Applications of Dendritic Polymers in Dentistry

Applications of Dendritic Polymers in Dentistry: History

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Subjects: Nanoscience & Nanotechnology

Contributor:

Michael Arkas

Michail Vardavoulias

Georgia Kythreoti

Dimitrios A. Giannakoudakis

Dendritic polymers represent the well-established 4th class of polymers, next to their conventional linear, branched, and cross-linked counterparts [5,6,7]. The main reason behind the increased scientific interest in this field is the multitude of applications [33]. Among the most recognized are water purification [43,44,45,46], separation systems [54,55] for instance chromatography [56,57], light harvesting [47,48] solvent extraction [62,63], catalysis [34,35,36,37,38], liquid crystals [39,40,41,42], textiles [49,50,51], dye nanocarriers [64,65], coatings [52,53], membranes [58,59], and gels [60,61], Perhaps the most important implementations derive from a similarity to biological molecules. These include biomimetic synthesis [68,69,70,71] and biomedical formulations based on the macromolecules themselves [72,73,74,75,76,77,78] and their composites [79]. Some possess intrinsic antimicrobial [80,81] and antiviral [82,83] properties. Functionalization [84,85] or inclusion of metal nanoparticles [86] into their cavities increases their potential. They are also capable of controlled [87], targeted [88], and stimuli-responsive [89] drug release [90,91,92]. Other nanomedicine uses [93,94] include theranostics [106,107] for example biosensors [102,103], cancer diagnosis [95,96] and therapy [97,98], magnetic resonance and bioimaging [104,105], therapy for inflammatory diseases, such as rheumatoid arthritis [110,111], gene transfection [99,100,101], treatment of central nervous system conditions [108,109], antimicrobial coatings for orthopedical implants [112,113], and antigen mimicry for vaccinations [114]. The properties of bio-dendritic polymers are ideally suited for tissue engineering [115,116,117,118] including the field of regeneretive dentistry.

dendrimer
biomaterial
cell scaffold
adhesion
differentiation
osseointegration

1. Interactions with Odontoblasts and Dental Pulp Cells

Teeth are the second most abundant hard tissue of the human body next to bones Hydroxyapatite is the most abundant component of both dentine and enamel [205] that are the predominant substances. , Research in the field of teeth reconstitution focuses on materials that interact with cells and inorganic component regeneration. A possible explanation is that, unlike the bone, for the formation of enamel, the tooth does not have the cell equivalent to the osteoblast that forms the organic matrix and mineralizes it. In the case of teeth, the ameloblast dies with the eruption of the tooth.

In this approach, G5 PAMAM was functionalized by peptides incorporating the typical RGD cell adhesion sequence (Arg-Gly-Asp). The α_νβ₃ integrin binding potential was expressed for human dermal microvessel endothelial cells (HDMEC), human vascular endothelial cells (HUVEC), and odontoblast-like MDPC-23 cells. However, the most important aspect of this work was the targeting capability of these compounds to the RGD receptors of the predentin of human tooth cultures, rendering them appealing carriers for tissue-specific cell delivery [206]. Similar G5 PAMAM RGD peptide composites synthesized by the intervention of fluorescein isothiocyanate also demonstrated a selective binding capacity to dental pulp cells and mouse odontoblast-like cells. On top of that, they modulated their differentiation toward the improvement of their odontogenic properties [207]. PAMAM derivatives phosphorylated via the Mannich-type reaction may also mediate odontogenic differentiation of the dental pulp stem cells and assist their proliferation [208].

2. Dentin Reconstitution

Established agents, such as the carboxy-terminated PAMAMs, are employed in a biomimetic attempt to imitate the role of non-collagenous proteins in the hierarchical intrafibrillar mineralization of dentine. Both in vitro and in vivo experiments in mice highlighted the effectiveness of G4 PAMAM-COOH in the sequestration of calcium and phosphate ions and the intrafibrillar templating of hydroxyapatite from amorphous calcium phosphate (ACP) [210]. The results were confirmed in vivo for G3.5 PAMAM-COOH and incubation in mice saliva after incorporation into the rat’s cheeks. On the side, the produced hydroxyapatite exhibited increased microhardness [211].

