, and other biomolecules. This section reviews the most recent advances in the development of polymer-based delivery nanosystems, depending on the carried moieties.

Drug Delivery
For a drug to be released to the targeted cell, it must be hydrophilic enough to travel through aqueous media and reach the cellular membrane but lipophilic enough to cross this barrier and pass inside the cell. Due to the broad range of available materials and the possibility of functionalization, polymeric materials can be tailored to adjust the hydrophilicity of the drug formulation and deliver the cargo at the desired site. Moreover, the versatility of polymer-based nanoparticles can ensure the delivery of encapsulated drugs through a variety of administration routes, including oral delivery, ocular delivery, nasal delivery, pulmonary delivery, buccal delivery, periodontal delivery, dermal and transdermal delivery, and vaginal delivery.
Given the wide range of possible applications, increasing research interest has been attracted to designing and testing polymer-based delivery platforms. Much effort has recently been put into developing antimicrobial delivery systems that would enhance cargo activity while overcoming drug resistance and diminishing systemic side effects [173,[180][181][182][183] ( Figure 4).
Several such novel polymer-based delivery systems are reviewed in Table 1.
delivery, pulmonary delivery, buccal delivery, periodontal delivery, dermal and transdermal delivery, and vaginal delivery. Given the wide range of possible applications, increasing research interest has been attracted to designing and testing polymer-based delivery platforms. Much effort has recently been put into developing antimicrobial delivery systems that would enhance cargo activity while overcoming drug resistance and diminishing systemic side effects [173,[180][181][182][183] (Figure 4). Several such novel polymer-based delivery systems are reviewed in Table 1.   Supported proliferation and growth of fibroblasts Sustained drug release Higher release rate in an alkaline pH compared to neutral pH during 10 days Suitable for severe wound infections [193] Antimicrobial agent: Vancomycin Polymer: Hyaluronic acid Other materials: Oleylamine Sustained drug release for 72 h Moderate antibacterial activity against Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) 1.8 times higher MRSA cell death than for free drug administration due to a stronger impact on the bacterial membrane [194] Antimicrobial agent: Triphala Churna (polyherbal formulation) Polymer: Starch Other materials: -Excellent antibacterial activity against Salmonella typhi and Shigella dysenteriae Antibiofilm activity against methicillin-resistant Staphylococcus aureus Neuroprotective potential [117] Antimicrobial agent: SET-M33 peptide Polymer: Dextran Other materials: -

Effective against Pseudomonas aeruginosa
Acceptable cytotoxicity Markedly improved lung residence time [195] [196] Antimicrobial agent: Pistacia lentiscus L. var. chia essential oil Polymer: PLA Other materials: Surfactants (poly(vinyl alcohol-PVA), lecithin-LEC) Higher encapsulation efficiency was recorded for PLA/PVA NPs than for PLA/LEC NPs A gradual release of the carried agent was noticed for the PLA/PVA NPs, while the PLA/LEC NPs exhibited a more immediate release [197] Antimicrobial agent: Rifampicin Polymers: PLA, Poly(L-lysine) Other materials: -High and superficial loading of the antibiotic Effective delivery with a biphasic release profile Slowed particle migration in the Staphylococcus aureus biofilm thickness Improved retention in the biofilm Better antibiotic efficacy than for uncoated particles [198] Antimicrobial agents: Rutin, Benzamide Polymers: PEG, PLGA Other materials: -Sustained release of rutin-benzamide for several days Antibacterial activity against Staphylococcus aureus and Pseudomonas aeruginosaAnti-biofilm activity through the disruption of the bacterial membrane and biofilm surface [199] Antimicrobial agent: Pseudomonas aeruginosa biofilm dispersal Worm-like particles are more effective in the long term; spherical NPs are better for faster delivery applications [205] As cancers remain one of the major health concerns worldwide, extensive research has been oriented to developing better therapeutics for this category of diseases. Chemotherapeutic drugs can be employed in the treatment of cancer patients, as they interfere with the cell cycle and the process of mitosis, causing a greater proportion of cell kill in tumor cells than in healthy tissues [206]. Nonetheless, large systemic doses of such aggressive drugs may lead to drug resistance and adverse effects, while their repeated administration requires a strict treatment schedule that must be adapted to the ability of healthy tissues to recover [207][208][209]. Thus, attention has been drawn to developing carrier systems that allow a controlled release at the tumor site. Due to recent findings concerning the tumor microenvironment, targeted solutions have been envisaged. Specifically, stimuli-responsive delivery systems have been created to target the acidic pH and/or hypoxic environment characteristic of tumor cells [210] (Figure 5).

