Hesun Zhu

Degradation Mechanism and Control of Silk Fibroin

Controlling the degradation process of silk is an important and interesting subject in the field of biomaterials. In the present study, silk fibroin films with different secondary conformations and nanostructures were used to study degradation behavior in buffered protease XIV solution. Different from previous studies, silk fibroin films with highest β-sheet content achieved the highest degradation rate in our research. A new degradation mechanism revealed that degradation behavior of silk fibroin was related to not only crystal content but also hydrophilic interaction and then crystal-noncrystal alternate nanostructures. First, hydrophilic blocks of silk fibroin were degraded. Then, hydrophobic crystal blocks that were formerly surrounded and immobilized by hydrophilic blocks became free particles and moved into solution. Therefore, on the basis of the mechanism, which enables the process to be more controllable and flexible, controlling the degradation behavior of silk fibroin without affecting other performances such as its mechanical or hydrophilic properties becomes feasible, and this would greatly expand the applications of silk as a biomedical material.

Deposition of diamond‐like carbon films by electrolysis of methanol solution

By electrolysis of the methanol solution, an attempt was made to deposit diamondlike carbon (DLC) films on silicon substrate at temperature of less than 60 °C. Substrates were negatively biased with a dc potential of 0 to −3000 V. IR spectra showed that the O–H, C–H, and C–O vibration bands of electrolyte decreased remarkably after electrolysis and a new peak characterized as the C=C bond appeared. The deposited films were characterized as DLC films by Raman spectroscopy.

Silk self-assembly mechanisms and control from thermodynamics to kinetics.

Silkworms and spiders generate fibers that exhibit high strength and extensibility. The underlying mechanisms involved in processing silk proteins into fiber form remain incompletely understood, resulting in the failure to fully recapitulate the remarkable properties of native fibers in vitro from regenerated silk solutions. In the present study, the extensibility and high strength of regenerated silks were achieved by mimicking the natural spinning process. Conformational transitions inside micelles, followed by aggregation of micelles and their stabilization as they relate to the metastable structure of silk are described. Subsequently, the mechanisms to control the formation of nanofibrous structures were elucidated. The results clarify that the self-assembly of silk in aqueous solution is a thermodynamically driven process where kinetics also play a key role. Four key factors, molecular mobility, charge, hydrophilic interactions, and concentration underlie the process. Adjusting these factors can balance nanostructure and conformational composition, and be used to achieve silk-based materials with properties comparable to native fibers. These mechanisms suggest new directions to design silk-based multifunctional materials.

Nanofibrous architecture of silk fibroin scaffolds prepared with a mild self-assembly process.

Besides excellent biocompatibility and biodegradability, a useful tissue engineering scaffold should provide suitable macropores and nanofibrous structure, similar to extracellular matrix (ECM), to induce desired cellular activities and to guide tissue regeneration. In the present study, a mild process to prepare porous and nanofibrous silk-based scaffolds from aqueous solution is described. Using collagen to control the self-assembly of silk, nanofibrous silk scaffolds were firstly achieved through lyophilization. Water annealing was used to generate insolubility in the silk-based scaffolds, thereby avoiding the use of organic solvents. The nano-fibrils formed in the silk-collagen scaffolds had diameters of 20-100 nm, similar with native collagen in ECM. The silk-collagen scaffolds dissolved slowly in PBS solution, with about a 28% mass lost after 4 weeks. Following the dissolution or degradation, the nanofibrous structure inside the macropore walls emerged and interacted with cells directly. During in vitro cell culture, the nanofibrous silk-collagen scaffolds containing 7.4% collagen demonstrated significantly improved cell compatibility when compared with salt-leached silk scaffolds and silk-collagen scaffolds containing 20% collagen that emerged less nano-fibrils. Therefore, this new process provides useful scaffolds for tissue engineering applications. Furthermore, the process involves all-aqueous, room temperature and pressure processing without the use of toxic chemicals or solvents, offering new green chemistry approaches, as well as options to load bioactive drugs or growth factors into process.

