## 5 Trends in the Development of Barrier Membranes
### Barrier Membranes from PRF  
Platelet concentrates have been a research topic for more than 20 years; platelet-rich plasma, discovered in the late 1990s, was the result of centrifugation of blood harvested on the spot from the patient, offering good advantages in oral and maxillofacial surgery [161]. This procedure, on the other hand, had disadvantages related to the use of anticoagulants that could interfere with the healing process. Subsequently, a new product without anticoagulant was developed, platelet-rich fibrin (PRF). PRF has been shown to significantly increase the potential for tissue regeneration, favoring the slow and gradual release of growth factors trapped in its fibrin matrix [162]. PRP has been used primarily for soft tissue regeneration rather than osteogenesis and requires more blood than PRF. PRP is centrifuged at a higher rate, causing all heavy white blood cells and stem cells to sink to the base of the tube, not being collected in the sample [163]. Further research has found that a higher platelet concentration with the inclusion of white blood cells and stem cells in the sample would be even more therapeutic. The inclusion of white blood cells helped prevent postoperative infection, and the presence of stem cells generated an obvious capacity for regeneration. PRF is centrifuged at a slower rate, which causes a higher concentration of white blood cells, stem cells and platelets to remain in the middle plasma layer. Thus, PRF has a platelet concentration almost double that of blood. Platelets are involved in the release of growth and clotting factors. If the release of the growth factor occurs rapidly in the case of PRP, it is slow for PRF, with an average duration of 7-10 days postoperatively [164]. [PERSON] et al. [165] investigated the association of PRF with bovine xenograft in filling periodontal infrabony defects in patients with bilateral defects, evaluating treatment with PRF alone or with PRF combined with bovine bone. Results after 6 months were more favorable when PRF was combined with bovine bone; they noticed a decrease in probing depth and gains in periodontal attachment and bone height. [PERSON] et al. concluded that increased efficacy during bone defect regeneration treatment is achieved by the combination of PRF and xenograft, rather than by PRF alone [165]. [PERSON] et al. [166] compared the efficacy of PRF and autologous bone graft to increase attachment and reduce infrabony defects. Subjects showed infrabony defects that were treated with either an access flap associated with PRF or an access flap supplemented with an autologous bone graft; the third control group followed only the access flap procedure. Plaque index, the gingival index, the loss of attachment, the gingival recession and the depth of the periodontal pockets were analyzed on the day of surgery and 9 months later. The use of PRF had a more efficient result than the access flap alone, but the combination of the regenerating material allowed for obtaining an even more efficient treatment [166]. PRF has also proven effective in treating gingival recessions. [PERSON] et al. [167] compared PRF and connective tissue grafting with lateral repositioned flap to cover Miller class II recessions. Recession coverage was complete for 91% of patients treated with PRF, compared with 66% for those treated with a connective graft [167]. [PERSON] et al. [168], in a treatment of Miller class I or II recessions, compared the results of the use of PRF membranes and connective tissue graft, combined with coronally repositioned flaps and connective tissue graft. The parameters studied were the size of the recession, attachment gain, the height of the keratinized gingiva, the pocket depth, the quality and speed of wound healing and the patient's discomfort; clinical parameters were evaluated at baseline and after 12 months. The results were similar between the use of a PRF membrane and a connective graft.