I am pleased to introduce the Biointerphases In Focus issue on Bio Surface Analysis. This collection of work reveals the latest developments in surface analysis of biological and biomaterial interfaces. Some of the papers demonstrate advances in tools and novel methods. Others feature applications using traditional surface analysis tools to improve our understanding of interfaces of critical concern, including proteins at surfaces, biofilms, and biomaterial surfaces. It is my pleasure to highlight some of the advances toward a better understanding of the biointerface in this Editorial.
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has made significant strides toward analysis of tissue. Identification and localization of biological species are now possible due to developments in cluster ion sources, and due to the molecular specificity, and good spatial resolution of the technique. A review article of biomedical studies using TOF-SIMS imaging gives several examples of mapping organic compounds, such as drugs and lipids, directly on tissue sections from animals and from human biopsies (Bich, Touboul, and Brunelle). A second review article adds another dimension, depth, and features bioimaging in three dimensions. Model membrane systems, plant single cell, and tissue samples are presented, and the capabilities and challenges of the 3D imaging using TOF-SIMS are discussed (Fletcher). It must be noted, however, that the experimental methods call for thoughtful consideration due to intricacies of the biological systems and the sensitivity of TOF-SIMS. Two articles detail tissue sample preparation methods. One article shares lessons learned in sample preparation, data acquisition, and data analysis methods, and demonstrates imaging of mouse and human tissue for identification of specific biological features and functions (Gamble et al.). Another article assesses different sample preparation routes for mass spectrometric monitoring and imaging of lipids in bone cells via TOF-SIMS (Schaepe et al.).
Neutron Reflectometry (NR) is another powerful tool for studying soft material, including solid–liquid interfaces and in situ living tissue. Due to the penetrability of neutrons, NR is useful for probing thin films (5–5000 Å) at buried interfaces. The authors illustrate the potential of NR in addressing the structural properties of soft materials, from simple supported lipid membranes in static and dynamic conditions to the adherence of complex endothelial cells under liquid shear stress (Junghans et al.).
Several research groups demonstrate novel methods to better understand and quantify protein and biological species at surfaces. Protein interactions with material surfaces play a critical role in many biotechnological applications, including diagnostics and delivery of therapeutics. One group investigated protein coatings on nanoparticles and compared different approaches for determining the thickness of the protein layer and number of molecules at the surface. They found that x-ray photoelectron spectroscopy (XPS) and liquid-based particle sizing techniques produce consistent measurement results even though they are very different approaches (Belsey, Shard, and Minelli). Another group was able to quantitatively monitor binding events of DNA and protein targets to aptamer functionalized particles using flow cytometry. They demonstrated how processing steps such as annealing and binding history of particle-immobilized aptamers can affect subsequent binding activity (Dunaway et al.). In the area of microarrays, researchers used TOF-SIMS imaging and multivariate analysis to identify correlations between the surface chemistry of the polymer microarray and protein adsorption. They were able to identify chemical moieties that affected high or low protein adsorption, including those from the polymer backbone and side groups (Hook et al.). Another team investigated how peptides bind and interact with biological membranes. They used XPS and TOF-SIMS depth profiles to demonstrate that a proline-rich amphipathic cell-penetrating peptide known as sweet arrow peptide is located at the outer perimeter of the model lipid bilayer (Franz et al.). Finally, don't miss an elegant overview demonstrating that complementary methods are necessary to address the orientation, conformation, and accessibility of bioactive sites and to understand the structure–function relationships of adsorbed proteins. The authors used circular dichroism spectropolarimetry, amino-acid labeling/mass spectrometry, and bioactivity assays to synergistically assess adsorption-induced changes in protein bioactivity. The authors showed how changes in protein structure influence the enzymatic activity of hen egg-white lysozyme on various model substrates (Thyparambil, Wei, and Latour).
The control and understanding of biofilms have long been of interest to this community. It is often desired to minimize biofilms to prevent corrosion; however, taking advantage of microorganisms is of growing interest for applications including fuel cells and bioreactors. In all cases, it is important to understand the surface chemistry and biofilm structure as they affect growth, development, and activity of the microorganisms. One group investigated marine bacterial biofilms and correlated their chemical composition to the corrosion damage of steel underneath them using laser ablation and solvent capture by aspiration mass spectrometric imaging combined with metabolomics high-performance liquid chromatography mass spectrometry analysis and light profilometry (Brauer et al.). The authors claim the first report of ambient chemical and metabolomic imaging of marine biofilms on corroding metallic materials. The method may potentially lead to a greater understanding of the mechanisms involved in microbially influenced corrosion. Another group used a combination of spectroscopic (XPS), microscopic, and electrochemical techniques to evaluate how electrode surface chemistry influences morphological, chemical, and functional properties of biofilms (Artyushkova et al.). The study has implications with microbial fuel cells where bacterial metabolic processes combine with electricity production. The microbial fuel cells are promising candidates to produce energy during wastewater treatment. The authors found positively charged, highly functionalized, hydrophilic electrode surfaces were beneficial for growth of uniform biofilms, yielding the most efficient electron transfer. The substratum chemistry was found to affect not only primary attachment of the microbes but also subsequent biofilm development and bacterial physiology. Another group evaluated methods to clean surfaces used in a bioreactor (Fingerle et al.). They showed microstructured titanium surfaces are beneficial for biofilm growth, while still being fully restorable after biofilm contamination. Oxygen plasma cleaning was found to be a reliable method to fully clean the biofilm from the surfaces, while not affecting the titanium microstructure.
Surface analysis tools are important for a detailed understanding of biomaterials and their surface chemistry and morphology. One group investigated the surface chemistry of dialyzer membranes using TOF-SIMS imaging and quantitative XPS line scans (Holzweber et al.). Knowledge of the distribution and composition of the components on the outer surface region is important for understanding biological response to material surfaces. Both flat and hollow fiber membranes consisting of polysulfone and polyvinylpyrrolidone were characterized. Another group investigated the surface degradation behavior of poly(l-lactic acid) (PLLA) nanosheets (Marchany, Gardella, and Kuchera). These membranes have potential applications in tissue engineering and in delivery of therapeutics due to their enhanced performance compared to thicker, more bulky PLLA systems. The authors used TOF-SIMS to show that nanofilms exhibit less segregation of shorter chains to the surface than microfilms, due to the constrained geometries. They also concluded that the degradation rate at the surface of nanofilms is related to the inverse of the initial molecular weight, as is the case in bulk systems. Surface analysis has also been used to characterize novel structures like carbon nano-onions (Spampinato, Ceccone, and Giordani). These multishell fullerenes display several unique properties, such as a large surface area to volume ratio, a low density, and a graphitic multilayer morphology. In their study, the authors demonstrate successful functionalization of CNOs with zinc porphyrin (ZnTPP) via click chemistry using TOF-SIMS and XPS. When functionalized with fluorescent probes such as ZnTPP, CNOs can be used to enhance cellular imaging. Another group used surface analysis in the synthesis and optimization of poly(N-isopropylacrylamide) (pNIPAM) coated surfaces (Cooperstein, Bluestein, and Canavan). pNIPAM is of interest as it undergoes a phase change at temperatures relevant to the human body. This property can be exploited to release cells and proteins, which makes pNIPAM advantageous for cell sheet and tissue engineering applications. In this case, the pNIPAM surfaces were adapted by incorporation of 5-acrylamidofluorescein, which generates fluorescence and allows the material to be distinguished from the extracellular matrix.
I hope this collection of publication enlightens and inspires you. Happy reading!
