Over the previous three decades, LbL assembly technologies have evolved and adapted in accordance with the materials used, and the products desired. Conventional polymer LbL assembly is based on the adsorption of oppositely charged polymers on to a surface via forces where entropic and enthalpic interactions play a governing role ^[1][2][1,2]. Many modifications have been implemented to conventional LbL assembly during the intervening years to enhance the properties of the sequentially deposited layers. This has resulted in new branches of LbL assembly, such as 3D bio-printing (unconventional LbL) and saloplastics (quasi-LbL) [1]. As a result of this, LbL assembly is now prevalent in many fields because of its ability to coat an array of different surfaces, regardless of the topography and geometry.

Significant research has been conducted on polyelectrolyte multilayers (PEMs) deposited on to particles via LbL. The polyelectrolytes used can be biodegradable, biocompatible, and can possess various (bio)properties, including anti-inflammatory and osteogenic properties [3]. Consequently, these particles have a plethora of applications, including but not limited to delivery devices, biomedical coatings, protective coatings, and photocatalysis ^{[4][5][6][7][8][9][10]}[4,5,6,7,8,9,10].

When fabricating these particles via electrostatic LbL deposition, the predominant interactions that influence the interfacial assemblies are ion pairing, and other interactions such as: van der Waals forces, guest-host interactions, base-pair interactions, hydrogen bonding, hydrophobic interactions, and polar interactions ^{[11][12][13][14][15]}[11,12,13,14,15]. What separates multi-layered particles in their corresponding fields, is the fact that compared to other systems, the decomposable cores used to template multilayer structures can be synthesised from numerous components such as organic particles, biomolecules, polymers and inorganic salts ^[9][16][17][9,16,17]. To illustrate, mesoporous vaterite CaCO₃ particles (commonly used as delivery devices) are frequently used for PEM particle preparation ^[18][19][18,19], and may be prepared from the inorganic salts CaCl₂ and Na₂CO₃ [20], ranging from submicron to tens of microns in size [21]. Moreover, for these particles, the vaterite core is mesoporous [3], allowing for the loading of drugs and other bioactive molecules into its pores prior to the formation of the PEMs on the particle’s surface ^[4][6][17][4,6,17].