Imaging Agent Delivery
Medical imaging is an essential part of clinical diagnosis, enhancing diagnostic accuracy, enabling a faster start of treatment, and improving survival rates in many diseases [258]. Moreover, synergistic outcomes can be obtained by combining conventional imaging techniques with nanotechnology, especially when using nanoparticles as contrast agents [259][260][261]. Nonetheless, uncoated metal-based nanoparticulate contrast agents may induce toxicological reactions through ROS generation, the release of free metal ions, and the production of aggregates that cannot be eliminated by the cells [261].
Thus, a convenient approach is to coat these NPs with biocompatible polymers. For instance, Vu-Quang et al. [262] designed a nanosystem, based on SPION core covered with a pluronic F127-folate coating, that can specifically target folate receptor-expressing cancer cells-a promising candidate as a contrast agent in MRI. Similarly, Kania et al. [263] have coated SPIONs with ultrathin layers of chitosan derivatives, obtaining suitable T2 contrast agents for liver disease diagnostic. In another study by Amendola et al. [264], bimetallic (silver-iron) nanoparticles were coated with PEG, offering promising results in terms of biopersistency and contrast efficiency.
Another promising strategy is to deliver conventional contrast agents by polymerbased vehicles. In this respect, Shao et al. [220] have proposed a carboxymethyl chitosan 4-hydroxymethyl-pinacol phenyl borate carrier encapsulated with indocyanine green and modified with RGD. Their ROS-responsive nanosystem can be employed in near-infrared imaging and photothermal therapy against gastric cancer. Another polymer-contrast agent system possibility is offered by Ponsiglione et al. [265], who have delivered Gd-DTPA with the aid of hyaluronic acid. Cheng et al. [266] have also approached Gd delivery using porous polymersomes (produced from self-assembly of polyethylene oxide-bpolybutadiene (PBdEO) and polyethylene oxide-b-polycaprolactone (PEOCL)). The Gd was conjugated to polyamidoamine (PAMAM) dendrimers via diethylenetriaminepentaacetic acid dianhydride (DTPA dianhydride) before polymersome encapsulation.
Modern medical imaging can also benefit from polymers tagged with radionuclides for molecular imaging of cancer in techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) [267]. For instance, Gill et al. [268] have reported the synthesis of PLGA NPs surface conjugated to DTPA-hEGF, encapsulating the ruthenium-based DNA replication inhibitor and radiosensitizer, and labeled with 111 In (Figure 6). The same radiolabel was used by Gorshkov et al. [269], who conjugated it on N-vinylpyrrolidone-N-vinylformamide copolymers. In a recent study, Huang et al. [270] have prepared 64 Cu-labelled polymer that can detect small occult tumors in mice's brain, head, neck, and breast at much higher contrast 18 F-fluorodeoxyglucose. conjugated it on N-vinylpyrrolidone-N-vinylformamide copolymers. In a recent study, Huang et al. [270] have prepared 64 Cu-labelled polymer that can detect small occult tumors in mice's brain, head, neck, and breast at much higher contrast 18 F-fluorodeoxyglucose.

Gene Delivery
Gene therapy and immune engineering are complex tasks that hold great promise in treating various disorders. In this respect, nucleic acids can be employed for overexpressing or knocking down specific genes and can be used as adjuvants or danger signals for modulating the behavior of immune cells. Nonetheless, the direct delivery of nucleic acids has several drawbacks, as naked nucleic acids are prone to extracellular degradation, and they face difficulties in passing through the cell membrane [19,271,272].
In this context, increasing research has recently been focused on creating innovative delivery systems that can ensure efficient and targeted delivery of nucleic acids. Among the various tested materials, nanoscale polymers can embed or electrostatically absorb nucleic acids at their surface through a suitable surfactant or cationic polymer addition [6]. Specifically, cationic polymers can form electrostatic nanocomplexes with nucleic acids, which are highly negative, to facilitate their permeation into desired cells. In contrast, other hydrophobic polymers can physically entrap nucleic acids within nanoparticles [19].
Having a positively charged chemical structure, PEI-based nanoparticles are extensively used in gene delivery. However, despite its buffering capacity that can overcome intracellular barriers, PEI use is limited by its toxicity [273,274]. Poly(L-lysine) is another material that has attracted early gene delivery research, as it allows efficient binding to the cargo. Nevertheless, it faces challenges in facilitating endosomal escape and releasing the carried agents inside the cells [19].
Currently, lipid-based nanoparticles (LNPs) are the most clinically progressed nanoplatforms for delivering nucleic acids. Nonetheless, Blakney et al. [275] have compared the efficiency of LNP to that of pABOL bioreducible polymer in self-amplifying RNA

