*3.3. Improved Physical/Mechanical Properties*

In addition to EGCG, chitosan, PDA, and AMTN, various materials have been employed to improve the physical and mechanical properties of the membrane [12,14,37,39,43,49].

3.3.1. Recombinant Spider Silk Proteins and Pectin Derivatives for Improved Cell Adhesion

Natural silk has been applied for dental fields due to the structure and features that make it biocompatible [105]. Synthetic polymer membranes are inert and biocompatible; however, they are hydrophobic and less prone to cellular adhesive physical properties [6]. Recombinant spider silk protein not only demonstrates great mechanical characteristics

such as strength and elasticity but also great biological characteristics such as biocompatibility, biodegradability, and improved wetting capacity [106,107]. In 2020, Tasiopoulos et al. reported recombinant spider silk protein with a cell-binding motif from a fibronectin (FNsilk)-coated PTFE membrane [45]. In this study, the FN-silk-coated membrane showed higher cell adherence and proliferation properties in both human keratinocytes from soft tissue and human osteosarcoma cells from bone [45].

Pectin is structurally and functionally the most complex polysaccharide present in plant cell walls [108]. Pectin plays important roles in not only mediating plant growth, morphology, and development, but also in gelling and stabilizing the polymers in various foods and medicines [108,109]. Boda et al. reported an oxidized pectin-coated chitosan membrane [44]. The pectin-coated side of the membrane showed a two-fold increase in the mucoadhesive property to the mucosal mimic porcine esophagus than the non-coated side. On the contrary, the non-coated side of the chitosan membrane showed a 3–4 fold stronger adhesion to hard tissue mimicking hydroxyapatite discs than the pectin-coated side [44].

#### 3.3.2. Metal Reinforcement—Titanium and Magnesium

Choy et al. reported a vapor-phase Ti-infiltrated collagen membrane via titanium oxide atomic layer deposition [46]. The Ti-coated collagen membrane led to enhancement in both the tensile strength and Young's modulus compared to the non-coated collagen membrane. Furthermore, the Ti-coated membrane was retained for a longer time than a non-coated collagen membrane that was rapidly degraded by up to 90% within 1 week [46].

Zhang et al. reported a Mg core-reinforced PLA membrane to improve the mechanical– physical properties [47]. The membrane was fabricated by combining two PLA membranes with a fluoride-coated AZ91 (9 wt% Al, 1 wt% Zn) (FAZ91) Mg alloy core by hot pressing. Compared to only the PLA membrane control group, the FAZ91—Mg-reinforced PLA membrane group showed a significantly higher maximum load, stiffness, and faster degradation because FAZ81-Mg promoted the absorption and the degradation of the PLA wrap but was not too delayed [47].