2. Biopolymer-Based PEM Particle Immobilisation

Matrix-type capsules are also common when using CaCO₃ templates. These particles are formed from mesoporous CaCO₃ cores being interpenetrated by polyelectrolytes during LbL deposition. For instance, confocal laser scanning microscopy (CLSM) displays that in vaterite CaCO₃ crystal pores, the polymers that have interpenetrated, occupy the internal volume of the core homogenously [3]. Compared to hollow-particles, in the matrix-type, the polymers that were distributed inside the core remain post-dissolution, but instead of arranging in a shell formation, they remain in the particle’s lumen [3]. This lumen occupation becomes specifically important regarding the release of cargo from such capsules. Hollow-type capsules can resemble cells, as they provide a large, bordered inner space with superior mechanical and thermal stability that is separated from the outside environment ^[22][70]. Recently, a plethora of polymer-combinations was used to form matrix-type particles, that were then immobilized on to a glass surface [3]. The matrix-type particles were either poly-L-lysine (PLL)-based, or protamine (PR)-based and paired with either of the polyanions: HA, chondroitin sulphate (CS), dextran sulphate (DS) or heparin sulphate (HS), Once the vaterite templates were dissolved via EDTA, the polymer matrices rearranged, reducing the size of the formed particles by up to factors of ~8 in diameter. The shrinkage of the particles was solely dependent on the polymer-pairs utilized. Synoptically, the particles based on PR displayed a higher degree of shrinkage compared to PLL. This was most likely due to the fact that highly charged PLL had a vast number of contact sites with the polyanions, resulting in slower chain dynamics during the dissolution of the core. There was an increasing shrinkage trend displayed for the equivalent polyanions in the PLL- or PR-based particles, following the series: DS ≈ HS < CS < HA. The purpose of referring to the degree of shrinkage in this scenario is because it directly correlates to the immobilization of these particles on the glass surface. In addition to this, the PR-based particles may have a large number of positive charges on their outer surface that are not compensated. This results in the strong electrostatic attraction between the PR-based particles and the glass surface, which is rich in negative silanol groups. On the other hand, even though PLL is polycationic, it has multiple contact points with the polyanions used, resulting in fewer available charged amino groups. This impedes the immobilization of PLL-based particles as the electrostatic attraction between them and the surface is weaker. Another contrast between the two types of particles was that the type of polyanion used impacted the immobilization of PLL-based particles, whereas it did not affect the PR-based particles. In detail, polyanions with more charged groups resulted in particles with lower immobilization percentages, because there were fewer available amino groups in the PLL. This signifies the importance of the polymer charge density when immobilizing particles. The immobilization of the particles may have also been aided via hydrogen bonds, formed between the glass silanol groups and the amino groups present on the polycations. In view of the successful adhesion on the glass surface, the same particles were immobilized on polystyrene and ibidi- hydrophobic/hydrophilic coated wells, displaying that the immobilization of these particles was not affected by altering the surface coating. Additionally, it further solidifies that the driving force behind the particle immobilization on to the glass substrate is their hydrophobic nature. With decreasing shrinkage coefficients, and hence decreasing water content with increased polyanion charge density, PLL-based particles displayed lower percentages of immobilization. On the contrary, PR-based particles demonstrated immobilization percentages close to 100% in all scenarios, and hence, did not display a relationship between immobilization and the degree of shrinkage and water content. The authors suggested this can most likely be attributed to PR’s globular structure, of which rearranges upon interaction with polyanions and immobilization upon the surface. Overall, these fully biopolymer-based PEM particles may be used for fully biocompatible coatings for future bioapplications (i.e., implant coatings). Moreover, 2D polymer multilayers themselves may be used as hosts for PEM-coated or PEM particles ^[23][71]. Previously, such 2D PEM coatings have been used to host particles such as micelles ^[24][72], liposomes ^[25][26][73,74] and nanoparticles ^[27][28][32,75], for applications such as tissue engineering and biosensing ^[11][29][11,76]. PEMs formed of PLL and poly-L-glutamic acid (PLG) have been used to host both (PLL/PLG)₅-coated CaCO₃ particles as well as capsules for drug delivery applications. Here, both PLG- and PLL-loaded CaCO₃ particles were used as components of the layer-by-layer assembly process, followed by exposure to EDTA to remove CaCO₃, forming capsules. Following preparation, a range of deliverables including bovine serum albumin (BSA), silver nanoparticles, histone and rhBMP-2 were successfully encapsulated and released over a period of two weeks or longer ^[30][77]. Similar studies have been reported for the targeted delivery of interkeukin 12p70 and bone morphogenetic protein 2 ^[31][78]. Such coatings are possible to form due to the electrostatic interaction between the polymers within the PEM film as well as the final layer of the PEM particles, demonstrating the simple potential immobilisation of PEM particles within an array of polyelectrolyte films for various bio-applications, including antimicrobial or cell adhesive films, for instance. Another type of biopolymer-based particle is encountered in mucoadhesion; a type of wet immobilization between a coated particle and a mucous tissue or membrane. These coated particles are ideal for mucosal drug delivery pathways as they: (1) protect the potential drug from digestive enzymes found in the gastrointestinal tract ^[32][33][79,80], (2) ensure its stability due to fluctuating pH levels and (3) are capable of mucopenetration in order to reach the epithelium. The exterior of the mucus is negatively charged due to the presence of sialic acid inside mucins ^[34][81]. Furthermore, negatively charged glycoproteins found inside mucin allow for the formation of hydrogen bonds, due to strong proton donor/acceptor capabilities ^[34][81]. Therefore, to establish mucoadhesion, the outer layer of the particles must be able to bind electrostatically and/or via hydrogen bonds. An example of this is chitosan (CS) and sodium tripolyphosphate (TPP) nanoparticles that were tailored to immobilize electrostatically on the mucus surface, in order to deliver daptomycin for ocular treatment of bacterial endophthalmitis ^[35][82]. In addition to this, the surface chemistry of the particles could be modified further to allow adhesion to the mucus surface via disulfide bonding and van der Waals forces ^[36][83]. In the case of disulfide bridges, polymers can be thiolated so that they bond to the cysteine groups of mucin ^[37][84]. This was displayed in nanoparticles composed of maleimide-chitosan-catechol-alginate (Mal-CS-Cat-Alg) immobilized via the cysteine groups and bonded covalently with amines and thiols on the bladder mucus ^[38][85].