Gene Delivery
Gene therapy and immune engineering are complex tasks that hold great promise in treating various disorders. In this respect, nucleic acids can be employed for overexpressing or knocking down specific genes and can be used as adjuvants or danger signals for modulating the behavior of immune cells. Nonetheless, the direct delivery of nucleic acids has several drawbacks, as naked nucleic acids are prone to extracellular degradation, and they face difficulties in passing through the cell membrane [19,271,272].
In this context, increasing research has recently been focused on creating innovative delivery systems that can ensure efficient and targeted delivery of nucleic acids. Among the various tested materials, nanoscale polymers can embed or electrostatically absorb nucleic acids at their surface through a suitable surfactant or cationic polymer addition [6]. Specifically, cationic polymers can form electrostatic nanocomplexes with nucleic acids, which are highly negative, to facilitate their permeation into desired cells. In contrast, other hydrophobic polymers can physically entrap nucleic acids within nanoparticles [19].
Having a positively charged chemical structure, PEI-based nanoparticles are extensively used in gene delivery. However, despite its buffering capacity that can overcome intracellular barriers, PEI use is limited by its toxicity [273,274]. Poly(L-lysine) is another material that has attracted early gene delivery research, as it allows efficient binding to the cargo. Nevertheless, it faces challenges in facilitating endosomal escape and releasing the carried agents inside the cells [19].
Currently, lipid-based nanoparticles (LNPs) are the most clinically progressed nanoplatforms for delivering nucleic acids. Nonetheless, Blakney et al. [275] have compared the efficiency of LNP to that of pABOL bioreducible polymer in self-amplifying RNA (saRNA) delivery. Both tested platforms induced enhanced levels of IFN-γ, IL-12, IL-5, and TNF-α 4 h after administration. The researchers obtained a higher humoral and cellular immunity for LNPs, whereas a higher protein expression was observed for pABOL carriers. Thus, each delivery vehicle is advantageous for a different niche of saRNA applications. Specifically, LNPs are more suitable for vaccine formulations, while pABOL nanosystems may be employed in protein replacement therapies.
Another promising approach for nucleic acid delivery is employing lipid-polymer hybrid nanoparticles (LPNs) [276][277][278]. For instance, Vencken et al. [279] have tested the delivery of miR-17 to bronchial epithelial cells by LPNs, composed of PLGA and cationic lipid 1,2-dioleoyloxy-3-(trimethylammonium)propane, noting minimal cytotoxic and proinflammatory effects. LPNs can also be employed in gene therapy against drug-resistant glioblastoma, as investigated by Yang et al. [280]. The researchers have recently constructed LPNs loaded with CRISPR/Cas9 plasmids, targeting the MGMT gene, modified with the cRGD peptide that effectively targeted overexpressed integrin αvβ3 receptors in tumor cells, and restored the sensitivity of glioblastoma cells to temozolomide.

Vaccine Delivery
In general, vaccination represents the main method of preventing virus pathogenicity, reducing the burden of many infectious diseases. Nonetheless, traditional vaccines encounter several limitations, as they are susceptible to degradation, have a short duration of action, and may cause side effects and inflammatory reactions at the injection site [6,11]. Moreover, an important number of infectious diseases and chronic disorders (e.g., human immunodeficiency virus (HIV), healthcare-associated infections (HAIs), cytomegalovirus (CMV), respiratory syncytial virus (RSV), tuberculosis, malaria, etc.) cannot be prevented by conventional vaccines [18]. Thus, in recent years, modern bio-nanotechnology started being involved in vaccine development towards creating new-generation formulations [12,281]. In particular, the use of polymer-based nanovaccines is considered a promising approach in improving cross-presentation and enhancing vaccine potency against cancer, intracellular bacteria, and virus infection [282,283]. The main advantages of polymer-based nanovaccines are synthesized in Figure 7. (saRNA) delivery. Both tested platforms induced enhanced levels of IFN-γ, IL-12, IL-5, and TNF-α 4 h after administration. The researchers obtained a higher humoral and cellular immunity for LNPs, whereas a higher protein expression was observed for pABOL carriers. Thus, each delivery vehicle is advantageous for a different niche of saRNA applications. Specifically, LNPs are more suitable for vaccine formulations, while pABOL nanosystems may be employed in protein replacement therapies. Another promising approach for nucleic acid delivery is employing lipid-polymer hybrid nanoparticles (LPNs) [276][277][278]. For instance, Vencken et al. [279] have tested the delivery of miR-17 to bronchial epithelial cells by LPNs, composed of PLGA and cationic lipid 1,2-dioleoyloxy-3-(trimethylammonium)propane, noting minimal cytotoxic and proinflammatory effects. LPNs can also be employed in gene therapy against drug-resistant glioblastoma, as investigated by Yang et al. [280]. The researchers have recently constructed LPNs loaded with CRISPR/Cas9 plasmids, targeting the MGMT gene, modified with the cRGD peptide that effectively targeted overexpressed integrin αvβ3 receptors in tumor cells, and restored the sensitivity of glioblastoma cells to temozolomide.

