Superhydrophobic surfaces for extreme environmental conditions

Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellency behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired.

Inspired by nature, microtextured and nanotextured surfaces have demonstrated unique droplet repellent properties (1), which are beneficial for self-cleaning (2), antiicing (3), and condensation enhancement (4). For many practical applications, repelling an impacting water droplet is important. Much work has been performed to understand how surface topography and composition stabilize the Cassie–Baxter wetting state (5), a prerequisite for high droplet repellency, under static (6, 7) and dynamic (8–24) conditions to preclude transitioning to the Wenzel wetting state (25). To prevent the Cassie–Baxter to Wenzel wetting-state transition, hereafter defined as impalement, the capillary (antiwetting, surface property) pressure must exceed the wetting (droplet) pressure (9, 17). In previous research, the latter has been attributed to the dynamic pressure (17), effective water hammer (10, 11, 16, 23), and deformation of the droplet by the compressed air layer leading to a ring-shaped pressure maximum (13, 26).It is established that the use of hierarchical surface texture and low-surface energy coatings are key components for achieving liquid repellency and preventing impalement (13, 15, 16, 27). Much of this understanding is based on work conducted under ambient conditions; however, work on droplet mobility that departs from ambient environmental conditions (28–31) [i.e., substrate cooling (13, 32), supercooled droplet impact (33, 34), ambient pressure reduction (35–37), and droplet heating (38)] is yielding new insight into, and unveils new requirements for, the rational design of superrepellent surfaces. Mechanisms for the loss of superrepellent behavior include condensation-based impalement in the presence of hot vapor (warm droplets) (28, 38), increased droplet viscosity (33), and rapid recalescent freezing inhibiting droplet recoil (cold droplets) (32, 34). Therefore, in addition to wettability, it is necessary to investigate important aspects such as nonstandard atmosphere environments and nucleation (3, 39, 40)—which affect the intervening gas-layer dynamic state during droplet impact and enhance droplet–substrate adhesion—to enable surface texture tailoring to counter such effects to preserve superhydrophobicity (38, 41–46). While the effects of droplet and environmental temperature (hot or cold) on superhydrophobicity for impacting droplets are being better understood, research into the effect of the environmental gas pressure, an equally important counterpart, is comparatively scant (35–37), as is the combined effect with humidity. Both are very important for defining the thermodynamic state of the intervening gas layer during droplet impact, necessary to determine its behavior. Previous work has demonstrated that decreasing the environmental pressure influences the droplet impact dynamics on smooth and rough surfaces (e.g., prompt vs. thin-sheet splashing) (26, 47–50), which may alter impact and recoil dynamics on superhydrophobic surfaces; however, this remains to be seen.Here, we examine, experimentally and theoretically, the combined effects of reducing environmental pressure and varying humidity on droplet impact and recoil from superhydrophobic surfaces, identify impalement mechanisms, and armed with this knowledge, rationally nanoengineer robust superhydrophobic surfaces that can repel impacting droplets across a wide range of environmental conditions. We demonstrate that the likelihood of impalement on textured surfaces increases as the ambient pressure decreases and provide rationales to explain this. Additionally, through variation of the relative humidity, we report, and theoretically underpin, a hitherto unknown mechanism for wetting-state transition through supersaturation of, and subsequent condensation within, the air layer resulting from the pressure increase beneath an impacting droplet. Finally, we demonstrate an alternative coating capable of resisting impalement within the working envelope experimentally explored, based on our accrued knowledge. We believe that our observations will have profound implications for all applications of superhydrophobicity in both low-pressure, such as those involving ice accretion on aircraft through superior repellency of supercooled drops, and naturally humid environments, including for self-cleaning materials such as textiles.     

Numbers needed to treat or harm and likelihood of being helped versus harmed for fremanezumab in patients who had prior inadequate response to two to four classes of migraine preventive medications: A post hoc analysis

Objective

This study aimed to determine the number needed to treat (NNT), number needed to harm (NNH), and likelihood of being helped or harmed (LHH) in a post hoc analysis of the phase 3b FOCUS trial.

Background

Fremanezumab, a humanized monoclonal antibody that selectively targets calcitonin gene–related peptide (CGRP), has demonstrated efficacy, tolerability, and safety in adults with episodic migraine (EM) or chronic migraine (CM), with documented previous inadequate response to two to four classes of migraine preventive medications.

Methods

In the 12-week double-blind period of the FOCUS study, patients were randomized (1:1:1) to quarterly fremanezumab, monthly fremanezumab, or matched monthly placebo. NNT was based on responder analysis, defined as ≥50% reduction in monthly average number of migraine days at 12 weeks. NNH was based on discontinuations due to adverse events (AEs).

Results

Among patients with CM (n = 509), response rates and discontinuation rates were 27% (45/169) and 0 for quarterly fremanezumab, 29% (50/173) and 2% (3/173) for monthly fremanezumab, and 8% (13/167) and <1% (1/167) for placebo, respectively. These results translated to NNTs of 5.3 and 4.7, NNHs of 1000 and 88, and LHHs of 188 and 19 for quarterly and monthly fremanezumab, respectively. Among patients with EM (n = 328), response rates were 47% (50/107) for quarterly fremanezumab, 43% (47/110) for monthly fremanezumab, and 10% (11/111) for placebo. Discontinuation rates were <1% (n = 1) in all three groups. These results translated to NNTs of 2.7 and 3.0, NNHs of 1000 and 1000, and LHHs of 368 and 328 for quarterly and monthly fremanezumab, respectively.

