GBR gained traction as a therapy in the 1980s, but in 1957 there was already an initial report of space creation with plastic cages for bone regeneration in femoral defects in dogs12. Other studies followed, including those focusing on the craniofacial area13,14, but the mechanism of action was initially hypothesized to be the protection of the blood clot. In the early 1980s, the guided tissue regeneration principle was established, explaining that a specific tissue can be rege ne rated when cells with the restorative ability for that tissue type are able to occupy the wound space9,15. Through space maintenance and the preclusion of faster growing cells from the gingival connective tissue and oral mucosal epithelium, osteogenic cells are allowed to infiltrate and form bone8,9,15-17.

By enabling new bone formation, GBR may obviate the use of autologous bone grafts. Although an excellent source, the use of autogenous tissue is not desirable due to the pain and morbidity associated with the graft harboring site4,18. To function properly, GBR membranes must meet certain requirements, including cell exclusion, space creation, scaffolding for progenitor cell in-growth, biocompatibility, host tissue integration, and clinical manageability1,11,17.

The first generation of barrier membranes was non-resorbable, with expanded polytetrafluorothylene (ePTFE) (Teflon; Gore, Flagstaff, AZ, USA) membranes becoming the most frequently used1,8,11. With its high stability, non-immunogenicity, and reliably successful results, ePTFE membranes have been considered a gold standard for bone regeneration11,19, despite the fact that ePTFE-membrane exposure to the oral cavity always resulted in a failure of the treatment.

Even with the favorable outcomes using non-resorbable ePTFE, the strong disadvantage of requiring a second surgery to remove the material encouraged the development of a second generation of GBR with degradable barrier membranes. Along with the morbidity associated with a second surgical procedure for non-resorbable membrane removal, there is a risk of tissue damage and disturbed healing19. Additionally, there are some post-surgical complications of membrane exposure with non-biodegradable membranes that lead to a high incidence of infection that can decrease bone regeneration20,21. Besides avoiding the second surgery, resorbable membranes are also advantageous because of improved soft tissue healing and lower bacterial contamination risk due to decreased exposure from the degrading membrane19. In addition to the requirements stated previously, there are also further properties that these barrier membranes must fulfill: biocompatible degradation products that don't interfere with bone regeneration, appropriate degradation profile to synchronize with new tissue growth, and sufficient persistent mechanical and physical properties to perform the barrier function and allow in vivo use3.

Amongst the most common bioresorbable membranes are synthetic polyesters (poly(lactic acid), poly(glycolic acid), and poly(caprolactone) and their copolymers)22-25 and tissue-derived collagen21,26-29. Polyester membranes display biocompatibility and possess a high degree of customization, with degradation rates and mechanical properties that can be adjusted based on polymer composition and concentration30,31. As a natural component of the extracellular matrix, collagen is biocompatible and cell adhesive. Although collagen isn't inherently mechanically stable, it can be modified through various means of crosslinking3. Poly(ethylene glycol) (PEG) is also known as a biodegradable and biocompatible polymer. Since many oral and maxillofacial defects require precise shapes, an injectable material is desirable, such as a PEG-based in situ forming gel for GBR that demonstrated effectiveness in a clinical trial32-34.

In addition to the type of material, the physical form also plays a role in determining a material's properties, which can affect degradation rates and tissue integration. Porosity is one of the most important characteristics. With some materials, such as poly(lactide-co-glycolide), the porosity can be imposed by using porogens that form the pores and then are removed in a subsequent processing step35. Electrospinning is a manufacturing technique that creates elongated fibers with a degree of control over properties including fiber length, width, and orientation and overall porosity3. Researchers have explored these techniques for improving GBR membranes.

Although the existing bioresorbable membranes fulfill many requirements, most of them cannot maintain adequate space to act as a barrier membrane over an extended period of time5,19. However, in combination with a bone substitute material, these composite membranes have shown success24.

Effective bone graft materials can biologically stimulate bone growth through either osteoconduction by allowing cell growth through a scaffolding mechanism or osteoinduction by recruiting osteoprogenitors into the defect space4. Although cancellous autogenous bone grafts act as an osteoconductive material, the associated morbidity of the graft site restricts its use. Other natural sources include both allografts and xenografts, which are processed to reduce immunogenicity36. Demineralized bovine bone matrix retains type-1 collagen, non-collagenous proteins as well as a small amount of osteoinductive growth factors37. One of the most common commercially used products is deproteinized bovine bone matrix, Bio-Oss (Geistlich Biomaterials, Wolhusen, Switzerland), which is stripped of all organic elements by pyrolysis, a high temperature sintering, leaving hydroxyapatite (HA) as the main component38. Unfortunately, high temperature sintering also affects HA and alters it from a biological HA to a more synthetic one39. Despite positive outcomes of naturally-derived materials, there are still concerns regarding disease transmission and immunogenicity4.

Synthetic graft materials supply various formulations of calcium phosphate without the limitations of the animal-derived products. Bone is composed of a majority of calcium phosphate and these synthetic substitutes are known to have biologically active surface chemistry for osseointegration40. Additionally, these biodegradable, inorganic materials have a crystallographic structure similar to bone and controllable porosity that is important for mimicking bone, making them effective substitutes41,42. HA has been utilized as granules42,43, incorporated as nanoparticles3, or as a coating44. Tricalcium phosphate (TCP), which resorbs faster than HA, is another commonly used inorganic bone substitute41,45. Bioactive glass is another inorganic bone graft material that displays osteoconduction and an ability to bond with bone through chemical linkages46,47.