Vaccine Delivery
In general, vaccination represents the main method of preventing virus pathogenicity, reducing the burden of many infectious diseases. Nonetheless, traditional vaccines encounter several limitations, as they are susceptible to degradation, have a short duration of action, and may cause side effects and inflammatory reactions at the injection site [6,11]. Moreover, an important number of infectious diseases and chronic disorders (e.g., human immunodeficiency virus (HIV), healthcare-associated infections (HAIs), cytomegalovirus (CMV), respiratory syncytial virus (RSV), tuberculosis, malaria, etc.) cannot be prevented by conventional vaccines [18]. Thus, in recent years, modern bio-nanotechnology started being involved in vaccine development towards creating new-generation formulations [12,281]. In particular, the use of polymer-based nanovaccines is considered a promising approach in improving cross-presentation and enhancing vaccine potency against cancer, intracellular bacteria, and virus infection [282,283]. The main advantages of polymerbased nanovaccines are synthesized in Figure 7.   [11,18,154,284].
One attractive approach is to employ polymer nanoparticles, in mucosal delivery of vaccines, as a strategy to overcome some of the drawbacks of conventional vaccines. Such nanovaccines can target both the mucosal and systemic immune systems, enhancing humoral and cell-mediated immune responses, ensuring a sustained release, and protecting the loaded freight against degradation [12]. In more detail, mucosal vaccine delivery may stimulate cytotoxic T-cell responses along with secreted IgA, helping the host organism identify and destroy pathogens before entering further into the body [11].
Due to their immunological activity and mucoadhesive properties, CS-based NPs have been widely investigated in developing vaccines against Clostridium botulinum type A neurotoxins, Naospora, hepatitis B virus, Newcastle disease, and more [6,285]. For instance, Zhao et al. [286] have encapsulated Newcastle disease viruses (NDV) in N-2-hydroxypropyl trimethyl ammonium chloride chitosan (N-2-HACC) nanoparticles and assessed their potential as a mucosal immune delivery carrier. The newly developed nanosystems have shown much stronger cellular, humoral, and mucosal immune responses than commercially available live attenuated NDV vaccines.
Another example is offered by Dhakal et al. [287], who have proposed an innovative vaccine delivery platform and tested it against several influenza A virus strains. The researchers evaluated the immune responses and cross-protective efficacy of intranasal administered CSNPs, encapsulated with inactivated SwIAV vaccine, in pigs. The results showed an enhanced IgG serum antibody and mucosal secretory IgA antibody responses in nasal swabs, bronchoalveolar lavage (BAL) fluids, and lung lysates that were reactive against homologous (H1N2), heterologous (H1N1), and heterosubtypic (H3N2) viral strains. Influenza vaccine formulations were also created by use of other bioadhesive polymers [6,65], such as hyaluronic acid [288,289], alginate [290], starch [291], and poly(acrylic acid) [291,292].
Another intranasal vaccine delivery system has been developed and investigated by Hamzaoui and Laraba-Djebari [293]. Their study focused on PLGA NPs, loaded with Cerastes venom for snake envenomation prevention, and their results confirmed this new nano-formulation represents a potent adjuvant system that improves humoral immune response while protecting against high lethal doses of viper venoms. A similar approach for developing an antivenom vaccine was tackled by Mirzaei et al. [294]. The researchers used CS NPs for loading Echis carinatus venom in order to stabilize it. Moreover, the obtained antivenom plasma had a considerably higher potency for neutralizing the venom than conventional delivery systems.
In an effort to prevent antibiotic-resistant pathogen infections, increasing attention has been drawn to developing antibacterial vaccines [295]. In this respect, various nanoparticlebased vaccines, against several bacteria, have shown promising results (Table 3). Increased expression of TLR 2, TLR 4, IFN-γ, TGF-β, and Il-4 mRNA expression in chicken cecal tonsils Significantly higher OMPs-specific mucosal IgA production Enhanced lymphocyte proliferation response [297] Salmonella Polymer: Poly (lactic acid) Other materials: Vi polysaccharide and r-flagellin of Salmonella typhi Generated a strong immune response Promoted antibody class switching Produced memory antibody response from single point immunization Enhanced secretion of pro-inflammatory cytokine TNF-α and IL-6, while decreasing IFN-γ production [298]

Pathogen Vaccine Formulation Results
Refs.

Streptococcus pyogenes
Polymers: α-Poly-(L-glutamic acid), Trimethyl chitosan (TMC) Other materials: Peptide antigen Higher systemic and mucosal antibody titers than antigen adjuvanted with standard mucosal adjuvant cholera toxin B subunit or antigen mixed with TMC Reduced bacterial burden in nasal secretions, pharyngeal surface, and nasopharyngeal-associated lymphoid tissue [299] Streptococcus

Streptococcus agalactiae
Polymer: Poly(lactic-co-glycolic acid) Other materials: CAMP factor Induced a sustained increase od antibody titers Mortality and bacteria counts were lower than in the control group No pathological lesions were detected [305] Pseudomonas aeruginosa Polymers: Poly(lactic-co-glycolic acid), Alginate Other materials: -

Significant increase in total IgG and IgM antibodies
No cytotoxicity in lung, kidney, and liver [306] Pseudomonas aeruginosa Polymer: Poly(lactic-co-glycolic acid), Alginate Other materials: -Significant decrease in the bacterial burden in the spleen Considerably increased opsonic activity [307] Pseudomonas

Role of Polymer-Based NPs in Vaccine Development
Due to their extraordinary versatility, polymers play more than just transporter roles in vaccine formulations. Polymeric nanoparticles may possess the dual capability of being both the adjuvant and delivery vehicle, helping in controlled antigen release, inducing rapid and long-lived immunity, prolonging shelf-life at elevated temperatures, enhancing patient compliance, and enabling the rapid development of vaccines for newly emerging infectious disease viruses [10,65,75,310].

