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Atomic Force Microscopy-Based Characterization and Design of Biointerfaces

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Atomic Force Microscopy-Based Characterization and Design of Biointerfaces REVIEWS Atomic force microscopy-based characterization and design of biointerfaces David Alsteens1, Hermann E. Gaub2, Richard Newton3, Moritz Pfreundschuh3, Christoph Gerber4 and Daniel J. Müller3 Abstract | Atomic force microscopy (AFM)-based methods have matured into a powerful nanoscopic platform, enabling the characterization of a wide range of biological and synthetic biointerfaces ranging from tissues, cells, membranes, proteins, nucleic acids and functional materials. Although the unprecedented signal-to-noise ratio of AFM enables the imaging of biological interfaces from the cellular to the molecular scale, AFM-based force spectroscopy allows their mechanical, chemical, conductive or electrostatic, and biological properties to be probed. The combination of AFM-based imaging and spectroscopy structurally maps these properties and allows their 3D manipulation with molecular precision. In this Review, we survey basic and advanced AFM-related approaches and evaluate their unique advantages and limitations in imaging, sensing, parameterizing and designing biointerfaces. It is anticipated that in the next decade these AFM-related techniques will have a profound influence on the way researchers view, characterize and construct biointerfaces, thereby helping to solve and address fundamental challenges that cannot be addressed with other techniques. Surfaces at which tissues, microorganisms, cells, viruses and multiparametric characterization of biointerfaces or biomolecules make contact with other natural or syn- in their native state or in an environment that simu- thetic materials are termed biointerfaces. Understanding lates physiological conditions, which is of particular and manipulating the sensing and interactions that occur importance to understand how tissues, cells and bio- at biointerfaces is an enterprise common to a host of sci- molecules function. Ideally, this information should be 1Institute of Life Sciences, entific fields spanning materials science to medicine, sys- provided from the microscopic to the (sub-)nanoscopic Université catholique de tems to synthetic biology, plant biology to pathology, and scale. Furthermore, it is of great importance to be able Louvain, Croix du Sud 4–5, oncology to the study of the origins of life1,2. Biointerfaces to directly modify the morphological, physical, chemi- bte L7.07.06., 1348 occur between cells and their surroundings, such as the cal and biological properties of biointerfaces at similar Louvain-la-Neuve, Belgium. 2Applied Physics, Ludwig- extracellular matrix (ECM), between populations of cells, resolution to that at which they can be imaged. Maximilians-Universität and between biotic and abiotic elements of engineered In 1986, atomic force microscopy (AFM) was invented Munich, Amalienstrasse 54, systems3,4. Biointerfaces are relevant over a wide range to contour non-conducting solid-state surfaces at atomic 80799 München, Germany. of length- and timescales, and their study is amenable to resolution by raster scanning a molecularly sharp stylus 3 Department of Biosystems a similarly diverse range of approaches, whether the aim over the surface5,6. Shortly thereafter, researchers began Science and Engineering, Eidgenössische Technische is to dissect fundamental physical and chemical mecha- to consider the AFM tip as a nanotool that allows the Hochschule (ETH) Zürich, nisms, to unravel functional significance or to directly imaging and manipulation of both living and non- Mattenstrasse 26, manipulate interfaces. living matter from the atomic to the microscopic scale7,8. 4058 Basel, Switzerland. Our understanding of biointerfaces has been greatly The elegant simplicity of AFM allowed users to estab- 4Swiss Nanoscience Institute, University of Basel, assisted by the development of equipment that probes lish various imaging modes optimized for the surface Klingelbergstrasse 82, their structural and functional properties at microscopic, of tissues, cells, viruses, proteins, nucleic acids and bio- 4057 Basel, Switzerland. or even nanoscopic, molecular resolution. Powerful materials9–12. In addition, imaging biointerfaces from the Correspondence to D.J.M. techniques have been established that find application microscopic to (sub-)nanoscopic scale at unprecedented [email protected] in the imaging or characterization of the physical, chem- signal-to-noise ratio, the AFM can be used to simulta- 5 doi:10.1038/natrevmats.2017.8 ical and biological properties of biointerfaces . However, neously quantify and map their physical, chemical or Published online 14 Mar 2017 relatively few methods allow the simultaneous imaging biological properties13–16. Examples