There are many variants of the above standard pattern, such as pretreatment of dentin samples with Ca(OH)₂ solution [221], or using amorphous Ca₃(PO₄)₂ nanoparticles [222]. The latter, in combination with G3 PAMAM-NH₂ [223], or G3 PAMAM-COOH [224] in an artificial saliva-lactic acid solution, release Ca²⁺ and PO₄³⁻ ions that strengthen the hardness of the restored dentin to the level of the healthy tissue. An extra beneficial effect of Ca₃(PO₄)₂ nanoparticles is that they may effectively neutralize an acidic environment (pH 4). Incorporated in a dental adhesive comprising pyromellitic glycerol dimethacrylate, 2-hydroxyethyl methacrylate, and ethoxylated bisphenol A dimethacrylate, they may be employed to cure conditions that involve an acidic oral environment, for instance, dry mouth [225,226]. The composition of the adhesive may be upgraded by inorganic fillers such as barium-boro aluminosilicate glass particles to yield templates with recharging capability and superb nucleation properties [227,228,229]. Other formulations for Ca₃(PO₄)₂ Nps have also been developed, such as a protein-repellent “bioactive multifunctional composite” comprising 2-methacryloyloxyethyl phosphorylcholine, dimethylamino hexadecyl methacrylate and silver nanoparticles for antimicrobial protection [230].

In a slightly different strategy, the phosphate functionalities were directly bound to the dendritic polymer in an attempt to imitate the role of phosphophoryn. This protein and dentin sialoprotein represent the two most abundant non-collagenous proteins of the dentin matrix [231]. G3 or G4 PAMAM-PO₃H₂ were initially chosen because they present similar topological architecture and size. Remineralization of intrafibrillar and interfibrillar reconstituted type I collagen [232] and demineralized human dentin was established in artificial saliva or amorphous calcium phosphate stabilizing, polyacrylic acid (PAA) solution [233] and in vivo in the oral cavity of rats [234] To more effectively simulate the operation of non-collagenous dentine matrix protein, phosphorylated G4 PAMAM was combined with the respective carboxylated counterpart. The mechanical properties of hydroxyapatite produced by this blend during the mineralization process of collagen fibrils of natural dentin [235] and recombinant type I collagen fibrils [236] were further improved, resembling at the nanoscale level those of the natural tissues.

Regardless of the remineralization mechanism, a common practice is the inclusion of drugs or other active ingredients into the cavities of the dendritic polymers. Loading of G4 PAMAM-COOH with triclosan antibiotic was proposed for synchronous and protracted antimicrobial protection of the damaged dentin substrate [239]. Phosphorylated G3 and G4 PAMAM proved able to solubilize hydrophobic antibacterial apigenin in its cavities to address dental caries. This was attained by preventing the erosion of dentine caused by Streptococcus mutans and simultaneous reparation [240]. In another example, chlorhexidine was combined with G4-PAMAM-COOH to lessen its cytotoxicity. The mixture was applied as a first coating. The dental adhesive formulation of the second layer was amorphous calcium phosphate nanoparticle adhesive fillers stabilized poly aspartic acid [241]. It was additionally revealed that chlorhexidine reversed the undesired excitatory effect of G4-PAMAM-COOH on matrix metalloproteinase and inhibited its activity [242]. Besides drugs, G3 PAMAM was applied to mechanically exposed pulp teeth to act as a host substrate for pulpine. Clinical trials in 12 patients lasting one and a half years revealed that this gradual layering procedure, apart from more effective dentin mineralization, also enhanced the restoration of the injured pulp tissue [243].

3. Enamel

Enamel, the other major hard tissue of the tooth, was submitted to treatment approaches similar to dentin with analogous successful results [244]. An investigation for both components was conducted synchronously and side by side, beginning with anionic carboxylated PAMAM. The third [245] and the fourth generation [246] counterparts induced the crystallization of rod-like hydroxyapatite crystals on the etched enamel surface in the same orientation as the long axis of enamel crystals. In contrast to dentin, the different PAMAM terminal groups produced a differentiation in the enamel lesion remineralization percentage: (PAMAM-NH₂ 76.42 ± 3.32%), (PAMAM-COOH 60.07 ± 5.92%), and (PAMAM-OH 54.52 ± 7.81%) was quantified in bovine enamel specimens in a simulated oral environment [247]. The enamel surfaces, remineralized by the fifth generation of the two best-performing organic templates (PAMAM-NH₂, PAMAM-COOH), were subsequently tested against adhesion and biofilm formation from Streptococcus mutans. Both dendrimers resisted bacterial attacks, highlighting their potential to prevent secondary caries [248].