Controlling Structural Symmetry of a Hybrid Nanostructure and its Effect on Efficient Photocatalytic Hydrogen Evolution

The existence of lattice strain between two different materials can be used to control the fine structural configuration in a hybrid colloidal nanostructure. Enabled by such, the relative position change of Au and CdX in Au-CdX from a symmetric to an asymmetric configuration is demonstrated, which can further lead to fine tuning of plasmon-exciton coupling and different hydrogen photocatalytic performance. These results provide new insight into plasmon enhanced photocatalytic mechanisms and provide potential catalysts for photoreduction reactions.

Synthesis of novel Sb2O3 and Sb2O5 nanorods

Sb2O3 and Sb2O5 nanorods have been prepared by using a microemulsion method for the system of AOT-water-toluene. X-ray diffraction and high resolution electron microscopy (HREM) were used to characterize the phase and size as well as shape of the Sb2O3 and Sb2O5 nanorods. These nanorods have a diameter of 10-50 nm and a length of up to 25 mu m. A possible mechanism for the formation of the nanorods is discussed

Silk Fibroin Electrogelation Mechanisms

A silk fibroin gel system (e-gel), formed with weak electric fields, has potential utility in medical materials and devices. The mechanism of silk e-gel formation was studied to gain additional insight into the process and control of the material properties. Silk fibroin nanoparticles with sizes of tens of nanometers, composed of metastable conformations, were involved in e-gel formation. Under electric fields the nanoparticles rapidly assembled into larger nano- or microspheres with size range from tens of nanometers to several microns. Repulsive forces from the negative surface charge of the acidic groups on the protein were screened by the local decrease in solution pH in the vicinity of the positive electrode. By controlling the formation and content of silk fibroin nanoparticles e-gels could be formed even from low concentration silk fibroin solutions (1%). When e-gel formation was reversed to the solution state the aggregated nano- and microspheres dispersed into solution, a significant observation related to future applications for this process, such as drug delivery.

Formation of crystalline carbon nitride powder by a mild solvothermal method

Crystalline carbon nitride powder has been successfully prepared via the liquid–solid reaction between anhydrous C3N3Cl3 and Li3N in benzene at 355 °C and 5–6 MPa for 12 h. X-Ray diffraction (XRD) indicated that the major part of our brown sample was composed mainly of α-C3N4 and β-C3N4 with lattice parameters of a = 6.49 A, c = 4.70 A for α-C3N4 and a = 6.43 A, c = 2.43 A for β-C3N4. X-Ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FTIR), electron energy-loss spectroscopy (EELS) and energy loss near edge structure (ELNES) strongly supported the existence of C–N covalent bonds. The relative N/C ratio is about 0.66. The temperature had an effect on the crystallization of carbon nitride powder. We have shown that the solvothermal technique might hold some promise for synthesizing pure crystalline α-C3N4 and β-C3N4.

Preparation of alumina films from a new sol-gel route

A new sol‐gel route was applied to prepare alumina films by using AlCl3·6H2O as starting material and acetylacetone (AcAc) as additive. Our research results show that AcAc and aluminum form a complex compound in the sol, which makes the sol very stable. Meanwhile, the complex structure prevents the decomposition going on quickly during the sintering process and the obtained films are free of cracks. The asprepared films were amorphous. When the sintering temperature reached up to 6008C, g-Al2O3 began to form and was transformed to a Al2O3 at temperatures over 1200 8C. q 1999 Elsevier Science S.A. All rights reserved.

Salt-Leached Silk Scaffolds with Tunable Mechanical Properties

Substrate mechanical properties have remarkable influences on cell behavior and tissue regeneration. Although salt-leached silk scaffolds have been used in tissue engineering, applications in softer tissue regeneration can be encumbered with excessive stiffness. In the present study, silk-bound water interactions were regulated by controlling processing to allow the preparation of salt-leached porous scaffolds with tunable mechanical properties. Increasing silk-bound water interactions resulted in reduced silk II (β-sheet crystal) formation during salt-leaching, which resulted in a modulus decrease in the scaffolds. The microstructures as well as degradation behavior were also changed, implying that this water control and salt-leaching approach can be used to achieve tunable mechanical properties. Considering the utility of silk in various fields of biomedicine, the results point to a new approach to generate silk scaffolds with controllable properties to better mimic soft tissues by combining scaffold preparation methods and silk self-assembly in aqueous solutions.