This paper presents a research on the chloride penetration behavior of engineered cementitious composites (ECC) under sustained flexural loads. Three load levels, i.e. 30 %, 60 % and 75 % of the ultimate flexural load were used. Chloride diffusion depth and concentration profile were measured 30, 60 and 150 days after the specimen was exposed to NaCl solution and compared with pre-loaded specimens. Influence of the sustained local bending stress and microcracks were investigated. It shows that under sustained loads, the relationship between the surface chloride content and maximum normal tensile stress can be described using an exponential equation. A binary model was developed to explain the correlation among the chloride ion diffusion coefficient, maximum normal tensile stress and exposure time. Changes of capillary pore structure and phase compositions were measured using mercury intrusion porosimeter and X-ray diffraction, respectively. Unlike mortar, the fiber bridging of ECC helps with limiting crack width and thus the diffusion process, and the measured results were used to explain the observed penetration behavior of ECC. It is believed that the current study provides theoretical foundation for the durable design of the ECC/concrete composite structure.

Modern advanced minimally invasive surgery has been implemented for most of the significant gastrointestinal diseases. However, patients with coagulopathy or unresectable tumors cannot be cured by current treatment methods. Moreover, other existing medical devices for targeted drug release are too large to be applied in gastric endoscope because the diameter of the biopsy channel is smaller than 3 mm. To address it, in this work, we developed a piezoelectric single crystal ultrasonic transducer (the diameter was only 2.2 mm and the mass was 0.076 g) to produce acoustic waves, which could promote the drug release in the designed position of the digestive tract through an endoscope. It exhibited the electromechanical coupling coefficient of 0.36 and the center frequency of 6.9 MHz with the -6-dB bandwidth of 23%. In in vitro sonophoresis experiment, the gastric mucosa permeability to Bovine Serum Albumin increased about 5.6 times when the ultrasonic transducer was activated at 40 \text{V} {{\text {pp}}} and 60% duty ratio, proving that employment of this transducer could facilitate drug penetration in the gastric mucosa. Meanwhile, the permeability could be adjusted by tuning the duty ratio of the ultrasonic transducer. The corresponding sonophoresis mechanism was related to the acoustic streaming and the thermal effect produced by the transducer. In addition, the measured maximum power density was 128 mW/cm2 and the mechanical index of the ultrasonic transducer was 0.02. The results held a great implication for applications of the transducer for targeted drug release in the gastrointestinal tract.