Conclusions

The NNT, NNH, and LHH for quarterly and monthly fremanezumab compare favorably with those for traditional oral preventive medications, including topiramate, valproate, and propranolol.     

New interstitial HDR brachytherapy technique for prostate cancer: CT based 3D planning after transrectal implantation.

We have developed a new interstitial HDR brachytherapy technique for the treatment of prostate cancer using CT based 3D planning after transrectal implantation of four non-parallel needles. CT based needle reconstruction, target definition, evaluation and documentation, including DVHs and 3D imaging, is a feasible, safe and well tolerated treatment concept.     

Evaluation of a TG-43 compliant analytical dosimetry model in clinical 192Ir HDR brachytherapy treatment planning and assessment of the significance of source position and catheter reconstruction uncertainties

A simple, time efficient, analytical model incorporating heterogeneities and body dimensions around a point 192Ir source is generalized for accurate dosimetry around commercially available 192Ir brachytherapy sources. The generalized model was verified in dosimetry of a clinical 192Ir high dose rate prostate monotherapy application, involving 16 catheters and 83 source dwell positions, through comparison with corresponding treatment planning system data. The computational time efficiency and accuracy of the proposed model allowed the assessment of the impact that uncertainties in source dwell positions and catheter reconstruction may have on dose distributions, and how these could potentially affect the clinical outcome. Results revealed that a 0.1 cm catheter reconstruction uncertainty and a 0.15 cm source position uncertainty along the catheter lead to a dose uncertainty of less than 2% for doses lower than 200% of the prescribed dose, reaching up to 5% for points lying in close proximity to the catheters. These uncertainties were found to have no impact (less than 1%) on dose volume histogram results of both the planning target volume and the urethra. A catheter reconstruction uncertainty as high as 0.2 cm results in a dose uncertainty greater than 2%, reaching up to 9%, only for points inside the 150% contour. However, even in this case, the impact on dose volume histogram calculations is less than 3%.     

CT guided interstitial high dose rate brachytherapy for recurrent malignant gliomas

This paper describes the technique and preliminary results of high dose rate (HDR) interstitial brachytherapy for recurrent grade III and grade IV gliomas. Although in the initial treatment of malignant gliomas brachytherapy has been shown to give better results than external beam therapy, this has previously always been with low dose rate (LDR) brachytherapy. Stereotactic frames are used for interstitial LDR brachytherapy but a CT image-guided technique does not require such a frame. The survival rates for our initial 53 patients do not significantly differ from LDR results. However, using HDR there are several advantages, including a much shorter treatment time with HDR than LDR and better patient comfort. HDR also allows better individualized optimization of the treatment than LDR.     

Polymer gel dosimetry using a three-dimensional MRI acquisition technique

In this work, three-dimensional (3-D) MRI techniques are employed in N-Vinylpyrrolidone-Argon-(VIPAR-) based polymer gel dosimetry. VIPAR gels were irradiated using a Nucletron microSelection 192Ir HDR brachytherapy remote afterloading system with single source dwell position and intravascular brachytherapy irradiation protocols. A single VIPAR gel and a single irradiation are adequate to obtain the full calibration curve needed. The 3-D dose distributions obtained with the 3-D MRI method were found to be in good agreement with the corresponding Monte Carlo calculations, for brachytherapy and intravascular irradiations. The method allows the reconstruction of isodose contours over any plane, with increased spatial resolution and accuracy following a single MR acquisition. VIPAR gel measurements using a 3-D MRI readout technique can be of particular use in the experimental dosimetry of brachytherapy sources, as well as for dose verification purposes when complex irradiation regimes and three-dimensional dose gradients are investigated.     

Navigation system for interstitial brachytherapy.

PURPOSE: To develop a computed tomography (CT) based electromagnetic navigation system for interstitial brachytherapy. This is especially designed for situations when needles have to be positioned adjacent to or within critical anatomical structures. In such instances interactive 3D visualisation of the needle positions is essential. METHODS AND MATERIALS: The material consisted of a Polhemus electromagnetic 3D digitizer, a Pentium 200 MHz laptop and a voice recognition for continuous speech. In addition, we developed an external reference system constructed of Perspex which could be positioned above the tumour region and attached to the patient using a non-invasive fixation method. A specially designed needle holder and patient bed were also developed. Measurements were made on a series of phantoms in order to study the efficacy and accuracy of the navigation system. RESULTS: The mean navigation accuracy of positioning the 20.0 cm length metallic needles within the phantoms was in the range 2.0-4.1 mm with a maximum of 5.4 mm. This is an improvement on the accuracy of a CT-guided technique which was in the range 6.1-11.3 mm with a maximum of 19.4 mm. The mean reconstruction accuracy of the implant geometry was 3.2 mm within a non-ferromagnetic environment. We found that although the needles were metallic this did not have a significant influence. We also found for our experimental setups that the CT table and operation table non-ferromagnetic parts had no significant influence on the navigation accuracy. CONCLUSIONS: This navigation system will be a very useful clinical tool for interstitial brachytherapy applications, particularly when critical structures have to be avoided. It also should provide a significant improvement on our existing technique.