A UV-cleavable supramolecular cross-linker was designed to effectively control the mechanical strength of photocurable resin plastics. The resin monomer-soluble polyrotaxane (PRX) cross-linker was synthesized by introducing a hydrophobic n-butyl group and a cross-linkable methacrylate group in α-cyclodextrin threading to a polyethylene glycol containing UV-cleavable end groups. The UVcleavable PRX cross-linker was completely dissolved in 2-hydroxyethytl methacrylate (HEMA) and camphorquinone, representative photocurable resin components. The dumbbelllike stiff resin plastic was prepared by irradiating the mixture with 450 nm blue light. The stiffened resin plastic maintained its ultimate tensile strength (UTS) under visible light irradiation. However, the UTS of the resin plastic was remarkably decreased to 40% of the original value once the plastic was exposed to 254 nm UV light. This indicates that the suggested UVcleavable PRX cross-linker is effective in modulating the mechanical strength of photocurable resin plastics. P hotocurable resins are widely used in industry, commodities, and the biomedical field because they possess good formability, mechanical strength, and cost-effectiveness. 1,2 However, the irreversible stiffening of photocurable resins often causes significant problems in material applications in many engineering fields. For instance, photocurable resin plastic is essential to support prosthetic devices in the field of dentistry. 3 However, the irreversible stiffening of composite resins often causes significant damage to the dental tissue during removal after treatment. 4 Therefore, the development of stimuli-degradable resin plastic is highly desirable in dentistry.The use of photolabile groups, such as o-nitrobenzyl, has many advantages in the design of degradable materials. 5−7 Unlike other degradable chemical groups, photolabile groups can be accurately triggered to cause degradation in both dry and wet conditions without additives. 8,9 The most general designs for photocleavable polymers use some form of photocleavable cross-linker. Both end groups of o-nitrobenzylcontaining chemicals can be modified with (meth)acrylate groups, and the modified chemical can be used as a photocleavable cross-linker to reversibly reinforce the mechanical strength of (meth)acrylic resins. 10 Structural materials requiring temporary mechanical strength, such as hydrogels, scaffolds, and photoetching materials, are generally designed using this concept. 11−16 Although the strategy is useful for the design of functional photocleavable materials, the efficiency of cleavage remains a critical problem. Because each photolabile group is designed to connect two (meth)acrylate groups, equivalent amounts of photolabile groups with cross-linkable sites must be used to prepare the materials. For this reason, several minutes of irradiation with ultraviolet (UV) light is required to induce drastic degradation; the required time becomes much longer when the material is a rigid solid, such as a cross-linked resin. 17 Polyr...
Notwithstanding the progress of dental material adhesion on tooth surfaces in the past several decades, debonding strongly adhered restorative materials from tooth surfaces remains a challenging issue. Herein, we propose the use of photodegradable polyrotaxanes (PRXs), which are supramolecular-interlocked polymers comprising α-cyclodextrin threaded along a poly(ethylene glycol) (PEG) axle, as a component of adhesive resin cements to attenuate debonding strength via light irradiation. We synthesized photodegradable PRXs (iNB-PRXs) using internally o-nitrobenzyl ester-introduced PEG as an axle polymer. Notably, approximately 60% of iNB-PRX degraded into its constituent molecules after 5 min of irradiation with ultraviolet (UV) light. Thereafter, iNB-PRX was combined with the clinically utilized adhesive resin cement, and a poly(methyl methacrylate) (PMMA) block was adhered on the surface of bovine dentin using an adhesive resin cement cross-linked with iNB-PRX. Although the PMMA block was successfully adhered onto the dentin with a clinically acceptable adhesive force, the tensile strength of the PMMA–dentin specimens decreased significantly upon UV irradiation for 2 min owing to the UV-induced degradation of the iNB-PRX cross-linker. According to these results, the adhesive resin cement containing photodegradable iNB-PRX cross-linkers is a promising candidate for facilitating the debonding of dental materials from tooth surfaces via UV light irradiation.
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