Vaccine Adjuvants
As many antigens are poorly immunogenic, adjuvants are added to vaccine formulations to elicit/potentiate the immune response, offer better protection against pathogens, and diminish the required antigen amount for obtaining immunity [12,100,281].
The most currently used adjuvants are aluminum-based (or alum compounds) adjuvants and Freund's adjuvants. However, despite their relative safety and long history of use, aluminum salts may produce adverse effects, including erythema, nodules, contact hypersensitivity, and granulomas. Other drawbacks of alum adjuvants are the bias towards humoral immunity, the necessity of multiple doses, and incompatibility with many antigens. Freund's adjuvants also present important disadvantages, as the paraffin oil used for these emulsions causes toxicity issues and produces severe local reactions [65,[311][312][313]. Hence, better solutions had to be developed.
There are two main adjuvants types: antigen delivery systems (or depots) and immunostimulatory agents [130]. Some materials are even able to perform both roles simultaneously.

Antigen Delivery
A variety of polymeric carriers have been investigated for protecting antigens from proteolytic degradation, enhancing antigen entrapment, obtaining a desirable release profile, and targeting antigen-presenting cells (APCs) [144,147,283,[314][315][316] (Figure 8). both the adjuvant and delivery vehicle, helping in controlled antigen release, inducing rapid and long-lived immunity, prolonging shelf-life at elevated temperatures, enhancing patient compliance, and enabling the rapid development of vaccines for newly emerging infectious disease viruses [10,65,75,310].

Vaccine Adjuvants
As many antigens are poorly immunogenic, adjuvants are added to vaccine formulations to elicit/potentiate the immune response, offer better protection against pathogens, and diminish the required antigen amount for obtaining immunity [12,100,281].
The most currently used adjuvants are aluminum-based (or alum compounds) adjuvants and Freund's adjuvants. However, despite their relative safety and long history of use, aluminum salts may produce adverse effects, including erythema, nodules, contact hypersensitivity, and granulomas. Other drawbacks of alum adjuvants are the bias towards humoral immunity, the necessity of multiple doses, and incompatibility with many antigens. Freund's adjuvants also present important disadvantages, as the paraffin oil used for these emulsions causes toxicity issues and produces severe local reactions [65,[311][312][313]. Hence, better solutions had to be developed.
There are two main adjuvants types: antigen delivery systems (or depots) and immunostimulatory agents [130]. Some materials are even able to perform both roles simultaneously.