of multiparametric NATURE REVIEWS | MATERIALS VOLUME 2 | ARTICLE NUMBER 17008 | 1 ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. REVIEWS imaging include measuring the binding of ligands to the scanning AFM stylus and the AFM topographs show receptors in real time, assessing the action of antibiotics the underlying rigid cellular architecture onto which the on bacteria, quantifying interactions between molecules membrane has been pressed. In the early days of AFM, and cell and tissue surfaces, and contouring the free- the user had to frequently adjust imaging forces and con- energy landscape of biomolecular reactions at inter- ditions to avoid the deformation of soft heterogeneous faces12,16. Moreover, AFM can be used as a nanoscopic biointerfaces, but modern AFM imaging modes contour toolbox that brings the molecular and cell biological lab- biological systems with sufficiently precise force control oratory to the stylus17,18 and directs molecular interac- to avoid sample deformation or destruction12,16. tions at biointerfaces. The AFM stylus has been used, for example, to build up 3D scaffolds with molecular preci- Observing biomolecular systems at work. Many AFM sion19,20, to control and direct enzymatic reactions21 or cell imaging modes have been developed. In most of them, division22, and to sculpt and functionalize biointerfaces23 the stylus is scanned over a sample while the cantilever used to guide cellular behaviour and tissue formation3. height is adjusted to avoid excessive force between the Insight and engineering possibilities gleaned from differ- stylus and sample (FIG. 1b). Plotting the height of the canti- ent AFM applications provide complementary perspec- lever for every pixel scanned results in a topo graphy that tives that are vital for building a full understanding of can approach (sub-)nanometre resolution for a range biointerfaces and how to engineer them. Many excellent of native biological systems, including mammalian reviews report the unique possibilities AFM offers to and bacterial cells15, cellular and synthetic membranes, study biological problems. However, so far they are all viruses25, fibrils, nucleic acids26,27, or water-soluble28 and rather specialized and focus on individual applications. membrane29,30 proteins (FIG. 1c–h). Recording time-lapse In addition, some of them date back as many as ten or topographs allows the molecular machinery of cells to be more years. Thus, newcomers and established scientists observed directly at work24. Studies report the observa- working on the characterization and design of biointer- tion of enzymatic subunits of ATP synthase31, commu- faces have to search through these publications to find nication channels32, pore-forming proteins33,34, toxins35, the information needed. In this Review, we provide an light-driven proton pumps36, potassium channels37,38, up-to-date overview of the most promising AFM-based membrane protein diffusion39,40 and motor proteins41 in techniques that can be used to image and characterize action. The process of fibrillar42,43 and filamentous44,45 biointerfaces of various origins, report the use of AFM- growth, and of cellular fibrillogenesis and remodelling of based biosensors to detect biomolecular reactions in real ECM proteins46,47 have also been recorded in time-lapse time, and conclude with AFM-based techniques that topographs. The spatial resolution achieved in these topo- allow the spatiotemporal analysis, manipulation and graphs strongly depends on the tip radius, the mechani- design of biointerfaces. cal properties and roughness of the sample, and the force applied to the AFM stylus. Basic principles of AFM imaging Invented to contour surfaces in air under ambient con- Common AFM modes for imaging biointerfaces. The ditions, AFM uses a cantilever with a molecularly sharp most commonly used AFM imaging mode is the contact stylus at the free end to raster scan and contour the sur- mode, in which a stylus is scanned across a surface while face of a sample6. Such contouring of a sample, which applying a constant force. The physical principle by is always corrugated on the molecular scale, deflects which it works is analogous to the operation of a record the cantilever and changes the position of a laser beam player turntable (FIG. 1a,b). A drawback is that a stylus reflected from the back of the cantilever onto a position- scanning across a surface can generate lateral forces that, sensitive photodiode (FIG. 1a). This information is read in many cases, are sufficient to deform or displace a soft by a feedback system, whereby the vertical distance sample. Considerable expertise is required to mitigate (height) between the stylus and sample adjusts as a con- this problem while imaging soft biological samples by sequence of the force that is measured between them. contact-mode
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