Robust And Responsive Silk Fibroin Microstructure Fabrication And Properties

Thin shell microcapsules fabricated by Layer-by-layer （LbL） self-assembly ofoppositely charged polyelectrolytes on decomposable particle have potentialapplications on the field of drug delivery, biosening, microreactor, food industryand surface modification. The most useful microcapsules for the bio-relatedapplication should be biocompatible, non-cytotoxic, mechanically robust andstable in a wide range of environmental conditions, efficiently encapsulation andcontrolled or switchable permeability. However, the major limitations of thisapproach are cytotoxicity of usually exploited synthetic cationic components andpoor stability of microcapsules under extreme acidic and basic conditions withcapsules heavily irreversibly deformed or dissolved completely. Thereby, thechoice of proper components for biocompatible microcapsules is limited, andstable and reversibly deformable microcapsules at extreme acidic and basicconditions are rare. Herein, a natural protein, silk fibroin, which is attractive dueto its biocompatible, biodegradable and extraordinary mechanical properties, hasbeen employed for preparing microcapsules. The reversible pH responsive,permeability, controlled loading/unloading and pH-dependent mechanicalproperties have been detailed addressed. Furthermore, we reported a novelmethod to fabricate a highly ordered array of self-formed “micro-bubble”constructions within silk fibroin thin film pattern, which can be utilized forencapsulation various components including hydrophobic, hydrophilic polymersand nanoparticles.Extremely stable and pH-responsive thin shell microcapsules from silk fibroinionomers modified with poly-L-lysine and poly-L-glutamic acid, are successfullyfabricated by LbL self-assembly based on ionic paring and covalent cross-linkingof functionalized pendant groups, which are composed of biocompatible silkmaterials in contrast with usually exploited synthetic polyelectrolytes. Themicrocapsules are remarkable stable in the unusually wide pH range from1.5to12.0, and show significant pH responsive behavior at pH below2.5and above11.0, with the maximum volume swelling reaching800%, Moreover, the capsulesindicate fully reversible swelling/deswelling properties without any residualdeformation at extreme pH conditions. Furthermore, shell morphologydemonstrates dramatic change in different pHs, thickness swells up to three-fold and microroughness increases by a factor of two in pH1.5and11.5as comparedwith the values obtained in neutral condition.The permeability of silk ionomers microcapsules can be finely tuned by bilayernumbers, assembly solution concentration and pH. Increasing bilayer numbersand solution concentration result in an increasing shell thickness, which causesdramatically reduction of molecular cut-off of FITC-Dextran through the capsulewell. High degree of reversible swelling/deswelling response in dimensionsfacilitating the reversible variation in microcapsule shell permeability that can beutilized for pH-controlled encapsulation and releasing.FRAP （fluorescent recovery after photobleaching） was employed for quantitativeanalysis of silk ionomers microcapsules diffusion coefficient. pH-inducedpermeability are accompanied by significant changes in diffusion coefficient. Asexposed to acidic （pH2.0） and basic （pH11.5） conditions, the diffusioncoefficient dramatically increased from8.5x10-12cm²/s （pH5.5） to3.4x10^-10cm²/s and1.8x10^-10cm²/s, respectively, indicating a two order of magnitudeincrease. The diffusion coefficients also increase with an increasing number ofbilayer. And as expected, a lower diffusion coefficient is observed with highermolecular weight Dextran. However, variation of cross-linking time had littleeffect on the diffusion behavior.Magnetic silk ionomers microcapsules are fabricated by the assembly of PEG-coated SPIOs onto hollow microcapsule. TEM images indicate the capsule shellsare uniform coated with high density of SPIOs, no aggregation and without anynoticeable capsule deformation, which facilitating excellent magnetic-responsivein the present of magnet field, which can be utilized for remotely control in drugdelivery system.Micromechanical properties of silk ionomers capsules were measured by SurfaceForce Spectroscopy with AFM. The shell stiffness are significantly reduced bymore than two orders of magnitude as exposing to liquid, to the range of around10MPa. And the microcapsules indicate pH-dependent elastic modulus. Theshells undergo remarkable softening with a drop in Young’s modulus by morethan an order of magnitude at acidic （pH1.5） and basic （pH11.5） conditions,decreasing from8.5MPa （pH5.5） to0.6MPa and0.4MPa, correspondingly,which can be attributed to the swelling, stretching and increase in porosity ofmicrocapsule shell. Furthermore shell stiffness can be further tuned by cross-linking time, a five-fold increase in shell elastic modulus is observed upon160min cross-linking time due to the increased amount of amide bonds. Individually addressable patterned silk fibroin thin films with self-formed“micro-bubble” constructions are successfully fabricated by confining dissolutionof polystyrene prelayer in the patterned polystyrene-silk fibroin multilayer thinfilm. Raman spectra reveals that the PS only presents in the “micro-bubble”region, indicating the successfully self-encapsulation of PS. The formed “micro-bubble” count and shape depended on silk fibroin pattern thickness and patterngeometry. The silk fibroin pattern with “micro-bubble” constructions not onlyshow remarkable highly ordered array, also indicate the capability forencapsulation various materials, such as hydrophobic, hydrophilic polymers andnanoparticles, which provide a novel method for the application in the field ofsolid-state delivery system, drug delivery and bioengineering.