To overcome the defect of conventional nonlinear piezoelectric cantilever vibration energy harvesters, in this paper we conceive an elastic-support model to study the performance of energy converters under two types of variable-intensity excitation conditions: filtered Gaussian noises and pink noises. When excitation intensity is insufficient, thanks to the system's variable potential function, frequent bistable transition oscillations between two wells occur in elastic-support systems, while only weak oscillations in either well could be observed in rigid-support systems. In practical applications, the structural parameters of energy harvesters are not allowed to make real-time changes. If considered remaining the magnet interval and the spring's elastic stiffness unchanged while receiving stable maximum output voltage, elastic-support systems can be made full use toward variable-intensity filtered Gaussian noises. It has been proven that elastic-support systems are capable of adapting to random excitations with variable intensity, through which maximum power output and sufficient electromechanical energy conversion of the system can be accomplished.

Angiogenesis, a complex biologic process, is regulated by a large number of angiogenic factors, including vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF-2). Whether Bone morphogenetic proteins-2 (BMP-2), the osteoinductive factor, could significantly reinforce the effect of VEGF and FGF-2 on angiogenesis has not been studied in detail. To study the positive effects of multiple growth factors on angiogenesis, HUVECs were treated with BMP-2, VEGF, or FGF-2 singly and in binary and ternary combinations. This study further investigates the optimal timing of the ternary combination of BMP-2, VEGF and FGF-2 for angiogenesis in the chorioallantoic membrane (FGF-2 CAM). Results of single applications of BMP-2, VEGF, or FGF-2 suggested that HUVECs angiogenesis could be promoted in a dose-dependent manner and that the optimal concentration of BMP, VEGF and FGF-2 was 10, 50 and 1 ng/mL, respectively. These results indicated that the angiogenic activity of VEGF and FGF-2 was amplified by combining with BMP-2. The ternary combination of BMP-2, VEGF and FGF-2 exhibited a positive and synergistic effect on HUVECs angiogenesis, with the lower concentrations of each factor (1 ng/mL of BMP-2, 25 ng/mL of VEGF and 0.1 ng/mL of FGF-2) being sufficient to show synergistic promotion. When VEGF and FGF-2 were added in the initial activation stage and BMP-2 was added in the maturation stage, both HUVECs angiogenesis in vitro and CAM angiogenesis in vivo could be enhanced more effectively. These results could provide a basis for the controlled release systems capable of delivering multiple factors sequentially to promote angiogenesis in tissue engineering.

Based on the research results of conventional rigid support nonlinear energy harvesters, in this paper we conceive a kind of structure with an elastic support external magnet with the intent to keep the system in the state of bistable oscillation, even under low-intensity excitation conditions. It has been proved that elastic support systems have better power output performance than rigid support systems when excited at low-intensity vibrations. In addition, elastic support nonlinear energy harvesters do not need real-time adjustment of the magnet interval towards the variable-intensity random excitation source, consequently achieving maximum power output and sufficient electromechanical energy conversion of the system.

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