X-Ray Studies of Biological Matter in Microfluidic Environments

Abstract

Biological systems such as cells and cellular components are governed by processes, which take place on nanometer to micrometer length scales. X-ray scattering, diffraction and imaging techniques are extremely well suited to study these processes as the spatial resolution extends well into the relevant length scales. At the same time, the investigation of physical and chemical properties and behavior of such systems requires well-defined and controllable sample environments. One successful way to establish such environments, including specified flow fields, concentration gradients and confinement regimes is by employing microfluidic technology tailored to the particular scientific question. This brief review focuses on microfluidic techniques that have been used to investigate biological matter by X-rays. In particular, we show how the characteristics of flow on the micron scale enable new scientific approaches as compared to macroscale experiments.

Scanning X-ray nano-diffraction on eukaryotic cells

“…However, the well-established microfluidic devices based on soft lithography [131,132], which are often used in combination with optical microscopy and are fabricated of polymers like polydimethylsiloxane (PDMS), bring about the problem of showing a high X-ray absorption and a strong background signal in the small-angle region [133]. A variety of X-ray compatible microfluidic devices based on different materials and fabrication techniques has been proposed in the past as reviewed by Köster and Pfohl [38]. In one example for a device design the channel geometry is defined by a thin film of a moldable polymer like PDMS and adhesive Kapton film is used as a window material [34,35].…”

“…Microfluidic devices, by contrast, provide an adaptable and well-defined sample environment allowing for sample manipulation by, e.g., changing the pH or the concentration of chemicals. A variety of microfluidic devices based on different materials and fabrication techniques has been proposed in the past to meet the requirements of X-ray experiments [34][35][36][37][38]. The specific demands of the studied objects impose further limitations on the device design and the material choice.…”

Brownian and advective dynamics in microflow studied by coherent X-ray scattering experiments

“…During recent years, the experimental repertoire has been extended to X-ray methods, which are emerging and powerful tools for studying soft and biological materials and offer a number of advantages: the possibility to focus X-rays to well below a micrometer offers the opportunity to probe microfluidic devices with high spatial resolution, while X-rays allow sample structures to be probed from the atomic up to the hundreds of nanometers range. These tools enable experimental strategies, such as analyzing the evolution of reactions, self-assembly processes in concentration gradients or the response of soft systems on flow and shear (Dootz et al, 2007;Martel et al, 2008;Kö ster & Pfohl, 2012;Toma et al, 2013;Trebbin et al, 2013;Weinhausen et al, 2014). The analysis of dynamical changes using X-rays is either done by examining changes of the scattering pattern in the course of the experiments (Merlin et al, 2011) or by spatially scanning the microfluidic device and thus transforming spatial into temporal changes (Graceffa et al, 2013).…”

“…In XPCS experiments, temporal intensity fluctuations of a speckle pattern, related to changes in the spatial arrangement of the scattering objects, are evaluated. X-rays can probe a large range of momentum transfers q and have a large penetration depth, which allows for probing a wide range of length scales and to analyze samples which are optically opaque (Leheny, 2012;Shpyrko, 2014). XPCS has been used for studying the dynamics of colloidal (Dierker et al, 1995;Burghardt et al, 2012;Westermeier et al, 2016), polymeric (Falus et al, 2005) and soft hybrid systems (Carnis et al, 2014;Herná ndez et al, 2015).…”

Hard x-ray nanofocusing by refractive lenses of constant thickness

“…To understand the function of nanoscale objects or nano-structured materials, such as organelles in their cellular environment, catalytic nano particles in industrial catalysts, or nano-electronic devices, it is crucial to understand their structure. Hard x-ray microscopy is ideally suited for this type of structure determination in terms of elemental composition, 1 chemical state, 2 and local atomic structure 3 with minimal sample preparation and inside of special sample environments, such as microfluidic cells 4 or chemical reactors. 5 The spatial resolution in x-ray microscopy is often limited by the x-ray optics.…”