Strain-Induced Nonlinear Frictional Behavior of Graphene Nanowall Films.

Graphene nanowall (GNW) films, a representation of three-dimensional (3D) carbon nanomaterial films, are emerging as promising candidates for applications in electric devices and composites, on account of their 3D structures and exceptional properties of graphene sheets. However, the frictional responses of GNW films, which exhibit significant influence on their performances, have seldom been reported. Herein, we reported a growth process of a GNW film by the chemical vapor deposition method and studied the frictional behavior of the GNW film for the first time. The results demonstrated the nonlinearity between the frictional force of the GNW film and normal load. Based on the structural evolution of the GNW film with normal load and frictional tests on precompressed GNW films, the influence of the strain property of the GNW film, namely, the strengthening effect, could be confirmed. The results of molecular dynamics simulations show that the bending force of GNWs in front of the tip plays a determinate role in the frictional force of the GNW film. Furthermore, the bending force is proportional to the bending contact area, which increases nonlinearly with the normal load due to the strengthening effect of the GNW film. The result suggests that the nonlinear increase of the bending contact area induced by the strengthening effect of the GNW film is the key factor that leads to its nonlinear frictional force. This study provides a novel insight into the frictional responses of GNW films, which would be beneficial for the design and application of electric devices and composites made of GNW and other 3D carbon nanomaterial films.

Microscopic Mechanisms Behind the High Friction and Failure Initiation of Graphene Wrinkles.

Wrinkling occurs on the surfaces of large-area graphene ubiquitously. Despite that the wrinkled structures are found to degrade the lubricous property, the behind mechanisms remain less understood. Here, atomic force microscopy is adopted to characterize the friction and wear properties of graphene wrinkles (GWs) with different heights by nanoscratch tests. We verify the phenomena of high friction and reduced load-carrying capacity of wrinkles and report the observation of lubrication deterioration with increased heights. Using molecular dynamics simulations, we reveal that the contact quality at the interface is a dominant role in the friction evolution of wrinkles. The high friction of wrinkles is determined by the increased contact area and commensurability caused by the wrinkle deformation and topography changes. The wrinkle failure initiates near the root of the formed bilayer configuration due to the increased lateral stiffness and reduced atomic distance between the wrinkle layers. The increased interlocking effect results in a local shear stress of 91 GPa and induces the phase transitions of carbon atoms easily. As the wrinkle height decreases, the unstable local configuration weakens the interlocking effects and cannot fail even at a high load. This investigation sheds light on the microscopic frictional contact of GWs and provides guidance for tuning the tribological properties of graphene by controlling the wrinkle structures.

Injectable pH and redox dual responsive hydrogels based on self-assembled peptides for anti-tumor drug delivery.

Traditional anti-tumor drugs still have some shortcomings, such as low solubility, poor selectivity and poor bioavailability, which decrease their anti-tumor efficacy and aggravate systemic toxicity and side effects. In this paper, pH/redox dual responsive IC1-R peptide hydrogels were designed as drug delivery materials for the anti-tumor drug paclitaxel (PTX). The physical and chemical properties of drug-loaded IC1-R peptide hydrogels were characterized by pH/redox sensitivity, drug release, physical description, encapsulation rate, circular dichroism, electron transmission microscopy, and rheological tests. In vitro cytotoxicity and in vivo efficacy were studied to evaluate the anti-tumor efficacy of the PTX-loaded hydrogel. IC1-R was found to have high sensitivity to pH/redox conditions, and the encapsulation rate can reach more than 98% at different PTX dosages. The structure of the IC1-R peptide was found to be a ß-sheet under neutral conditions, which met the requirement for nanofiber network formation. Transmission electron microscopy and rheology tests confirmed that the suitable meshwork structure and improved mechanical and injectable properties of this hydrogel. In vitro and in vivo results showed that the blank hydrogel had good biological safety and confirmed the pH/redox sensitive properties of IC1-R-PTX, which allowed sustained delivery of the drug and enhanced tumor inhibition. In conclusion, this kind of PTX-loaded peptide hydrogel, which was formed in vitro, can be injected into tumors and can continuously and slowly release anti-tumor drugs under the stimulation of the tumor microenvironment to achieve the best anti-tumor effect and reduce toxicity and side effects. This biofunctional material has broad prospects in the field of drug delivery.

