AVS2018 Session BI-ThA: Biolubrication and Wear / Women in Bio-surface Science

The coming end of earth's fossil energy is pressing humanity to develop more efficient and environmentally friendly technologies. Control of wear and fatigue of machine parts has become one of the most important field of research to meet the outstanding energy crisis the world is currently facing. The design of lubricating fluids able to protect surfaces against wear and high friction has been one the several tools used by engineers to improve machines’ life time and decrease energy consumption. Inspired by many different biological systems that can resist fatigue wear for decades such as our synovial joints, different coating/lubricating technologies involving polymer brushes either in their molecular form or grafted on the surface have emerged. All these strategies require the lubricating or wear protecting molecules to be strongly anchored to the surfaces in order to avoid close contact between the surfaces. Strong anchoring of molecules on surfaces requires a good knowledge of the chemistry and the structure of the surface which complicates dramatically the translation of these technologies towards industrial settings.

The “fly-fishing” and breaking of single molecular bonds to study their properties has been extensively studied via Atomic Force Microscopy (AFM) and optical tweezers. A good example for this are various ligand-receptor bonds or surface to molecule bonds such as the gold-amine bond, for which a free energy of ~ 37 k_BT has been determined. The experimental setup and design has evolved over the years, and so have the technology and analysis strategies involved. In the last 15 years, using Jarzynski‘s equality emerged as a powerful theoretical tool for estimating interaction free energies via the analysis of non-equilibrium work distributions from single-molecule pulling experiments with optical tweezers and AFM. [1] However, some of the questions remain the same and others appear as the field becomes broader. For example, what happens when the chemical model used to connect the probe with the interacting surface and head groups is varied? We recently tested the variation of linker lengths and changing pulling speeds [2] and found strong correlations that confirm the predictions of bias in such experiments by Gore et al. [3] in 2003. In particular, the longer the length of the polymeric chain, the more work dissipates during the retraction of the tip, and so does in turn the estimated ∆G₀, which leads to an increasing bias between the average values and those calculated using Jarzynski’s equality. This is also reflected in the broadening of work distributions when using the same sample size. Longer polymeric chains show no convergence, unless millions or even billions of events were used. With this order of magnitude in mind, we started our very own “pitch drop experiment”: an ambitious project which aims to collect an ever-increasing number of single-molecular force runs for a single system, which will allow us to directly and step-by-step further evaluate equations, work-distributions, convergence behavior, and expected biases in single-molecule experiments. This work will continue along the PhD times of many students - first non-converged results will be discussed in detail and compared to systems that are well converged. Part of this mammoth task is the development of an automated single-molecule recognition algorithm that is capable of distinguishing with high reliability very low work single-molecule events from thermal noise. Some of our advances in this direction will be discussed in detail as well.

References:

[1] Jarzynski, C., JSMTE 2004, 2004 (09), P09005.

[2] Moreno Ostertag, L.; Utzig, T.; Klinger, C.; Valtiner, M., Langmuir 2018, 34 (3), 766-772.

[3] Gore, J.; Ritort, F.; Bustamante, C., PNAS 2003, 100 (22), 12564-12569.

Bio and aqueous applications of ionic liquids (IL) such as catalysis in micelles formed in aqueous IL solutions, lubrication or extraction of chemicals from biologic materials rely on surface-active and self-assembly properties of ILs. Here, we discuss qualitative relations of the interfacial and bulk structuring of water-soluble and highly surface-active ILs on chemically controlled surfaces over a wide range of water concentrations using both force probe and X-ray scattering experiments. Our data indicate that IL structuring evolves from surfactant-like surface adsorption at low IL concentrations, to micellar bulk structure adsorption above the critical micelle concentration, to planar bilayer formation in ILs with <1 wt % of water and at high charging of the surface. Interfacial structuring is controlled by mesoscopic bulk structuring at high water concentrations. Surface chemistry and surface charges decisively steer interfacial ordering of ions if the water concentration is low and/or the surface charge is high. We also demonstrate that controlling the interfacial forces by using self-assembled monolayer chemistry allows tuning of interfacial structures. Both the ratio of the head group size to the hydrophobic tail volume as well as the surface charging trigger the bulk structure and offer a tool for predicting interfacial structures. Based on the applied techniques and analyses, a qualitative prediction of molecular layering of ILs in aqueous systems is possible. Potential applications in biomedical applications will be discussed.