The phosphorylated PAMAM option was also implemented on human tooth enamel. The G4 counterpart possesses the same peripheral phosphate groups and size as those of amelogenin, the most important protein in the natural process of enamel. For this reason, G4 PAMAM-PO₃H₂ is tightly adsorbed on the enamel substrate and the bioinspired remineralization process in artificial saliva or the oral cavity of rats proceeds more efficiently in comparison to PAMAM-COOH, with a crystalline hydroxyapatite layer thickness of about 11.23 μm instead of 6.02 μm [249]. Another amphiphilic PAMAM dendron bonded with stearic acid at the focal point and coupled peripherally with aspartic acid moieties forms spherical organizations in solution. These undergo further aggregation as a function of concentration to linear chains such as amelogenin. In this way, HAP nucleation may follow desired orientations similar to those encountered in enamel [250]. Even a simple carboxy G4.0 PAMAM-COOH derivative may present a microribbon hierarchical organization similar to the amelogenin prototype, with a suitable ion chelating cation, such as in an aqueous ferric chloride solution [251]. A similar decoration of G3.5 PAMAM-COOH with alendronate groups bearing two phosphate functionalities was made to enhance the adsorption of the dendrimer to the enamel layer [252].

In a contiguous attempt to imitate the role of another protein (salivary statherin), G4 PAMAM was modified by the N-15 peptide. Then, it was incorporated into formulations containing calcium phosphate nanoparticles and the same adhesive resin used as described above [225] for the remineralization of dentin, with equally successful performance [253]. An enamel-specific water-insoluble antibacterial and antibiofilm agent, honokiol, may also be included in PAMAM-COOH and then released in a controlled profile. Anticaries’ activity was established through planktonic growth assays and in vivo in male Sprague Dawley rats (Figure 26b) [254]. The research on the potential of dendritic polymers for use in dentistry is summarized in Table 4.

Table 4. Dendritic polymers in Dentistry.