3. Surface Modification via Sol-Gels

Using the theory introduced thus far, a more complex immobilization pathway, involving patterned SiO₂ particles on to an aluminium surface will now be discussed. Their adhesion to the solid surface is in the form of a film; a hybrid epoxy functionalised ZrO₂/SiO₂ and sol-gel. To begin with, polyethylene imine (PEI) and PSS layers are deposited onto the SiO₂ particle, via LbL assembly. The surface ζ-potential of SiO₂ is negative (−29 mV) ^[39][86]. Hence, the first deposited layer is that of the polycation PEI (resultant surface ζ-value of +36 mV), followed by the polyanion PSS (ζ −32 mV) ^[40][87]. The third layer added was that of the anticorrosion agent benzotriazole, resulting in a surface ζ-potential of −4 mV. Compared to previous examples of PEM particles, the SiO₂ does not encapsulate its cargo in a hollow-lumen or matrix. Instead, the benzotriazole is entrapped within the PEM during LbL assembly, to prevent it from interacting with the layer matrix. At this stage, it should be noted that compared to linear LbL assemblies, the benzotriazole should not be thought of as a complete outer layer, as it is interwoven into the PSS layer. The inhibitor immobilizes the PSS via electrostatic attraction and hydrogen bonding between the amino and sulfonate groups. Once the particles were prepared, they were embedded into hybrid ZrO₂/SiO₂ sol-gel films utilising zirconia- and organosiloxane-based sols, following the sol-gel procedure ^[39][40][86,87]. These steps resulted in the creation of a film that was deposited on the aluminium alloy via dip-coating. This provided the alloy with a ‘self-healing’ coating. When corrosion commenced, a pH change was triggered, degrading the PSS/benzotriazole complex and hence releasing the anti-corrosion agent. The inhibitor formed a thin layer over the impaired metal surface and hindered further corrosion by replacing the damaged Al₂O₃ layer. It would be expected from the theory introduced previously that the spherical particles would repel each other (Coulombic force) and due to their small surface contact area, this agglomeration would not be favoured. However, the authors mention the dense, homogeneity of the PEM particles within the sol-gel film. This was due to crosslinking agents that were added whilst the particles were still in solution to ensure that when the method got to the drying stage, the crosslinking agents chemically bonded with the particles to form crack-free coating ^[41][89]. Overall, in this scenario, the majority of the forces discussed in this review would still influence particle immobilization, but the effect would not be as dominant. Coulombic forces between spherical particles would still try and prevent clustering, whilst the capillary force would influence the mobility of the particles before gelation, but the inclusion of cross-linkers solidifies the efficiency of this method. Moreover, in a later study, particle PEMs were decorated with silver nanoparticles, allowing them to become both pH and laser-stimulated corrosion inhibitors ^[42][26]. This demonstrates the potential for such PEM particles as anti-corrosive materials in various coatings, including sol-gels and paints, for instance.

4. Polymer Brushes and Scaffolds with Incorporated Particles

Another and very different strategy of surface modification is based on grafting polymers on a substrate with subsequent polymerization. Functionalizing the surfaces with polymeric brushes is advantageous since it allows to modify and switch the surfaces with a very thin layer at the interface ^[43][90]. Responsiveness of polymeric brushes to different stimuli is another essential feature of such coatings; in this regard, incorporation of nanoparticles into polymeric brushes revealed a possibility of altering hydrophobicity/hydrophilicity, switchable mass transport, motions, reversible assembly disassembly of nanoparticles, etc. ^[44][91]. Immobilization of gold nanoparticles onto poly-(2-vinylpyridine) brushes has been shown to lead enhance plasmonic effect ^[45][92]. Recently, non-fouling poly(di(ethylene glycol)methyl ether methacrylate) brushes were functionalized with calcium carbonate particles ^[46][93], which can be used to control cell adhesion to such surfaces. In fact, similar conclusions were drawn from studying cell adhesion on cell non-friendly hydrogel coatings, where surface-incorporated calcium carbonate particles were shown to serve as adhesion centers ^[47][94]. In addition, particles have been functionalized with brushes. For example, silica particles were incorporated and extensively characterized ^[48][95]. Subsequently, silica particles functionalized with brushes were used for drug delivery ^[49][96]. Additionally, quantum dots were shown to be functionalized with brushes leading to vesicle formation ^[50][97]. Further, other matrices such as polycaprolactone scaffolds were functionalized with calcium carbonate particles, which were shown to stimulate vascularization in vivo ^[51][98].

