## 3 Absorbable Membranes
### Absorbable Membranes Made of Natural Polymers  
The most commonly used natural polymers for barrier membranes are type I collagen and type III collagen. BioMend Membrane (Zimmer Biomet Dental, Warsaw, IN, USA) was the first collagen-based membrane produced for guided tissue and bone regeneration application. Natural collagen is obtained by a decellularization process followed by steps of removal of any antigenic components [55]. Sources for collagen include human skin (Alloderm(r), LifeCell, Branchburg, NJ, USA), porcine skin (Bio-Gide(r), Geistlich, Shirley, NY, USA), porcine peritoneum (MemGuide(r), Ace, Boston, MA, USA) or Achilles' tendon (Cytoplast(r) RTM Collagen, Franklin Lakes, NJ, USA) [14,56]. After harvesting from the primary source, the product is subjected to decellularization, crosslinking and sterilization processes. The crosslinking treatment aims to increase the resistance of the membrane and can be achieved by different methods: UV irradiation or chemical processing. Glutaraldehyde, carbodiimide, hexamethylene diisocyanate, diphenyl-phosphoryl azide, formaldehyde or genipine can be used in chemical crosslinking [12,57]; other studies have also shown that crosslinking promoted prolonged biodegradation, reduced epithelial migration, decreased tissue integration and decreased vascularization [14,58]. The main concern related to the use of chemical agents is given by the potential toxic and inflammatory reactions produced in the recipient organism [59]. To overcome these disadvantages, other cross-linking agents with low cytotoxicity have been developed, such as (a) diphenylphosphorylorylazide, with biocompatibility and good handling characteristics, without cytotoxic effects [60]; (b) 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, with high resistance and low cytotoxicity [61] and (c) epigallocatechin-3-gallate, with better mechanical properties and anti-inflammatory effects [62]. Moreover, although crosslinked collagen membranes have prolonged degradation times, they also have significantly higher membrane exposure rates of up to 70.5% [63; 64]. As already mentioned, premature exposure of membranes is often associated with bacterial invasion, as demonstrated by [PERSON] [65]. Collagen membranes can perform hemostatic functions, allowing and stimulating the attachment and proliferation of fibroblasts and osteoblasts due to the presence of arginine-glycine-aspartic acid (Arg-Gly-Asp) and GFOGER sequences (glycine-phenylalanine-hydroxyproline-glycine-glycine) integrin-specific [66]; data suggest that cellular activity begins 3-5 days postoperatively [67]. At the same time, the membrane allows the signals from the cells associated with the membrane to be transferred to the cells in the bone defect, thus creating a favorable environment for bone neo-formation [68]. They can also be easily integrated with soft tissues and are permeable, so nutrient diffusion can be conducted easily [69; 70]. There are data on the performance of non-crosslinked type I collagen membranes with non-absorbable PTFE membrane that claim that non-crosslinked collagen membranes promote a high rate of vascularization at two months postoperatively [71]. Other studies have compared the rate of bone formation of collagen membranes with e-PTFE membrane, both of which show similar results [72; 73; 74; 26]; stromal cells attached to the collagen membrane promoted the production of fibroblast growth factor (FGF-2) and bone morphogenetic protein (BMP-2), stimulating bone regeneration. The influence of the use of collagen membrane in the regeneration of calvarial defects on Wistar rats was investigated; defects that were covered with collagen membranes demonstrated better bone regeneration than defects without membrane [75]. The resorption time differs for each membrane, from 8 to 38 weeks [76; 77]. Accelerated resorption of the barrier membrane can compromise the success of surgery.