Dendritic Polymer	Modification	Substrate	Function	Ref.
G5 PAMAM	Arg-Gly Asp	-	Dermal microvessel endothelial, human vascular endothelial, odontoblast-like MDPC-23 cells	[206]
G5 PAMAM	Fluorescein isothiocyanate Arg-Gly Asp	-	Dental pulp cells differentiation mouse odontoblast-like MDPC-23 cells	[207]
G3, G4 PAMAM-COOH	Carboxylic acid functionalization	Collagen fibrils	Dentin remineralization	[210]
G3.5 PAMAM	Alendronate	Demineralized enamel	Enamel remineralization	[252]
G4 PAMAM-COOH	Butanedioic anhydride	Ferric chloride solution	Aggregates with a microribbon structure	[251]
PAMAM G3	Carboxylic acid functionalization	Demineralized enamel	Enamel remineralization	[245]
G1 PAMAM Dendron	Aspartic acid, hexadecyl chain at the focal point	Biomimetic amelogenin nanospheres	Hydroxyapatite orientation Enamel	[250]
G4 PAMAM	Dimethyl phosphate	Demineralized enamel	Enamel remineralization	[249]
G3 PAMAM-NH2		Demineralized dentin	Dentin remineralization	[217]
G4 PAMAM-COOH	Triclosan	Demineralized dentin	Dentin remineralization, anti-bacterial	[239]
G4 PAMAM		Demineralized dentinal tubules	Dentin remineralization	[215]
G3 PAMAM-NH2		Type-I collagen fibrils	Mineralization of collagen fibrils	[219]
G2 G4-PAMAM-OH		Demineralized dentin	Dentin remineralization	[212]
PAMAM-COOH	Ca(OH)₂	Demineralized dentin	Dentin remineralization	[221]
G4.5 PAMAM-COOH		Demineralized enamel	Enamel remineralization	[246]
G2 PAMAM	Glutaraldehyde	Demineralized dentin	Dentin remineralization	[213]
G3 PAMAM	Phosphorylation polyacrylic acid	Demineralized dentin	Dentin remineralization	[233]
G4 PAMAM	Phosphorylation	Demineralized dentin	Dentin remineralization	[234]
G3 PAMAM	Amorphous calcium phosphate	Demineralized dentin	Dentin remineralization	[223]
G3.5 PAMAM		Demineralized dentin	In vivo Dentin remineralization	[211]
PAMAM-COOH	Nano-hydroxyapatite	Demineralized dentin	Dentin remineralization	[237]
PAMAM-OH, PAMAM-COOH, PAMAM-NH2		Demineralized dentin	Dentin remineralization	[218]
G3 PAMAM	Nanoparticles of amorphous calcium phosphate (NACP)	Demineralized dentin	Dentin remineralization	[225]
G3-PAMAM-NH2	Nanoparticles of amorphous calcium phosphate (NACP)	Demineralized dentin	Dentin remineralization in an acidic solution	[226]
PAMAM 3.5	NACP resin	Demineralized dentin	Dentin remineralization	[227]
PAMAM		Demineralized dentin	Dentin remineralization	[216]
G3 PAMAM	bioactive multifunctional composite	Demineralized dentin	Dentin remineralization	[230]
G3 PAMAM	Glutaraldehyde	Demineralized dentin	Dentin remineralization	[214]
G3 PAMAM-NH2	Amorphous calcium phosphate nanoparticles	Ethoxylated bisphenol A, dimethacrylate pyromellitic glycerol dimethacrylate resin	Dentin remineralization	[228]
Phosphorylated G3, G4 PAMAM	Apigenin	Demineralized dentin	Dentin remineralization, anti-bacterial	[240]
G4 PAMAM		Mesoporous bioactive glass nanoparticles	Dentin remineralization	[238]
G4 PAMAM	SN15 salivary statherin protein-inspired peptide NACP resin	Demineralized dentin	Enamel remineralization	[253]
PAMAM	Nanoparticles of amorphous calcium phosphate	Demineralized dentin	Dentin remineralization	[222]
G3 PAMAM-COOH	Nanoparticles of amorphous calcium phosphate	Demineralized dentin	Dentin remineralization	[224]
PAMAM-NH2, PAMAM-COOH, PAMAM-OH		Enamel	Enamel remineralization	[247]
G5 PAMAM-NH2, PAMAM-COOH		Demineralized enamel	Enamel remineralization	[248]
PAMAM-PO₃H₂		Demineralized dentin and type I collagen matrix	Hydroxyapatite, Dentin remineralization	[232]
G3 PAMAM	Pulpine	Demineralized dentin	Dentin remineralization, pulp tissue repair	[243]
PAMAM	Honokiol-loaded	Demineralized dentin	Enamel remineralization, anti-bacterial	[254]
G4 PAMAM-COOH	Chlorhexidine	Demineralized dentin	Dentin remineralization	[242]
G3 PAMAM-NH2	Nanoparticles of amorphous calcium phosphate, pyromellitic glycerol dimethacrylate, ethoxylated bisphenol A dimethacrylate resin	Demineralized dentin	Dentin remineralization in an acidic solution after severe fluid challenges	[229]
G4 PAMAM- PO₃H₂ G4 PAMAM-COOH		Demineralized dentin	Dentin biomimetic remineralization and dentinal tubule occlusion	[235]
G4 PAMAM-COOH	Amorphous calcium phosphate nanofillers, chlorhexidine	Demineralized dentin	Dentin remineralization, anti-bacterial	[241]
PAMAM- PO₃H₂			Dental pulp stem cells	[208]
PAMAM- PO₃H₂ PAMAM-COOH		Collagen fibrils	Mineralization of type I collagen fibrils	[236]
PAMAM–COOH	Nano-sized amorphous calcium phosphate	Collagen fibrils	Mineralization of dentin type I collagen fibrils	[220]

This entry is adapted from the peer-reviewed paper 10.3390/pharmaceutics15020524

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