Antigen Delivery
A variety of polymeric carriers have been investigated for protecting antigens from proteolytic degradation, enhancing antigen entrapment, obtaining a desirable release profile, and targeting antigen-presenting cells (APCs) [144,147,283,[314][315][316] (Figure 8). For instance, Wusiman et al. [317] have prepared antigen delivery carriers made of CS-modified PLGA NPs, PEI-modified PLGA NPs, and ε-Poly-L-lysine (εPL)-modified For instance, Wusiman et al. [317] have prepared antigen delivery carriers made of CS-modified PLGA NPs, PEI-modified PLGA NPs, and ε-Poly-L-lysine (εPL)-modified PLGA NPs. The particles were loaded with AHPP and OVA, exhibiting positive charge after surface cationic polymers modification and demonstrating improved antigen loading capacity and stability ( Figure 9). Moreover, these formulations allowed greater OVA adsorption capacity, leading to a significantly increased lymphocyte proliferation, improved CD4+/CD8+ T cells ratio, and secretion of cytokines (TNF-α, IFN-γ, IL-4, and IL-6), antibodies (IgG), and antibody subtypes (IgG1 and IgG2a) in immunized mice. PLGA NPs. The particles were loaded with AHPP and OVA, exhibiting positive charge after surface cationic polymers modification and demonstrating improved antigen loading capacity and stability ( Figure 9). Moreover, these formulations allowed greater OVA adsorption capacity, leading to a significantly increased lymphocyte proliferation, improved CD4+/CD8+ T cells ratio, and secretion of cytokines (TNF-α, IFN-γ, IL-4, and IL-6), antibodies (IgG), and antibody subtypes (IgG1 and IgG2a) in immunized mice. Cruz et al. [318] have also tackled the benefits of PLGA NPs antigen encapsulation. The researchers have co-encapsulated resiquimod and tetanus toxoid peptide antigen in PLGA NPs, obtaining a prolonged controlled release in the endosome. Their findings demonstrated that the slower kinetics of antigen release is more effective for major histocompatibility complex (MHC) class II and I cross-presentation in dendritic cells, producing stronger and more durable immune responses than soluble components.
By conjugating PLGA with PEG through a peroxalate ester bond and adding PEI as a cationic adjuvant, Liang et al. [319] have synthesized an antigen delivery system that is both ROS responsive and facilitates antigen uptake while diminishing the toxicity associated with cationic adjuvants. The tested nanocarrier proved excellent loading capacity, in vitro stability when encapsulating OVA model antigen, enhanced dendritic cell maturation, improved antigen uptake, increased lysosomal escape, antigen cross-presentation, upregulation of CD4+ and CD8+ T cell proportions, and increased memory T-cell generation.
PLGA has also shown promising results in combination with inorganic materials. In particular, Saengruengrit et al. [320] have reported the successful synthesis of a delivery system based on biocompatible nanocomposite particles of PLGA and superparamagnetic iron oxide nanoparticles (SPIONs). When an external magnetic field was applied, the SPI-ONs-PLGA system presented superparamagnetic activity, low toxicity, and good uptake in macrophages and bone-marrow-derived primary dendritic cells (BM-DCs). Moreover, the nanodelivery platform did not induce BM-DCs secretion of TNF-α, but it upregulated MHC II, CD80, and CD86 expression and IL-12 and IFN-γ production.
Another widely studied biopolymer for antigen delivery is chitosan. In this respect, Bussio et al. [321] have developed a core-shell structure, with an oily core and a surrounding CS shell of a lower size, for transcutaneous vaccination ( Figure 10). CS polymeric corona offered protection to the cargo and exhibited high stability in different storage conditions, along with a significant association of OVA as the model antigen. Cruz et al. [318] have also tackled the benefits of PLGA NPs antigen encapsulation. The researchers have co-encapsulated resiquimod and tetanus toxoid peptide antigen in PLGA NPs, obtaining a prolonged controlled release in the endosome. Their findings demonstrated that the slower kinetics of antigen release is more effective for major histocompatibility complex (MHC) class II and I cross-presentation in dendritic cells, producing stronger and more durable immune responses than soluble components.
By conjugating PLGA with PEG through a peroxalate ester bond and adding PEI as a cationic adjuvant, Liang et al. [319] have synthesized an antigen delivery system that is both ROS responsive and facilitates antigen uptake while diminishing the toxicity associated with cationic adjuvants. The tested nanocarrier proved excellent loading capacity, in vitro stability when encapsulating OVA model antigen, enhanced dendritic cell maturation, improved antigen uptake, increased lysosomal escape, antigen cross-presentation, upregulation of CD4+ and CD8+ T cell proportions, and increased memory T-cell generation.
PLGA has also shown promising results in combination with inorganic materials. In particular, Saengruengrit et al. [320] have reported the successful synthesis of a delivery system based on biocompatible nanocomposite particles of PLGA and superparamagnetic iron oxide nanoparticles (SPIONs). When an external magnetic field was applied, the SPIONs-PLGA system presented superparamagnetic activity, low toxicity, and good uptake in macrophages and bone-marrow-derived primary dendritic cells (BM-DCs). Moreover, the nanodelivery platform did not induce BM-DCs secretion of TNF-α, but it upregulated MHC II, CD80, and CD86 expression and IL-12 and IFN-γ production.
Another widely studied biopolymer for antigen delivery is chitosan. In this respect, Bussio et al. [321] have developed a core-shell structure, with an oily core and a surrounding CS shell of a lower size, for transcutaneous vaccination (Figure 10). CS polymeric corona offered protection to the cargo and exhibited high stability in different storage conditions, along with a significant association of OVA as the model antigen.
Wang et al. [322] have investigated a system based on polydopamine nanoparticles (Pdop-NPs) for subcutaneous antigen delivery as a vector in cancer immunotherapy. OVA model antigen was grafted onto the nanoparticles to form a carrier system able to migrate to lymph nodes and penetrate APCs. Furthermore, OVA-encapsulated Pdop-NPs promoted the maturation of DCs, activated OVA-specific cytotoxic CD8+ T cells, and induced the production of memory CD4+ and CD8+ T cells, thus considerably suppressing tumor growth. Wang et al. [322] have investigated a system based on polydopamine nanoparticles (Pdop-NPs) for subcutaneous antigen delivery as a vector in cancer immunotherapy. OVA model antigen was grafted onto the nanoparticles to form a carrier system able to migrate to lymph nodes and penetrate APCs. Furthermore, OVA-encapsulated Pdop-NPs promoted the maturation of DCs, activated OVA-specific cytotoxic CD8+ T cells, and induced the production of memory CD4+ and CD8+ T cells, thus considerably suppressing tumor growth. Another promising delivery system tested for OVA encapsulation is based on lignin nanoparticles. This adjuvant developed by Alqahtani et al. [121] was proven to be a safe stabilizer for antigen formulation during preparation and storage. Moreover, the OVAencapsulated lignin particles showed no cytotoxicity, significantly higher antigen uptake in dendritic cells, and stronger IgG antibody response than that induced by free OVA alum-adjuvanted OVA, being a potential candidate for the induction of long-term immunity.
Lipid-polymeric hybrid delivery systems have also started to draw increasing scientific interest. For instance, Miura et al. [323] have created a cholesterol-pullulan self-assembly nanogel that they further modified by carboxylic group substitution to become negatively charged. This innovative system has been shown to target APCs and release the loaded antigen, inducing considerable adaptive immunity.

