Preface to Special Topic: Surface Modification, Wetting, and Biological Interfaces (Guest Editors: John Ralston and Jingfang Zhou)

Abstract

This Special Topic section of Biomicrofluidics on “Surface Modification, Wetting, and Biological Interfaces,” is discussed. The topic is very timely and one that is tremendously relevant to the microfluidics and nanofluidics community.

It is a great pleasure for us to guest edit this special issue of Biomicrofluidics on the topic ”Surface Modification, Wetting, and Biological Interfaces.” This is a very timely topic and tremendously relevant to the microfluidics and nanofluidics community. A body of papers from leading experts in this field, both fundamental and applied, constitutes a critical mass that will provoke further discussion in this broad interdisciplinary area. We appreciate the efforts and enthusiasm of the contributing authors for this topic, and acknowledge those who were prepared to contribute, but were unable to do so at this time. We trust that this special issue will stimulate new ideas, methods and applications in ongoing advances in this growing area of strong international interest. Three of the papers in this special issue apply computational methods to address fundamental aspects of biofilms or polymer films. Biofilm formation as well as protein adsorption involve molecular assembly processes occurring at interfaces. Many biotechnologies, including biosensors, enzyme immobilization, antibody attachment, and tissue engineering, rely on the effective control over this process. Schmitt et al.¹ investigate the adsorption kinetics of three proteins, focusing on the role of van der Waals forces and mutual interactions between the adsorbing and the adsorbed biomolecules. Their results suggest two adsorption modes and are further explored using Monte Carlo (MC) simulation to support their mechanism. The paper by Milchev, Dimitrov, and Binder² is devoted to a complex system composed of a polymer brush, which is exposed to a colloidal solution. They use molecular dynamics simulations, with a dissipative particle dynamics (DPD) thermostat, to address the problem of how the density distribution of colloidal particles within the brush behaves under shear. They discover that the density profile of colloidal particles within the brush is very weakly affected by the shear, unless the polymer grafting density is small. Thakkar and Ayappa³ formulate and extend the dissipative particle dynamics simulations to investigate the structural and mechanical properties of polymer-grafted lipid bilayers, which has profound implications in pharmacology, cell biology, and advanced materials. In their study, they achieve successfully both constant pressure and a tension-free bilayer state in the simulations using an Anderson barostat. They also present an approach to computing elastic properties of membranes by proposing a novel Delaunay triangulation analysis of the lipid head groups. Their simulations account for not only the effect of grafted polymers on the mechanical properties of lipid membranes, but also the change in the structural properties of the membrane itself, such as expansion of the membrane or the main phase transition.

Surface modification and functionalization of the microchannels have facilitated many biomedical and chemical applications. Microchannel walls are no longer simply guides for fluid flow but rather are important instruments in the functionality of microchips. Polydimethylsiloxane (PDMS) is used as one of the major substrates to build microfluidic devices. However, the inherent hydrophobic nature of the material prevents many applications. Tan et al.⁴ explore a novel technique for modifying PDMS microchannel wettability using a second oxygen plasma treatment after microchip bonding. The approach provides a simple way to create a desired surface property, especially for biological applications. Thierry et al.⁵ functionalized disposal PDMS devices use the plasma polymerization of an epoxy-containing monomer, which is subsequently conjugated with herceptin to capture HER2–positive circulating breast cancer cells. They achieve a high density of antibody immobilization and less nonspecific binding, as well as efficient capture (∼80%) of HER2-positive cells from full blood. The paper by Priest⁶ summarizes the advances in postbonding techniques for surface patterning of microchannels, including laminar flow and capillarity, photolithography, microplasma, and electrochemical biolithography.

Here we also report four on-chip applications, including antibody immobilization, single-cell attachment, DNA hybridization, and separation. Ohashi et al.⁷ describe a method that utilizes packed microbeads in the microfluidic channel and introduces an immobilization process on the bead surface by applying the avidin-biotin surface chemistry. This immobilization method can be applied not only to immunoassays but also to various applications, such as DNA analysis, enzyme assay, and so forth. In this way, they conduct a rapid and highly sensitive immunoassay on a chip. The other paper by his group employs light as an external stimulus to induce in situ cell micropatterning inside the sealed microchannels. In their approach, microfluidic systems, single-cell patterning, and cell culture techniques are combined to achieve sensitive and highly efficient single-cell contact-based analysis under environmental conditions. They discover that the 20-μm photomask is optimal for the formation of adherence patterns of single endothelial cell in the microchannel.⁸ Wang and Li⁹ present a method for gold nanoparticle (GNP)-assisted detection of DNA hybridization. In particular they use microchannels to fabricate the well-ordered lines of GNPs, which can then interrogate in an orthogonal direction by switching the PDMS channel direction. The authors report the influence of DNA length on the hybridization on the GNPs, as well as their release from GNPs and subsequent hybridization and fluorescence detection. With the aid of GNPs, they discriminate successfully between two PCR amplicons with one-base-pair difference. Finally, the use of a novel DNA separation matrix, cholesterol-bearing pullulan (CHP) nanogels, is reported for microchip electrophoresis by Kondo et al.¹⁰ The gel is easily introduced into the microchannel. Excellent resolution is observed, especially for small DNA fragments from 100 to 1500 bp at high concentration of CHP gels and the separation mechanism is proposed using Ogston and reptation models.