Biofilm Removal by Reversible Shape Recovery of the Substrate

Bacteria can colonize essentially any surface and form antibiotic resistant biofilms, which are multicellular structures embedded in an extracellular matrix secreted by the attached cells. To develop better biofilm control technologies, we recently demonstrated that mature biofilms can be effectively removed through on-demand shape recovery of a shape memory polymer (SMP) composed of tert-butyl acrylate (tBA). It was further demonstrated that such a dynamic substratum can sensitize the detached biofilm cells to antibiotics. However, this SMP can undergo shape change only once, limiting its application in long-term biofilm control. This motivated the present study, which aimed to prove the concept that biofilm can be effectively removed by repeated on-demand shape recovery. Reversible shape memory polymers (rSMPs) containing poly(ε-caprolactone) diisocyanatoethyl dimethacrylate (PCLDIMA) of varying molecular masses and butyl acrylate (BA) as a linker were synthesized by using benzoyl peroxide (BPO) as a thermal initiator. By comparison of several combinations of PCLDIMA of different molecular masses, a 2:1 weight ratio mixture of 2000 and 15000 g/mol PCLDIMA was the most promising because it had a shape transition (at 36.7 °C) close to body temperature. The synthesized rSMP demonstrated good reversible shape recovery and up to 94.3 ± 1.0% removal of 48 h Pseudomonas aeruginosa PAO1 biofilm cells after three consecutive shape recovery cycles. Additionally, the detached biofilm cells were found to be 5.0 ± 1.2 times more susceptible to 50 μg/mL tobramycin than the static control.

Heating of Polymer Films Induced by HIFU: Study of Acoustic and Thermal Effects

This article presents the study of the thermal and acoustic effects occurring in polymer samples of different thicknesses receiving high-intensity focused ultrasound (HIFU). Whereas the heating mechanisms in polymer plates immersed in water are well known, the physical mechanisms enabling the heating of polymer films using a solid waveguide transducer remain not fully understood. A coupled acoustothermal finite-element simulation is conducted to model the sound field and the heat generation inside polymer samples of different thicknesses. To validate the acoustic model, the acoustic particle velocities at the transducer waveguide tip are measured by vibrometry and compared with the simulation results. The heating effects in the samples are monitored using an infrared thermography system and compared with the measured particle velocities and with the acoustic and thermal simulation results. Correlations among particle velocities, sound intensity, and polymer heating are investigated. A qualitative and quantitative correlation between the simulation and the measurement results is found. Experiments show that the heating effects depend on the sample thickness. In samples thinner than 1 mm, the maximum temperature is lower than the one observed in samples thicker than 1 mm but rises faster. The simulation shows that the sound intensity in polymer samples thinner than 1 mm decreases sharply with the decrease in the thickness of the sample. This study contributes to the understanding of the challenges in heating thin polymer films by HIFU in a dry environment, using only a force to couple the transducer to the polymer films.

HIFU induced particles redistribution in polymer matrix via synchrotron radiation X-ray microtomography

High-intensity Focused Ultrasound (HIFU) was used to stimulate the embedded copper sulfate (CuSO₄) particles to release from the crosslinked poly (methyl methacrylate-co-butyl acrylate) copolymer solid matrix. In order to better understand the ultrasound release mechanism for drug/polymer delivery systems, the synchrotron radiation X-ray computed microtomography (SR-CT) was used to non-destructively investigate the structure of drug/polymer delivery systems after different HIFU treatment time. For the first time, we clearly demonstrate that ultrasonic waves can overcome the constraints of the polymer chain and drive the filler to move from the strong region to the weak region in the solid polymer matrix, thus resulting in a change in distribution of the filler in solid polymers. This result also demonstrates that SR-CT is a powerful technique which can be used to quantitatively study the 3D structure of fillers/polymers composite as it can take a broader and overall view than the conventional localized two-dimensional analysis method such as SEM, TEM.

High-intensity focused ultrasound selective annealing induced patterned and gradient crystallization behavior of polymer

High-intensity focused ultrasound (HIFU) was developed as a spatial selective annealing method to control the crystallization behavior and performance of polymer using amorphous polyethylene terephthalate (PET) as an example for demonstration. The spatial crystallization and morphological details of HIFU induced crystallization areas at the lamellar level and spherulite scale were studied by Micro-Focus hard X-ray diffraction, small angle X-ray scattering and optical microscopy. According to the distribution of crystallinity of PET, we can indirectly detect the history of thermal distribution of the ultrasonic focal point, which is hard to obtain by other methods. The crystallinity and the area of the crystalline region of PET sample increased with ultrasound power or irradiation time. Different from common crystalline structure of polymer materials, HIFU induced crystallinity of PET has a significant gradient distribution. The gradient crystal structure leads to a better mechanical performances, which can realize the good balance between toughness and strength. Ultrasound annealing, as a complement and development of the traditional annealing technology, has the characteristics of high efficient and spatial selectivity, showing great application prospect in post processing field.

Field responsive materials: photo-, electro-, magnetic- and ultrasound-sensitive polymers

Recent advances in field-responsive polymers, which have emerged as highly promising materials for numerous applications, are highlighted.

Focused ultrasound actuation of shape memory polymers; acoustic-thermoelastic modeling and testing

Controlled drug delivery (CDD) technologies have received extensive attention recently.

Investigations on the heating effect of PE-LD induced by high-intensity focused ultrasound

High-intensity focused ultrasound is widely applied in tissue treatment as well as for heating of solid polymer materials. Previous studies investigating the heating effect in polymer materials utilized sound transmission through water or other fluids at low HIFU power. In this study, the ultrasonic transducer possesses a solid sound conductor made of aluminum and a high HIFU power of above 100W was applied to heat solid PE-LD samples. Temperature measurements were performed by calibrated non-invasive infrared thermal imaging. A strong heating effect with heating above melting temperature and evaporation temperature within less than 1s of irradiation was observed. Furthermore, the acoustic coupling defined by the force applied by the ultrasonic applicator to the polymer material was found to be fundamental to induce the heating effect. This investigation reveals HIFU for new applications in the field of polymer processing.

Therapeutic-Ultrasound-Triggered Shape Memory of a Melamine-Enhanced Poly(vinyl alcohol) Physical Hydrogel

Therapeutic-ultrasound-triggered shape memory was demonstrated for the first time with a melamine-enhanced poly(vinyl alcohol) (PVA) physical hydrogel. The addition of a small amount of melamine (up to 1.5 wt %) in PVA results in a strong hydrogel due to the multiple H-bonding between the two constituents. A temporary shape of the hydrogel can be obtained by deformation of the hydrogel (∼65 wt % water) at room temperature, followed by fixation of the deformation by freezing/thawing the hydrogel under strain, which induces crystallization of PVA. We show that the ultrasound delivered by a commercially available device designed for the patient's pain relief could trigger the shape recovery process as a result of ultrasound-induced local heating in the hydrogel that melts the crystallized PVA cross-linking. This hydrogel is thus interesting for potential applications because it combines many desirable properties, being mechanically strong, biocompatible, and self-healable and displaying the shape memory capability triggered by a physiological stimulus.

Superior shape memory properties and microstructure evolution of poly(ether-b-amide12) elastomer enhanced by poly(ε-caprolactone)

While the recovery ratio of PEBA decreases marginally with PCL content, the maximum recovery stresses (σ_max) increase greatly despite of the deformation temperature, indicating an enhancement of the recovery stress.