Immunomodulation
One way of enhancing the immune responses is to use a targeted delivery approach to immune cells [324]. In this respect, Dowling et al. [325] have encapsulated a Toll-like receptor (TLR) 8 agonist inside various poly(ethylene glycol)-bl-poly(propylene sulfide) (PEG-bl-PPS) polymer-based nanostructures, allowing direct intracellular release after selective uptake by DCs. TLR 8 agonist polymersomes led to similar newborn DC maturation profiles to those induced by BCG and stronger IL-12p70 production, holding promising potential for early-life immunization against intracellular pathogens. Following a similar strategy for stimulating cellular immunity, Rajput et al. [326] have designed an Reprinted from an open-access source [321]. Another promising delivery system tested for OVA encapsulation is based on lignin nanoparticles. This adjuvant developed by Alqahtani et al. [121] was proven to be a safe stabilizer for antigen formulation during preparation and storage. Moreover, the OVAencapsulated lignin particles showed no cytotoxicity, significantly higher antigen uptake in dendritic cells, and stronger IgG antibody response than that induced by free OVA alumadjuvanted OVA, being a potential candidate for the induction of long-term immunity.
Lipid-polymeric hybrid delivery systems have also started to draw increasing scientific interest. For instance, Miura et al. [323] have created a cholesterol-pullulan self-assembly nanogel that they further modified by carboxylic group substitution to become negatively charged. This innovative system has been shown to target APCs and release the loaded antigen, inducing considerable adaptive immunity.

Immunomodulation
One way of enhancing the immune responses is to use a targeted delivery approach to immune cells [324]. In this respect, Dowling et al. [325] have encapsulated a Toll-like receptor (TLR) 8 agonist inside various poly(ethylene glycol)-bl-poly(propylene sulfide) (PEG-bl-PPS) polymer-based nanostructures, allowing direct intracellular release after selective uptake by DCs. TLR 8 agonist polymersomes led to similar newborn DC maturation profiles to those induced by BCG and stronger IL-12p70 production, holding promising potential for early-life immunization against intracellular pathogens. Following a similar strategy for stimulating cellular immunity, Rajput et al. [326] have designed an inulin acetate-based nanodelivery system to target DCs. The tested material exhibited potent vaccine adjuvant properties, activating TLR 4 on multiple immune cells to secrete various cytokines. Widmer et al. [327] proposed a novel carrier nanosystem that can ensure the targeted delivery of resiquimod to the lymph node. The researchers successfully encapsulated this TLR 7 ligand into methoxy poly(ethylene glycol)-b-poly(DL-lactic acid) (mPEG-PLA) and mixed poly(DL-lactic-co-glycolic acid) (PLGA)/mPEG-PLA nanoparticles obtaining good results in terms of cell (i.e., dendritic cells and macrophages) targeting and uptake.
Moreover, the investigated particles are non-inflammatory and non-toxic on immune cells, making them promising candidates for cancer immunotherapy.
Another strategy is to take advantage of the intrinsic immunostimulatory properties of certain materials [9,154]. Several polymers, including PLGA, PS, CS, cellulose, lentinan, and dendrimers, can enhance the immune effects of vaccine formulations [14,154,328,329].
The beneficial properties of such polymers can be harnessed for improving the immune response for a broad range of vaccines. For example, inhalable polymeric particles were designed for pulmonary delivery of the hepatitis B vaccine. Thomas et al. [330] have created porous PLGA, as well as PLA NPs loaded with a specific antigen (i.e., HBsAg) that induced enhanced immune responses. Dewangan et al. [331] have also designed an HBsAg PLGA-loaded nanovaccine that demonstrated sustained release and better internalization in macrophage and MRC-5 cell lines. The researchers have tested several single-dose administration routes, obtaining the best results, in terms of immune-stimulating activity, for the intramuscular route; particularly, the nanovaccine administered in this way produced better humoral and cellular responses. An alternative intramuscular delivery system for HBsAg antigen was proposed by Liu et al. [332], who produced PLA microparticles modified with didodecyldimethylammonium bromide that absorbed hepatitis-specific antigens onto their surface. After three intramuscular injections with these particles, the level of pro-inflammatory cytokines (IL-1β, IL-6, CCL2, and CXCL1) increased at the injection site, the vaccine exhibiting ten times higher antigen-specific IgG titers than the group treated with commercial alum-adjuvanted antigen.
Another vaccine, for which polymers have been shown to potentiate the immune response, is tuberculosis (TB) vaccine. Khademi et al. [333] have combined the vaccine for this disease with chitosan and tested the novel formulation on mice. The CS-based TB vaccine demonstrated how parenteral and non-parenteral immunization lead to appropriate immune responses, inducing both protective and cell-mediated (CD4 and CD8) immune responses in the immunized animal models. Moreover, due to the mucoadhesive properties of CS, non-parenteral immunization can be considered as a more effective administration route.
Another highly researched topic is the development of an effective HIV vaccine. In this respect, Dacoba et al. [334] have investigated if the covalent attachment of a protease cleavage site (PCS) peptide to polysaccharide-based nanoparticles, together with the administration of polyinosinic:polycytidylic acid, enhanced the immune response. The study obtained promising results, with strong activation of APCs, concluding that both nanoparticle composition and the conjugation of the HIV peptide antigen contributed to the generated humoral and cellular immune responses.

