Direct force measurements between two mica surfaces in aqueous electrolyte solutions over broad ranges of LaCl3 concentrations and pH values were carried out with a surface forces apparatus. Charge inversion on mica surfaces is detected once the LaCl3 concentration reaches a critical value. With the continual increase of LaCl3 concentrations, the mica surface will be overscreened by the counterions. It is demonstrated that the two mica surfaces may experience the jump-in contact even at high LaCl3 concentrations, which is seldom seen in monovalent salt solutions. The strong adhesion cannot be attributed to the van der Waals force alone, but should include the ion-ion correlation forces. Through adjusting the pH values in LaCl3 solutions, the ion-ion correlation force can be evaluated quantitatively. These results provide important insight into the fundamental understanding in the role of ion-ion correlations in ion screening mechanism and interactions between charged objects.

The structure profiles and physical properties of the adsorbed water film on a mica surface under conditions with different degrees of relative humidity are investigated by a surface force apparatus. The first layer of the adsorbed water film shows ice-like properties, including a lattice constant similar with ice crystal, a high bearing capacity that can support normal pressure as high as 4 MPa, a creep behavior under the action of even a small normal load, and a character of hydrogen bond. Adjacent to the first layer of the adsorbed water film, the water molecules in the outer layer are liquid-like that can flow freely under the action of external loads. Experimental results demonstrate that the adsorbed water layer makes the mica surface change from hydrophilic to weak hydrophobic. The weak hydrophobic surface may induce the latter adsorbed water molecules to form water islands on a mica sheet.

In this paper, the effects of annealing temperature on both radial supporting performance and axial flexibility of poly(L‐lactic acid) (PLLA) braided stents are studied. Stents are annealed at a series of temperatures ranging from 80 to 160°C for 1 h, then indicators of shaping effect, radial supporting performance, and axial flexibility are compared. Stents not annealed and annealed at 80°C cannot be completely shaped. In contrast, stents annealed at 100 to 160°C are well shaped showing radial shrinkage rate of 1.0 ± 0.2% and almost no axial elongation. The radial compressive force and axial force are gradually increased by 51.2% and 89.2%, respectively with the annealing temperature gradually increasing from 80 to 160°C, indicating that the radial supporting performance is improved but axial flexibility is weakened by a higher annealing temperature. Taken together, PLLA braided stents can be annealed at 100 to 120°C to obtain sufficient radial force and lower axial force simultaneously for clinical applications. Moreover, tensile test, X‐ray diffraction, and differential scanning calorimetry are performed for monofilaments annealed at different temperatures to further explore the effect mechanism of annealing temperature on the mechanical properties of stents. This study may provide helpful suggestions for the manufacture of biodegradable braided stents.

Mechanical properties of poly(L-lactic acid) (PLLA) braided stents are closely related to the performance of the weaving monofilaments. In this study, the PLLA monofilaments were prepared via solid-state drawing (SSD) method and annealed with different constraint methods. Experimental results show that mechanical properties of fabricated monofilaments have been greatly improved by SSD method. With the constrained annealing method, mechanical properties can be further improved while negative effect is shown with unconstrained annealing method. The physicochemical properties of PLLA monofilaments were further characterized, and it is found that the draw ratio of SSD and annealing method can affect the crystallinity and crystallite size, which can directly influence the mechanical properties of the monofilaments.This work tells that it is necessary to consider collectively the effects of SSD method and annealing method of the monofilaments in order to obtain optimized mechanical properties of the braided stents.

In this article, the mechanical properties and physicochemical indexes of a poly(L-lactic acid) (PLLA) braided stent during in vitro degradation in bile for 270 days are characterized to investigate its applications in treating biliary strictures. The PLLA braided stent always maintains structural integrity well macroscopically without cracks or pits microscopically. During degradation, radial compressive force and chronic outward force are increased by 25.1% and 14.3%, respectively, bending stiffness and axial force are increased by 3.0% and 4.9%, respectively. These results indicate that PLLA braided stent can keep sufficient radial force and good bending performance during degradation in bile for 270 days, showing promising application prospects in treating severe benign biliary strictures demanding support for over half a year. Furthermore, almost no mass loss and 42.4% attenuation in molecular weight indicate the degradation mechanism of bulk erosion without obvious autocatalysis, causing decrease in bile pH. Degradation rate constant is 0.002 day À1 and crystallinity is increased by 8%. These results explain that PLLA braided stent can keep high mechanical properties during degradation in bile for 270 days, which is in the early stage of degradation. This study may bring new ideas into the applications of PLLA braided stents in treating biliary strictures.