Redox-Sensitive Ultrashort Peptide Hydrogel with Tunable Mechanical Properties for Anti-Tumor Drug Delivery.

In this study, we report a new ultrashort peptide (LOC), which forms a redox-sensitive hydrogel after cross-linking with the mild oxidant H2 O2 and used it for tumor-targeted delivery of doxorubicin hydrochloride (DOX). LOC gelled within a few minutes in low-concentration H2 O2 solution. The concentration of H2 O2 significantly altered the gelation time and mechanical properties of the hydrogel. The in vitro micromorphology, secondary structure and rheology characterization of cross-linked hydrogels confirmed the sensitivity and injectability to reducing agent. The cross-linked hydrogel had a strong drug loading capacity, and the drug was released in a GSH concentration-dependent manner, following the Fick diffusion model. In addition, the cross-linked hydrogel showed no cytotoxicity to normal fibroblasts, and no damage to the subcutaneous tissue of mice was observed. In vitro cytotoxicity experiments showed that the DOX-hydrogel system exhibited good anti-cancer efficacy. In vivo studies using 4T1 tumor-bearing mice showed that the DOX-hydrogel system had a significant inhibitory effect on tumors. Therefore, the newly designed redox-sensitive hydrogel can effectively enhance the therapeutic efficacy of DOX and reduce toxicity, making it an attractive biological material.

Double-Vacancy Controlled Friction on Graphene: The Enhancement of Atomic Pinning.

The vacancy-enhanced contact friction of graphene is mainly attributed to the vacancy-enhanced out-of-plane deformation flexibility of the graphene and the climbing of the tip out of the vacancy trap (which actually acts as a step edge). However, this mechanism does not apply for explaining the enhanced friction caused by small-sized vacancies that are unable to accommodate the tip, such as single vacancy and double vacancies, which also commonly exist in the graphene. In the present study, by performing a set of classic molecular dynamics simulations, we demonstrated that the double-vacancy defect in graphene substantially enhanced the contact friction when the tip slides over it and the pinning effect of the reconstructed lattice of the double-vacancy defect with atoms at the bottom of the tip dominated such an influence. The underlying mechanism of such an atomic pinning effect and the influence of the normal load, sliding direction, and the sliding velocity were unveiled by analyzing the obtained friction evolution and the atomic configuration and interaction between the tip and the graphene. We believe that the findings presented in this study complete the state-of-art understanding of the nanoscale friction behaviors of vacancy-defected graphene, which is essential for the implementation of their potential control.

Effect of particle size of hydroxyapatite nanoparticles on its biocompatibility.

Nano-particulate biomaterials have been used in clinical diagnosis and treatment, as drug carrier or in cosmetics because of their excellent performance properties. The toxicity and biocompatibility of nanoparticles (NPs), however, are always a focused concern for a doctor or a scientist. At present, there is almost no systemic evaluation standard or testing methods of safety for nanoparticles. In this study, two kinds of hydroxylapatite, (HAP) NPs with different particle sizes were selected. A number of biocompatibility tests in vivo or in vitro were conducted. They were cytotoxicity (MTT assay), genotoxicity (Ames, Mouse Lymphoma Mutagenesis Assay), and systemic toxicity (Acute and Subacute). The results indicated that, under the concentration of 100 mg/L, both HAP NPs could cause significant inhibition of cell growth. The size of NPs might have close tie with cell response. The mutagenic test in vitro was negative in this study. Histopathological findings showed that both kinds of HAP NPs could induce pseudotubercles in lung. Moreover, smaller size of nanoparticles resulted in a vacuolar degeneration of nephric tubule epithelium at 7 days post-intraveneous injection. The results implied that the size of NPs might play an important role in the biocompatibility of the materials. The kidney might be the main organ of discharge of nanoparticles from body.