Selenium (Se) and Tellurium (Te) are two elements under-utilized in medicinal treatments that naturally occur in the human body. Selenium is a bioessential element that exists as a micronutrient throughout most biological systems. In mammalian species, selenium is found in the form of selenocycteine, an amino acid found in selenoproteins. Selenoproteins play an important role in cell metabolism and is an active participant in anti-oxidant glutathione peroxidase mechanism which aids in DNA synthesis. Given selenium’s crucial role within biological functions, recent efforts have investigated the antimicrobial properties of selenium as a means to reverse, suppress or prevent the development biofilms. Tellurium, belonging to the same family as oxygen, sulphur and selenium, has been far less studied for its bioactivity. In part, this is due to its classification of being a non-essential biological element. However, recent evidence points to the possible existence and role in biological activity, abet to a lesser extent than selenium. Given that Selenium (Se), a bioessential element, and tellurium (Te), its related analog, are under-utilized elements in the medicinal libraries of antimicrobial treatments, recent research on these elements reveals that they may have numerous therapeutic applications beyond antimicrobial effects including for anti-inflammatory, anti-fouling and anti-cancer treatments. The focus of the work has been to develop novel nano-alloys composed of Se and Te by bio-friendly and chemical free synthesis methods and to evaluate their antimicrobial effects in conjunction with complementary studies on their toxicity against normal cells. Using nano-alloy formulations of these elements will form the basis of a new type of nature-inspired microbial prevention. We demonstrate that the Seand Te nano-alloys provide a reduced toxicity to normal cells while providing enhanced therapeutic efficiency of these compounds towards biofilm inhibition including on surfaces. The short-term impact of this work will provide novel approaches to inhibiting biofilm formation. The long-term impact of this work will provide the basis for treatment of some difficult to treat nosocomial infections caused by Staphylococcus aureus, Pseudomonas aeruginosa, E. coli and Candida albicans.

In 1978, the Worth Health Organization's Alma Ata Declaration asserted individuals' "right and duty to participate individually and collectively in the planning and implementation of their health care". The expansion of these policies began in the 1980s and by the 1990s, social movements across the world demanded greater public accountability and the inclusion of regular citizens in the decision-making process. Today, patient-centered healthcare has continued to progress, and in 2009 a new definition was made by the president of the Institute for Healthcare Improvement Donald Berwick "The experience (to the extent the informed, individual patient desires it) of transparency, individualization, recognition, respect, dignity, and choice in all matters, without exception, related to one's person, circumstances, and relationships in health care". These ideas have transformed over time to become “patient-centered design.” In patient-centered design, the focus is to redefine how people experience healthcare by focusing on their needs. The focus of the design is on the wants, needs, and skills of the products’ end-users, including patients, doctors, nurses, caretakers, and others.

Issues such as the rise of antibiotic resistance highlight the need for constant innovation in the field of point of care (POC) microbial diagnostics. Current approaches that do not require the use of energy for storage or detection are hindered by low sample concentration and adhesion challenges which arise when handling these often “sticky” organisms. To overcome these limitations, we combine two approaches in complex analyte handling: infused polymers, which provide a universal anti-adhesion surface against microorganisms, and paper-based microfluidics, which present a lightweight, rugged, and low-cost platform for POC diagnostics, to create paper-supported liquid-infused polymer surfaces. The results showed that the liquid-infused system could be created on multiple different types of paper, including commercially-available silicone release paper which is already manufactured at an industrial scale. Folding the paper liquid-infused surfaces produced chambers which could be used to concentrate the organisms into single point via evaporation with >60% efficiency, compared to <20% efficiency for controls without an infused polymer layer. Sample containing bacteria could be moved from point to point without the loss of cells due to surface adhesion. Finally, integrated proof-of-principle tests showed that the use of liquid-infused surfaces to handle bacteria in this way resulted in positive colorimetric indication of Staphylococcus aureus significantly faster than control surfaces. These results demonstrate the use of paper-supported liquid-infused surfaces for improved microorganism handling in POC diagnostics.

Controlling and understanding the interaction behavior of biomolecules with polymer surfaces is one key aspect for the design of new biomaterials. Thin hydrogel coatings offer a huge variety of possibilities to tune physical and chemical surface properties and to create functional biocompatible interfaces supported on a substrate material. Among polymer architectures studied for biocompatible systems are dendritically structured polycations decorated with oligosaccharide shell [1, 2], and zwitterionic copolymers based on phosphorylcholine groups [3]. We prepared two types of thin hydrogel films: 1) based on dendritic polymer core-shell nanoparticles of hyperbranched poly (ethylene imine) (PEI) with maltose shell and 2) based on the statistical copolymer poly[(2-methacryloyloxyethyl phosphorylcholine)-co-(glycidyl methacrylate)] (MPC-co-GMA). For both surface types swelling and the interaction with selected biomolecules from small drug molecules to proteins and phospholipids was analyzed quantitatively by in-situ spectroscopic ellipsometry and quartz crystal microbalance with dissipation monitoring. The adsorbed amount of biomolecules was correlated to changes in hydration, thickness and viscoelastic properties of the films to obtain new insights into the specific interaction processes.

References

[1] M. Warenda, K.-J. Eichhorn, B. Voit, D. Appelhans et Al., Macromol. Rapid Commun. 33 (2012) 1466.

[2] D. Wrobel, D. Appelhans, B. Voit et Al., Colloid. Surf. B: Biointerfaces 152 (2017) 18.

[3] A. S. Münch, K.-J. Eichhorn, P. Uhlmann et Al., J. Mater. Chem. B 6 (2018) 830.