Starch-based Microcapsules Designed Through Layer-by-layer Assembly And Their Application For The Protein Controlled Delivery

One of the key scientific problems to improve the stability and bioavailability ofbioactive factors is how to obtain the suitabledelivery carrier materials with high loadingefficiency and effective controlled release functionality for the bioactive factors. In this work,by using a layer-by-layer self-assembly technique, the starch-based assembly carrier materialswere assembled with the model protein. Then, the effects of the property and quantity offunctional groups, the molar mass and the internal ordered aggregation structures of theassembly carrier materials, as well as their variations in the liquid food system and digestivetract environment on the stability and in vitro controlled release behaviors of the assemblymicrocapsules were evaluated. In this way, the protocol for regulating the molecular chaincharacteristics and aggregation structures of the starch-based assembly carrier materials wasacquired. This provides theories for establishing the assembly microcapsule systems with highloading efficiency and controlled release behaviors.The charge, molecular chain characteristics and aggregation structures of thestarch-based assembly carrier materials were modulated by introducing the anionic andcationic groups using carboxymethylation and quaternary amination and by enzymolysisusing pullulanase enzyme. Finally, the modified starches with a molar mass in the range of2.010×106to2.345×107g/mol and with DS of carboxymethyl group in the range of0.041to0.245were selected as the anionic starch-based assembly carrier materials, together with themodified starches with a molar mass in the range of6.154×103to2.662×104g/mol and withDS of quaternary amine group in the range of0.143to0.283as the cationic starch-basedassembly carrier materials. It is found that except for the case of being in a solution of pH=1.2, the anionic starch-based assembly carrier materials displayed negative charge which wasenhanced as the DS of carboxymethyl group increased, whereas the cationic starch-basedassembly carrier materials showed positive charge and its quantity could be also increased byan increase in the DS of quaternary amine group. Small angle X-ray scattering (SAXS)analysis revealed that while the internal ordered aggregates of all anionic starch-basedassembly carrier materials displayed a mass fractal structure, the anionic starch-basedassembly carrier material with Mw=4.373×106g/mol showed a surface self-similarorganization with high compactness. Nevertheless, negligible or no self-similar characteristicscould be observed for the cationic starch-based assembly carrier materials except the materialwith a Mwof1.344×104g/mol) having a mass fractal structure.Microcapsules with different layers were prepared by assembling BSA with the starch-based assembly carrier materials differing in degree of substitution and in molecularmolar mass. Then, the variations in the zeta potential, and particle size and its distribution ofthe microcapsules while being stored in the buffer solutions with a pH value in the range of2.5to5for different times were investigated. Moreover, the changes in the zeta-potential,particle size, morphology and internal ordered aggregation structures of the microcapsules invitro digestive tract environment were also understood. Based on this, by using an in vitrodissolution tester, the BSA release behaviors from the assembly microcapsules was studied,and thus how the variations of the structural characteristics (size, charge, morphology, andaggregation structures, etc.) influence the release behaviors of BSA was explored. The resultsshow that the anionic starch-based assembly carrier material with Mwof6.049×106g/mol andwith a DS of0.157had the highest encapsulation efficiency (i.e.,63.69%) and the highestloading rate of70.61%for BSA. Following this case, the three-layer microcapsule werefurther obtained by assembling the above microcapsule with other two materials, i.e., thecationic starch-based assembly carrier material (Mw=7.407×103g/mol, DS=0.261) and theanionic starch based assembly carrier material (Mw=4.373×106g/mol, DS=0.04).Noteworthily, the newly established thriple layer microcapsule system had the desiredcontrolled release property with a1.3%leakage rate in the sprite and with a release amount ofBSA being56.79%in the mimic colon fluid. It is noted that the internal ordered aggregates ofthis three-layer assembly microcapsule displayed a mass fractal structure at different pHvalues, with the maximum fractal dimension at pH=1.2. Under pH=1.2, the microcapsulespresented dark black polygonal shape, and the surface of these microcapsules showed fuzzyboundaries and an increased size while being at the pH of6.8and7.2.Starch based assembly microcapsules delivery system with various structural propertieswere developed by adjusting the molecular molar mass, DS of anionic and cationic starchand different assembled layers.Furthermore, the relationship between effective loading orcontrolled release of protein and the molecular structure of starch based assembly carriedmaterial as well as the nano-structural changes of assembly microcapsules undergastrointestinal tract environment were established. Starch based assembly carried materialswith protein controlled release properties were obtained and all these results will promote thepotential application of starch based assembly carried materials in control the release ofprotein in the liquid food system.