In the plant seed, phospholipids and amphiphilic oleosin on the surface of oil body to stabilize the organelles, and oleosins prevent the oil body from gathering by steric hindrance [33–34]. The mixed surface membrane is adsorbed around triglyceride core, which is mainly two layers, one layer was considered to be composed of a primary mixed layer of phospholipids and oleosin, and a secondary layer was germ protein. Among them, a germ protein plays an important role in the long-term stabilization of oil body against assembling . Therefore, the oil body is stable in the seed for a long time. However, we found that the oil body ruptured after being stored in vitro for a period of time. This may be due to the lack of germ protein in the separate oil body or the interactions, which lead to the formation of droplet networks and its rearrangement took place at a relatively high rate, leading to instability [14, 35]. Therefore, xanthan gum is chosen as the matrix, which is participate in the process of wound healing [36–37]. Xanthan gum can be enriched in the external surface of adjacent oil body and interacted with each other to protect the stability of oil body, which may form bridge flocculation with oil body through hydrophobic bonding. The repulsive steric force is enhanced by the interaction of oil and xanthan gum, which in turn forms hydrogen bonds and makes it more resistant to external forces. Based on this, it may be the reason why the OBEME has good stability. In addition, xanthan gum is able to increase the temperature range of decomposition and the formed rate of volatile decomposition products during heating was slow .
Water gradually forms ice crystals at low temperature, and oil droplets are close to each other in the unfrozen water phase, which may lead to the fracture of the interface layer and promote the accumulation of oil droplets . However, xanthan gum is a polyhydroxy compound with very strong water absorption and strong water retention in the composite system, so that the free water in the composite system is less, reducing the generation of ice crystals . On the other hand, the protein around the lipid droplets is thick and forms a protective layer that prevents ice formation, fat crystallization, interfacial phase transitions, and biopolymer conformations. So, the OBEME prepared with xanthan gum as a matrix maintained high stability after freeze-thaw cycles.
The oil body linked hEGF has better transdermal absorption effect, hEGF accumulation in the skin was the highest at 60min and decreased at 90min, indicating that hEGF had been released . However, hEGF accumulation in skin tissue was the highest after treated with the OBEME for 90min, which significantly improved its sustained release effect. It may be due to the fact that the OBEME is a storable microgel that distributes drug molecules in the inner layer or specific regions of the microgel through self-assembly or hydrophilic interactions. In addition, the oil body is rich in oleic acid, which is commonly used as a permeation promoter and may play a role in transdermal penetration. On the other hand, the structure of the oil body is similar to that of liposomes, which increases the permeability of the skin to embedded drugs . Its diameter was smaller, which is conducive to transdermal absorption and the skin treated by the OBEME can deliver the drug more effectively through the stratum corneum into the deeper layer. The xanthan gum as the excipient had the slow-release effect for example, the floating chitosan-xanthan beads were used to control the release of glipizide . Because the skin tissue has a highly hydrated water environment, which the OBEME is more easily permeated into the skin than oil body. Xanthan gum has the characteristics of improving the biodistribution of drugs, prolonging the half-life of drugs, and protecting drugs from degradation in the microenvironment. Therefore, it blocks the possibility of EGF being degraded by proteases. As a transdermal drug, skin toxicity is also very important. In previous studies, it was proved that OBEME has non-irritating and sensitizing effects, and showing good safety .
Wound healing is a complex biological process, involving inflammation, re-epithelialization and tissue remodeling . When tissues are injured, the pathogen are released, resulting in complex inflammatory response. A large number of macrophage precursors are recruited, which affect the speed of wound healing. However, the OBEME effectively covered the wound, removed exudates, and reduced the expression of macrophage marker CD68. It may be related to the fact that the OBEME is rich in unsaturated fatty acids such as oleic acid and linoleic acid, which can inhibit inflammatory factors , thereby shortening the inflammatory period of the wound after the OBEME treatment. On the other hand, EGF regulates early inflammatory response to promote wound re-epithelialization . The OBEME promoted the expression of CD31 and K5 and K19 and thus accelerated angiogenesis and epithelialization. In the proliferative phase, the wound is repaired by the proliferation and migration of epithelial cells, fibroblasts and vascular endothelial cells. In the later period of healing, the quantity, thickness and density of collagen were changing all the time. Collagen is rich in COLIII, which plays an essential role in the formation of new capillaries . COL1 is a protein in the ECM, which acts as a structural scaffold in tissues. Collagen can increase the density of covalent cross-linking and cross-linking with non-collagen components. So, the increase of collagen contributes to the rapid healing of wounds . In the remodeling phase, the OBEME enhanced the expression of COLI and COLIII, thus providing structural integrity for new tissues. In the proliferative phase, the increase of myofibroblasts by up-regulation α-SMA is helpful to reduce the wound area, however, with the prolongation of wound healing time, α-SMA expression level directly affects the degree of tissue fibrosis. The expression of α-SMA was down-regulated on day 15, suggesting that the decrease of α-SMA expression level could reduce the trend of scar formation and improve the quality of wound repair.