3.3 Photothermal effect and E2 release from bilayer scaffolds
The application of PDA particles in smart drug delivery systems for controlled and sustained release of biomolecules has been considered as an efficient way to promote tissue regeneration[39]. Previous studies have already indicated that oral administration of E2 would have a very poor therapeutic effect in treating uterine injuries as compared to in situ controlled release [40]. PDA is an excellent photothermal agent with good biocompatibility and biodegradability. Moreover, due to the inherent molecule structure, PDA particles exhibit a pH-sensitive behavior. Therefore, the release of biomolecules from PDA particles can be easily regulated by NIR laser irradiation and environment pH. In the current study, since E2 as a biomolecule for uterine regeneration was encapsulated in PDA particles and the synthesized PDA@E2 particles were incorporated in electrospun fibers, the release of E2 from S+F-PDA@E2 bilayer scaffolds could be precisely tuned by NIR laser irradiation and/or environment pH. Firstly, the photothermal effect of S+F and S+F-PDA@E2 bilayer scaffolds were investigated. Fig.6(A)(B) displays temperature changes of S+F and S+F-PDA@E2 bilayer scaffolds under the irradiation of the NIR laser at the energy density of 0.5 and 1.0 W/cm2, respectively. S+F-PDA@E2 bilayer scaffolds could be heated up to 35 ℃ and 49 ℃ after being irradiated by 0.5 W/cm2 and 1.0 W/cm2 NIR laser, respectively, for 3 min. Such high temperatures caused by NIR laser could accelerate Brownian movements of E2 molecules. The accelerated movement of E2 molecules would further result in more E2 released from bilayer scaffolds. Indeed, with the irradiation of NIR laser, more E2 was released in comparison to unirradiated samples [Fig.6(C)].
On the other hand, because human uterus presents a dynamic pH environment, it is essential to study E2 release kinetics in buffer solutions of different pH values. In the current study, the E2 release behavior were studied in solutions of pH 4.5, 7.4, and 9.0, respectively. As shown in Fig.6(D), E2 could be sustainably released in long-term (over 28 days), and the E2 release kinetics could match the Higuchi model (Fig.S12). The sustained release of E2 would have significant therapeutic effect on uterine regeneration. It is well-known that a long-term biomolecule delivery can prolong the beneficial effect and thus promote tissue regeneration [41]. Moreover, in contrast to an acidic environment, E2 could be quickly released in a basic solution, which may be attributed to the disintegration of PDA particles and degradation of PLGA/GelMA electrospun fibers in an alkaline environment.
Estrogen plays a critical role in regulating menstruation and endometrium regeneration. The safety and efficacy of E2 for endometrium regeneration has been clinically proved [42]. Endometrium stromal cells and glandular epithelial cells in the endometrium have many estrogen receptors. When E2 binds to their receptors, it enables increases in expressions of angiogenic growth factors such as VEGF, bFGF and TGF-β1 [43]. These angiogenic growth factors would facilitate endothelial cell migration, proliferation, differentiation, tube formation, and thus increase angiogenesis and blood vessel density, thereby improving the reconstruction of uterine tissues. Previous studies have investigated the possibility in using hydrogels or scaffolds to deliver E2 in situ for uterus injury treatment [40, 44]. However, the problem was that E2 released from these hydrogels or scaffolds could not reach a controlled and sustained long-term delivery state. In the current study, the in-site release of E2 from bilayer scaffolds could not only be regulated by the NIR laser irradiation but also be modulated by environment pH, which is highly beneficial for uterine regeneration.