In Vitro Study of Polypeptide Catalyzed Biosilicification of Dentin Surfaces

Introduction:  Resin bonding agents may deteriorate with time and become ineffective in sealing restorative-tooth interfaces and in preventing dentin hypersensitivity.  In situ formation of mineral particles to occlude dentin tubules might reduce such clinical failures.  As a model system we investigated formation of silica on dentin from tetramethylorthosilicate (TMOS) by the amine-containing macromolecular catalysts polylysine (PLL), an analog of the protein catalyst responsible for silica formation in primitive marine species.

Objectives:  To evaluate if biocatalyzed silicification could be conducted on a dentin surface and occlude dentine tubules.

Methods:  Dentin disks (n=6) nominally 500µm thick were prepared from defect/caries-free human molars.  After removal of smear layer 1mM 30-70kDa PLL(Aldrich) was applied layer-by-layer with HCl pre-hydrolyzed 100mM TMOS.  Smear-free disks(n=4) served as controls.  Disks were examined with SEM (JSM-6320F,JEOL) and XPS (VG Escalab MkII).  The permeability of four smear-free disks was measured at baseline and after silicification, and results were compared with a matched-pair t-test.

Results:  Smear (Fig-a) and smear-free (Fig-b) morphologies were as expected.  The PLL/TMOS treated disks exhibited a partial coating of 0.2µm particles (Fig-c), whose morphology and XPS atomic percentages (below) were consistent with silica.

	O1s	C1s	N1s	Ca2p	P2p	Na1s	Si2p
Control	38.3	29.4	4.7	14.8	11.1	1.6	0.0
TMOS/PLL	46.5	17.7	2.9	5.9	4.9	0.0	22.4

Mean(sd) permeability decreased from 1.40(2.40) to 0.56(0.97)x10^-3 µL/cm²-min-cmH₂O, but the difference was not statistically significant (p=0.325), possibly due to technique, dentin variation, insufficient mechanical integrity or adhesion of the particles, which were pushed from the tubules (Fig-d).

Conclusions:  The results demonstrate that the polypeptide retained its catalytic activity on dentin and the formed particles bridged the patent tubules.  Further understanding of the reaction parameters for silica or other biocatalyzed minerals could provide control of particle properties and location of deposition for novel sealing or resin reinforcement applications.