Applications of zein-based material in nano-encapsulation, pickering emulsions, microspheres and amorphous solid dispersions
Creators: name: Zhang, Hongwei
Publisher: name: No Publisher Supplied
Year: 2020

description: With the increasing environmental concerns about the synthetic plastic materials, prolamine originated from grains have come back to people’s sight due to their unique characteristics: GRAS (Generally Recognized as Safe) nature with good biocompatibility and biodegradability, highly hydrophobic but soluble in 60%-80% aqueous alcohol, easy fabrication of particulate carrier at micro even nano level, and relatively slower digestibility for oral route-controlled release design, etc. Among them, zein is the first commercial prolamine originated from maize byproduct, which is usually produced as a white or light yellowish powder. As one of the historically manufactured plant proteins, zein has various applications in the industry like coatings, binders, fibers, etc. In the decade, zein’s advantages make it one potential biomaterial with intensive explorations in nutraceutical and medical delivery use, as well as an environmental friendly structural material for the soft matter. However, there is still the specific limitation or scarce study for zein based materials to fulfill its application potentials in each trending field, including nano-encapsulation, Pickering emulsions, microspheres and amorphous solid dispersions. My Ph.D. thesis exploits the solutions to break through the instability limitations of zein-based material in nano encapsulation and Pickering emulsions via chemical modification, and also fills its research gap for scaling up bioactive compound encapsulation using microsphere system and amorphous solid dispersions. Firstly, hydrophilic modification was successfully performed on zein material, in order to improve its colloidal stability by intrinsically enhancing its amphiphilic nature. The zein was conjugated with hydrophilic carboxymethyl dextran (CMD) through covalent linking, and a novel amphiphilic zein-based material, zein-carboxymethyl dextran (Zein-CMD), was successfully synthesized to self-assemble nano-micelles for delivery potential application. The results suggested that about 2 CMD molecules were conjugated per zein chain, which resulted in the transformation from secondary α-helix to β-sheet and random coil structure. After modification, Zein-CMD still took a rod-like conformation but more elongated than pristine zein in aqueous alcohol solution. The modified zein-based material could self-assemble spherical nano-micelles by conventional anti-solvent method, with a good control on colloidal stability and particle size within a physiological pH environment, regardless of zein’s precipitation tendency due to its isoelectric point of ~ 6.2. Secondly, hydrophilic modified zein-based material was self-assembled into nano-sized micelles to enhance the encapsulation efficiency of lipophilic phytochemicals. Dihydromyricetin (DMY) loaded Zein-CMD nano-micelles, DMY/Zein-CMD, was prepared through anti-solvent method. The Dihydromyricetin load was up to 30 wt.%, and a better colloidal stability was achieved overwhelming solely zein carrier or zein with CMD as the outer. In addition, DMY encapsulated in the Zein-CMD carrier was confirmed as amorphous status, with hydrogen bonding found to synergistically prevent recrystallization of DMY with hydrophobic interactions. In comparison, dissolution profile of DMY/Zein-CMD was significantly improved as compared to pristine zein in bio-relevant media. In the following section, hydrophobic modification on zein material was also conducted to resolve the unstable issues of Pickering emulsions (PE) based on pristine zein particles. The typical hydrophobic lauryl chains were successfully grafted onto zein to adjust its hydrophile-lipophile balance (HLB) to better stabilize PE. It is indicated that the HLB of lauryl-zein conjugate could be tuned by 1-8 lauryl chains per zein chain, which was visually quantified by water contact angle as well. Through anti-solvent and ultrasonic treatments, modified zein conjugate particles were successfully prepared with narrowly size distribution. These particles at low concentration of 1 wt% could better stabilize 70% oil internal phase than that at 50% oil phase, and overwhelmed the breakage of pristine zein based Pickering emulsions after 1 week’s storage at the ambient environment. The potential mechanism was discussed that the increase of hydrophobicity through the lauryl grafting correspondingly enhanced the surface tension. A new insight was introduced on PE stabilization containing high oil internal phase, using hydrophobic modified zein conjugate particles without other additives. Besides chemical modification on zein material, encapsulation of poor soluble bioactive compounds at scale-up level was also discussed on zein-based microsphere delivery system and amorphous solid dispersions. A novel zein-based microsphere delivery system containing amorphous resveratrol was prepared by anti-solvent method and following freeze drying technology. The results showed that up to 20 wt% of amorphous resveratrol was stabilized in zein microspheres possibly via hydrogen bonding, and the amorphous formulation could maintain stable for 3 months. A better dissolution performance with enhanced solubility from amorphous resveratrol was achieved from zein microspheres as compared to equivalent crystalline resveratrol. Free drying technology can be a way to manufacture hydrophobic phytochemical loaded microspheres based on zein material, but a limit of the initial load could be a concern regarding on the high crystallization tendency. Lastly, amorphous solid dispersions (ASD) containing felodipine and polymeric carrier zein were produced by spray drying technology. The solid state characterization results demonstrated that amorphous origin of ASD was maintained under 3 months’ accelerated stability study, with spherical particles of about 1 um were observed without any birefringence in the micro condition. Only one single glass transition (Tg) was detected around 128.6 °C without exotherms or endotherms, indicating the good miscibility of felodipine in polymeric zein through spray drying. Based on cumulative bioaccessibility of felodipine through TIM-1 in vitro digestion model, a 6-8 times increased bioaccessibility from ASD was achieved as compared to equivalent crystalline felodipine mixed with zein. The spray dried amorphous solid dispersions using zein as polymeric excipient was proved to maintain saturation status of felodipine and enhance its bioaccessibility in simulated upper intestinal tract.