Kempen, Diederik H.R.

Biomaterials that act as both protein delivery vehicle and scaffold can improve the safety and efficacy of bone morphogenetic protein-2 (BMP-2) for clinical applications. However, the optimal scaffold characteristics are not known. The osteoinductive and osteoconductive capacity of a fixed electrically charged surface is thus far unexplored. Therefore, in this study, we aim to investigate the effect of different electrical states on BMP-2-induced bone formation in oligo[(polyethylene glycol) fumarate] (OPF) hydrogels. Neutral, negatively, or positively charged scaffolds were fabricated using unmodified OPF (neutral charge), sodium methacrylate crosslinked OPF (negative charge), or [2-(methacryloyloxy) ethyl] trimethylammonium chloride crosslinked OPF (positive charge), respectively. To allow investigation of surface charge for different BMP-2 release rates, three BMP-2 release profiles were generated by protein encapsulation into poly(lactic-co-glycolic acid) microspheres and/or adsorption on the OPF composite. Release of radiolabeled 125 I-BMP-2 was analyzed in vitro and in vivo and bone formation was assessed after 9 weeks of subcutaneous implantation in rats. Negatively charged OPF generated significantly more bone formation compared with neutral and positively charged OPF. This effect was seen for all three loading methods and subsequent BMP-2 release profiles. Along with charge modifications, a more sustained release of BMP-2 improved overall bone formation in OPF composites. Overall, this study clearly shows that negative charge enhances bone formation compared with neutral and positive charge in OPF composites. Impact Statement Biomaterials can play a dual role in bone regeneration: they enable local sustained delivery of growth factors, such as bone morphogenetic protein-2 (BMP-2), while they provide structural support as scaffold. By better imitating the properties of native bone tissue, scaffolds may be both osteoconductive and osteoinductive. The latter can be achieved by modifying the electrical charge of the surface. The present work uses tunable oligo[(polyethylene glycol) fumarate] hydrogel and demonstrates that negative charge enhances BMP-2-induced bone formation compared with neutral or positive charge. Altogether, this indicates that tissue-specific surface charge modifications of biomaterials hold great promise in the field of tissue regeneration.

The main challenge in bone morphogenic protein 2 (BMP-2)-based application lies in finding strategies that prolong its effective period as it has a short biological half-life. Several BMP-2 release profiles have shown to enhance bone formation at various application sites. However, it remains to be determined which BMP-2 release profile best augments bone formation and whether this effect is location dependent. Therefore, the aim of this study was to investigate the effect of BMP-2 release from oligo[(polyethylene glycol) fumarate] bis(2-(methacryloyloxy)ethyl) phosphate (OPF-BP) composites on the osteoinductive efficacy at ectopic versus orthotopic application. By varying the BMP-2 loading method, three different OPF-BP composites were created with varied release profiles. The composites were compared with unloaded OPF-BP as negative control, and to the clinically used Infuse ® absorbable collagen sponge (ACS) as positive control. Bone formation was assessed by microcomputed tomography after 9 weeks of subcutaneous implantation and 3, 6, and 9 weeks of orthotopic implantation in rats ( n = 48). Whereas a BMP-2 burst release of >49% generated significantly more bone compared with sustained release (burst release <30%) at the subcutaneous implantation site, differential release did not affect bone formation at the orthotopic site. Furthermore, all BMP-2 containing OPF-BP composites showed significantly more bone formation compared with ACS in the orthotopic implantation site. In conclusion, this study clearly shows that the osteoinductive effect of different BMP-2 release profiles is location dependent. In addition, more bone formation in OPF-BP compared with ACS at both application sites emphasizes the role of biomaterials as a scaffold to achieve proper bone tissue formation. Impact Statement The main challenge in bone morphogenic protein 2 (BMP-2)-based application lies in finding strategies to prolong its biologic activity as it has a short biological half-life. The present study uses a phosphate-modified oligo[(polyethylene glycol) fumarate] hydrogel that can be tuned to achieve differential release profiles of biologically active BMP-2 release. We demonstrate that this platform outperforms Infuse ® , currently used in the clinic and that the osteoinductive effect of BMP-2 is location dependent. Altogether, this study stresses the importance of evaluating efficacy of bone tissue engineering strategies at an orthotopic location rather than subcutaneously. Even more so, it emphasizes the role of biomaterials as a scaffold to achieve proper bone tissue engineering.

AbstractTo investigate the effect of sustained bone morphogenetic protein‐2 (BMP‐2) release kinetics on bone formation in poly(propylene fumarate) (PPF) scaffolds, different poly(lactic‐co‐glycolic acid) (PLGA) microspheres were used as delivery vehicles. All PPF scaffolds had the same 75% porous structure, while the degradation rate of the embedded PLGA microspheres was changed to tailor BMP‐2 release by varying the lactic‐to‐glycolic acid (L:G) ratio in the copolymer. Four PLGA microsphere formulations with 50/50, 65/35, 75/25, and 85/15 L:G ratios and varying in vivo degradation rates were fabricated. The in vitro and in vivo BMP‐2 release kinetics were determined by analyzing radiolabeled 125I‐BMP‐2. Biological activity of released BMP‐2 was tested using a W20‐17 cell culture model in vitro and a subcutaneous rat model in vivo. Corresponding outcome parameters were defined as capacity to increase the in vitro AP activity in weekly consecutive cell cultures over 14 weeks and the in vivo bone formation after 7 and 14 weeks. The PLGA/PPF composites showed similar biological activity and BMP‐2 release profiles in vitro. In vivo, PPF/PLGA 85:15 composite released significantly less BMP‐2 per time point in the first weeks. Micro‐CT and histological analysis after 7 and 14 weeks of implantation showed bone formation, which significantly increased over time for all composites. No significant differences were seen between the composites. Overall, the results of this study show that small differences in BMP‐2 sustained release had no significant effect on BMP‐2 osteogenic efficacy in PPF/PLGA composites. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 477–487, 2018.

To satisfy different mechanical requirements in hard and soft tissue replacements, a series of biodegradable and crosslinkable copolymers of poly(propylene fumarate)‐co‐polycaprolactone (PPF‐co‐PCL) are synthesized and employed to fabricate 2D disks and 3D scaffolds via photocrosslinking. Thermal properties such as the glass transition temperature (Tg) and melting temperature (Tm) of the PPF‐co‐PCL networks can be controlled efficiently by varying the PCL composition (φPCL). As a result, their mechanical properties vary significantly from hard and stiff materials to soft and flexible ones with increasing φPCL, making them attractive candidate materials for bone and peripheral nerve regeneration, respectively. Several PPF‐co‐PCL formulations are selected to perform in vitro cell studies using mouse pre‐osteoblastic MC3T3‐E1, rat Schwann cell precursor line (SPL201), and pheochromocytoma (PC12) cells, and in vivo animal testing in the rat femur bone defect model and in the rat sciatic nerve transection model. The formation of new bone in the porous bone scaffolds with a low φPCL and guided axon growth through the nerve conduits with a higher φPCL suggest that crosslinked PPF‐co‐PCLs have appropriate compatibility and functionality. Furthermore, the role of surface stiffness in modulating cellular behavior and functions is verified on the crosslinked PPF‐co‐PCL surfaces without any pretreatments.