COVID-19 Immunization
As severe acute respiratory syndrome-associated coronavirus 2 (SARS-CoV 2), also known as coronavirus disease of 2019 (COVID-19), has produced a public health crisis worldwide with huge human and economic losses, concerted global efforts have been employed in designing efficient vaccines [335][336][337]. As the genetic sequence of SARS-CoV 2 was made available in record time (within weeks after its discovery), the current vaccines were developed with unprecedented speed, with the clinical trials of promising candidates being completed within only a few months [17,336,338].
Nanomedicine played a tremendous role in COVID-19 vaccine development [339]. Moreover, the virus can be regarded as a functional nanomaterial, due to its nanometric size and core-shell nanostructure [340,341]. Thus, various nanoplatforms, such as lipid nanoparticles, polyplexes, dendrimers, cationic polysaccharide particles, and cationic nanoemulsions, were tested for delivering nucleic acids in vaccine formulations [17,275,338,342]. Out of the plethora of possibilities, lipid nanoparticles (LNP) are the most clinically advanced, both Pfizer/BioNTech and Moderna COVID-19 vaccines being LNP formulations [17,275].
Nonetheless, polymer-based vaccine alternatives have also shown promising results. For instance, Volpatti et al. [343] have created a subunit nanovaccine by conjugating SARS-CoV-2 Spike protein receptor-binding domain on the surface of polymersomes susceptible to oxidation. This vaccine formulation conducted to strong humoral neutralizing response to SARS-CoV-2 and robust T cell immunity.
Another strategy was adopted by Zhang et al. [344], who developed a core-shell nanostructure with a core made of PLGA and a human-cell-derived shell sourced from cells that are naturally targeted by SARS-CoV 2. The researchers demonstrated that the virus is neutralized, after incubation with these nanosponges, and can no longer infect cells.
Polyamidoamines (PAMAMs) represent another promising strategy in the treatment of COVID-19 [340], as it was demonstrated that they could prevent the cleavage of angiotensin and acute respiratory distress syndrome by binding to the ACE2 receptor [345]. Alternatively, chitin and chitosan can be used as delivery vehicles, as they have intrinsic antiviral activities and immune-boosting effects [346]. Other antiviral macromolecules of interest for COVID-19 drugs and vaccines are poly(vinylbenzoic acid), poly(vinylphosphonic acid), PVP, and cyclodextrins [347].

Conclusions and Future Perspectives
To summarize, a multitude of natural and synthetic polymers can be used to design useful delivery nanosystems for diverse therapeutics, imaging agents, antigens, and other biomolecules. Their versatility and property tunability can be exploited for carrying the necessary moieties to the desired site, even if the cells/tissues are challenging to reach by conventional drugs. Moreover, polymeric nanoparticles allow a targeted and controlled cargo release in response to changes in the pH, the oxygen level in the tissues, or binding with specific receptors. Therefore, polymer-based systems are suitable for many therapies against infections and chronic diseases, offering accurate diagnosis possibilities. This review also explores the role of polymers in developing novel and improved vaccines, especially mucosal administered formulations, for preventing various conditions, including envenomation, hepatitis, tuberculosis, cancer, and COVID-19 infection.
Considering the recent advances in this field, it can be expected that the particles, experimentally validated on animal models, would move to clinical trials. Nonetheless, further research is required, as a small subset of the immune-activation cascade is usually examined, while overall effects on human health may be neglected. Another challenge that has to be soon overcome is translating from the lab to scale-up synthesis of polymeric nanocarriers without compromising their quality and fine-tuned properties.
Furthermore, interesting possibilities arise at the convergence of nanotechnology with other innovative fields, such as artificial intelligence and data analytics, that are promising perspectives towards attaining personalized therapeutic and vaccine formulations.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.