5. Surface Patterning and Microcapsule Arrays

There are several patterning techniques that allow the positioning nano- and microparticles onto solid surfaces. In general, surface patterning is represented by top-down micro- and nanolithographic techniques and bottom-up chemical methods, including self-assembly of micro- and nano-structure ^[52][53][99,100]. While lithography allows the fabrication of patterned thin films with desired geometries and dimensions, bottom-up approaches enable the positioning of nano- and microparticles (pre-fabricated or self-assembled directly on the surface) onto solid surfaces. Depending on the scale, surface patterning techniques belong either to the methods of microfabrication, if the processes can reliably produce features of microscopic size such as ten micrometres or less; or nanofabrication, if the processes can produce nanoscale features, such as less than 100 nanometres. Microlithography is classically used in the semiconductor industry and also for the manufacture of microelectromechanical systems. In recent decades, it also found its niche in Biomaterials, for their manufacture at small scales ^[53][100]. Nowadays, neither self-assembly nor top-down lithographic approaches can adequately fulfil all the requirements for surface patterning. Lithographic technologies represent a powerful apparatus for surface modification, providing high uniformity and consistency in terms of controlling specific size, shape, and functionality of the patterns. This is in contrast to the self-assembled patterns, which often exhibit dynamic instability and therefore present challenges in manipulation of exact size, shape, and encapsulated drug doses ^[53][100]. The assembly of defect-free arrays with true long-range order remains challenging. From the other side, nanolithography has not yet demonstrated success in constructing less then tens of nanometer patterns. Although significant progress has been made in terms of nanolithography even at smaller length-scales ^[54][55][101,102], such advances are accompanied by the technological barriers and the use of unique state-of-the-art instrumentation ^[52][55][99,102]. Besides, the lithographic approach typically yields primitive planar geometries and lacks architectural diversity, while self-assembly allows the formation of morphologically and compositionally sophisticated structures. The latter is especially important for biomedical applications. Recent reports also more and more often relied on a hybrid strategy that utilizes lithographically defined masks to direct the self-assembly ^[52][99]. There are a few studies where lithographic techniques met LbL technology. Thus, microporous polymer nanofilms were grown in a sequential LbL manner and patterned by photo lithography during their growth using the photomask ^[56][103]. In ^[57][104], polymer capsules were deposited on a solid support and the surface was then patterned by electron beam lithography. Electron beam irradiation resulted in the increase of the capsule adhesion to the surface, while the capsules in the non-irradiated areas were washed out. Most often, when the PEMs are assembled with the assistance of lithographic techniques, this is about soft lithography ^[58][59][105,106]. In most of the studies, PEMs are self-assembled on the surfaces without any pattern, forming homogeneous thin coatings ^[60][107]. Actually, it is of note that the widely accepted assertion about the smoothness of the LbL films is not always true: the LbL coatings could also be rough and even spontaneously form patterns during film build-up under certain conditions ^[61][62][108,109]. Using the micro-structured silicon rubber mould ^[58][105] and microfluidics-assisted growth ^[63][110] of the LbL films, controlled fabrication of micro-patterned PEMs can be achieved. For instance, HA/PLL films were grown in 3D with the pattern D providing advanced support for selective cell growth ^[63][110]. However, patterning at smaller length-scales using these approaches remains impossible. This challenge may be avoided if using the immobilization of prefabricated nano- and micro-particles made of PEMs instead of PEM fabrication on the surface. Another advantage of immobilization is that it provides an intriguing opportunity to mix the particles of various compositions (e.g., microcapsules hosting various functional payloads), providing extremely high diversity in tailoring surface properties. In this respect, patterned immobilization of nano- and micro-particles made of PEMs might become a powerful surface modification, if control over the immobilization is achieved. For instance, semi-controlled immobilization of the microparticles on a LbL-coated silicon wafer is illustrated and discussed in the corresponding section. An example of the patterning of PEM microcapsules with electron-beam irradiation has also been mentioned above. More commonly, electrostatic coupling of the microcapsules to the surface is employed. Thus, glutaraldehyde-cross-linked PSS/PAH microcapsules templated on 4–6 µm vaterite crystals were supported onto the surface pre-treated with PAH ^[64][111]. The microcapsules reacted with the amino groups on the surface via free aldehyde groups on their outer layer (Schiff base –CH=N– linkages). In ^[65][112], biotinylated PSS/PAH microcapsules templated on ca. 15 µm vaterite crystals were immobilised onto avidin patterns on a poly(ethylene terephthalate) (PET) film. The immobilisation was driven by biotin-avidin affinity. The particles described above were fabricated using the LbL assembly of polymers onto solid substrates with or without the removal of the substrate. Structures that can be assembled using the LbL polymer coating may also include free-standing multicomponent or one-component structures made the same way using decomposable cores such as protein aggregates ^[66][67][113,114] or vaterite crystals ^[68][115]. Plenty of other structures can be obtained such as pure protein particles using the LbL assembly and hard templating on vaterite crystals ^[69][116]. This brings almost no limit to a variety of structures for immobilization onto solid surfaces and huge opportunities for further research and applications (also biologically related) in this filed.