## Conventional GTR and GBR
### Membrane
#### 2.1.2 Second-Generation Membranes  
In order to eliminate the necessity for secondary surgery, an absorbable membrane was suggested as a barrier membrane for GTR and GBR. The absorbable membranes are divided into two groups based on their origin: natural membranes and synthetic membranes.  
Natural membranes mainly consist of collagen or chitosan from animal sources. Tissue-derived collagen-based membranes are made from human skin (Alloderm(r), LifeCell, Branchburg, NJ, USA), bovine achilles tendon (Cytoplast(r) RTM Collagen, Franklin Lakes, NJ, USA), or porcine skin (Bio-Gide(r), Geistlich, Shirley, NY, USA) [18; 19]. Collagen has many biological activities such as hemostasis, an attraction of periodontal ligament and gingival fibroblast cells, augmentation of the soft tissue, biocompatibility, biodegradability, and cell affinity. Most of the commercial collagen membranes are developed from type I collagen or a combination of type I and type III collagen [14]. Collagen-based membranes showed inferior performance in vivo, as the membrane starts to degrade [20]. Additionally, disease transmission and ethical or religious problems can be possible risks. Biomechanical properties and stability of the collagen matrix can be enhanced by physical and chemical cross-linking, ultraviolet light, hexamethylene diisocyanate (HMDIC), glutaraldehyde(GA), diphenyl-phosphoryl azide (DPPA), formaldehyde(FA) plus irradiation, and genipin (Gp) [21; 22]. The addition of a natural cross-linking agent, Gp, into the AlloDerm(r) rehydration protocol affects the mechanical properties and stability of the collagen matrix [21]. A significant enhancement in tensile strength compared to control was observed when Gp exposure time was increased from 30 min to 6 h. Also, other studies showed that cross-linking is associated with prolonged biodegradation, reduced epithelial migration, decreased tissue integration, and decreased vascularization [23]. Also, the membrane made from silk fibroin showed a biocompatible reaction with osteoblastic-like MG63 cells and it could be an alternative barrier membrane for GBR [24].  
Polymeric membranes are either based on polyesters such as polyglycolic acid (PGA), polylactic acid (PLA), poly \(\epsilon\)-caprolactone (PCL), their copolymers, and tissue-derived collagens [5; 25]. These materials are biocompatible, biodegradable, and easier to handle than e-PTFE membranes. In general, the membrane should maintain its function for more than 4-6 weeks for successful periodontal regeneration [26]. However, they are not inert, since some tissue reactions may be expected during degradation. Although the initial tensile strength of membranes was high (12-14 MPa), it completely decreased below 1 MPa after 14 days of exposure [27]. There is also variability and lack of control over the rate of membrane absorption, which is influenced by factors such as the local pH and material composition [27]. The polymeric membrane has other drawbacks, including an inflammatory reaction due to macrophages and leukocytes around the membrane during absorption [28]. These membranes are processed by melting or solvent casting/particulate leaching and phase inversion [27].