Biomedical Engineering Reference
local ingress of water. With time, local domains of the methacrylate network may
become sufficiently degraded and/or hydrophilic to permit access by esterases,
which greatly accelerate ester bond hydrolysis. The esterase-catalyzed degradation
of monomethacrylates, dimethacrylates, and commercial dental resins has been
documented in solution [ 107 - 110 ], in saliva [ 109 , 111 , 112 ], and in vivo [ 101 ].
The breaking of covalent bonds by addition of water to ester bonds is considered to
be one of the main reasons for resin degradation within the hybrid layer [ 90 , 91 ].
Degradation of methacrylate ester groups produces carboxylic acids—the same
functional group that is the culprit in lactic acid-induced decay.
7.6 Water-Compatible Esterase-Resistant Adhesives
Water is ubiquitous in the mouths of healthy patients and thus it is imperative that
we develop restorative materials that can function adequately in the presence of
water. Forty years ago, researchers were discussing the detrimental effect of water
on bonding dental materials to the tooth; to date, this problem has not been resolved
[ 113 ]. One approach to the problem of bonding to wet dentin has been to increase
the relative hydrophilicity of dentin adhesives with a goal of promoting increased
wetting of the collagen. However, hydrophilic polymers absorb more water than
more hydrophobic resins [ 91 ]; the consequence of this increased water sorption is
lowered mechanical properties [ 105 ] and increased degradation under wet
conditions [ 98 , 101 , 114 ].
There are several strategies for reducing hydrolytic degradation of methacrylate
adhesives. One strategy involves selectively modifying methacrylate side chains so
that they are both water-compatible and esterase-resistant [ 115 - 117 ]. This can be
accomplished by the use of bulky and/or branched functional groups that are poor
esterase substrates but are sufficiently hydrophilic to be water-compatible (e.g., by
incorporating polar functional groups such as -OH). Published reports on the
reduced esterase susceptibility of urethane-modified BisGMA [ 108 ] and of
acrylates with branched or aromatic side chains [ 110 ] support this approach.
Another strategy involves increasing matrix hydrophobicity following initial mono-
mer penetration into the dentin layer. Secondary cross-linking of polar functional
groups on methacrylate side chains could be employed to achieve this goal.
Increasing the extent of conversion of methacrylate resins will reduce susceptibility
to esterase hydrolysis by reducing the number of unreacted pendant groups
[ 81 , 118 ].
Adhesive phase separation causes incomplete and differential infiltration of the
demineralized dentin matrix [ 71 , 119 ]. The collagen fibrils are not completely
protected by the hydrophobic resin polymers and they will be susceptible to
degradation [ 25 , 44 , 49 , 120 , 121 ]. Water-compatible components in adhesive
formulations have to be considered, especially the partition of these components
in the aqueous environment when phase separation occurs. In our laboratory,
several approaches, such as new monomers with branch structure [ 115 , 117 , 122 ],