Membranes and graft materials can act as a barrier to fibrous tissue ingrowth and a scaffold to support bone and some even provide some osteoinductive activity, but a further critical role of biomaterials in improving oral and maxillofacial bone regeneration is delivering bioactive molecules. Growth factors are signaling proteins that regulate cellular growth, proliferation, and differentiation. Enamel matrix derivative and platelet rich plasma are both biologically-derived products that contain multiple growth factors and have demonstrated enhanced healing in periodontal tissues even though the mechanism of action is not understood3,48,49. Platelet rich plasma is comprised of various autologous growth factors that have individually been identified to enhance bone regeneration, including platelet-derived growth factor50, fibroblast growth factor51, and insulin-like growth factor52, but its positive effect on regeneration processes appears to be restricted to soft tissue healing49,53.

The class of bone morphogenetic proteins (BMPs), first identified to generate extraskeletal bone formation in bone extracts in 196554, have now been extensively studied and shown to be critical for the induction of bone55. The class of BMPs consists of 15 variants, with BMP-2, BMP-4, BMP-7, and BMP-12 shown to be particularly effective in bone regeneration56. BMP-2 and BMP-7 have been recombinantly produced for commercial use45. BMP-2 has shown impressive potential to regenerate bone in animal studies and the clinic, including in oral applications57. GBR in conjunction with BMP-2 delivery by a bone substitute material has been shown to be an effective strategy in humans as well58. However, milligram doses that are orders of magnitude higher than normal physiological levels are required59. An enhancer of BMP-2, such as N-methyl pyrrolidone (NMP), can possibly avoid the high cost and possible side effects and safety concerns of the large dose60. In a rabbit calvarial model, NMP enhanced bone regeneration over a polylactide membrane alone, emphasizing the importance of the bioactivity61.

Besides enhancers, controlled release of growth factors can increase their efficiency. In fact, acting as a vehicle for local delivery of growth factors and protecting them from degradation and inactivation are major roles of biomaterials. A slow, controlled release BMP-2 delivery system has been shown to induce and sustain bone formation62. Many of the same materials used as bone grafts have also been explored as delivery systems: collagen63, calcium phosphates64, and polyesters like polycaprolactone65.

Growth factors can non-covalently bind or covalently attach to the carrier66. Non-covalent growth factor delivery systems can function through adsorption (e.g., collagen sponge63 and deproteinized bovine bone matrix38), ion complexation with charged polymers (e.g., poly-L-ornithine and poly-L-arginine complexes67) or physical entrapment (e.g., polyesters68 and PEG38). The materials that physically incorporate growth factors can take various forms from liposomes69 to nanoparticles70 to hydrogels71. In contrast to these modes of delivery, covalent systems retain growth factors at the site of action until cleaved off or the carrier is degraded, extending the residence time66. Additionally, immobilization enables spatial growth factor delivery, limits side effects by constraining the growth factors to the site of action, and mimics physiological matrix-bound situations72. Growth factors can be covalently tethered to a material directly73,74 or through a linker75,76. Another strategy is genetically engineering fusion growth factors that include an attachment site outside the active protein sequence. BMP-2 was engineered to contain amino acid domains that enable both enzymatic covalent attachment to and release from fibrin-based substances, creating a system that mimics physiological binding and liberation77. Although it is possible that the growth factor may lose some activity through all of these covalent immobilization methods, this loss may be mitigated by the higher retention and other advantages.

In addition to allowing the delivery of bioactive molecules, biomaterials are also critical for enabling cell-based therapies. To encourage proliferation and differentiation, cells require an artificial extracellular matrix, which can be supplied through an appropriately designed biomaterial36. Including a cell source can further encourage oral and maxillofacial bone growth through direct tissue growth and bone repair as well as growth factor secretion from the cells78. Although this area has only been actively pursued relatively recently, due to advancements in biological cell research, there are some promising studies.

Mesenchymal stem cells (MSCs), multipotent adult stem cells that can be harvested from mesenchymal tissues such as bone marrow, have been suggested as a cell source for tissue engineering56. Since they are easily attained and expanded, bone marrow MSCs (BMSCs) are the most commonly explored MSCs36. A number of studies have demonstrated alveolar bone regeneration with BMSCs79-81. A study also showed the positive effect of combining stem cells with growth factor release through BMP-2 expressing BMSCs82.

In addition to BMSCs, there are several other cell types that are being explored for oral and maxillofacial bone tissue engineering. Umbilical cord MSCs are another easily obtainable reservoir of stem cells and initial studies show promising results for bone regeneration with this source83-86. Adipose-derived stem cells are other extraoral and non-craniofacial cells that are easily accessible and were successfully used to regenerate bone87,88. The periodontal ligament and dental pulp are both sources of stem cells in the oral cavity that have been isolated and characterized56,89. Stem cells from both of these tissues have demonstrated bone regeneration capabilities90-92. However, these cells are more difficult to harvest36.

With all of these stem cells sources, the biomaterial that supports them is a critical aspect for facilitating bone regeneration. Many of the same materials that have been developed as bone graft materials and bioactive molecule delivery vehicles have been explored as cell scaffolds. HA93, collagen94, fibrin87, and poly(lactide-co-glycolide)95 as well as composites such as HA/TCP96, chitosan-gelatin79, and calcium phosphate cement-chitosan-polyglactin86 are amongst these scaffolds.