Manuel Palacio

Bioadhesion: a review of concepts and applications

Bioadhesion refers to the phenomenon where natural and synthetic materials adhere to biological surfaces. An understanding of the fundamental mechanisms that govern bioadhesion is of great interest for various researchers who aim to develop new biomaterials, therapies and technological applications such as biosensors. This review paper will first describe various examples of the manifestation of bioadhesion along with the underlying mechanisms. This will be followed by a discussion of some of the methods for the optimization of bioadhesion. Finally, nanoscale and macroscale characterization techniques for the efficacy of bioadhesion and the analysis of failure surfaces are described.

Normal and Lateral Force Calibration Techniques for AFM Cantilevers

Atomic force microscopy (AFM) is a useful tool, not only for imaging but also for quantification of normal and lateral forces exerted on the AFM tip while interacting with the surface of materials. In order to measure these forces, an accurate determination of the normal and lateral forces exerted on the AFM cantilever is necessary. To date, there is no generally accepted technique for the force calibration of AFM cantilevers. In this paper, we present a critical review of various techniques for measuring cantilever stiffness in the normal and lateral/torsional directions in order to calibrate the normal and lateral forces exerted on AFM cantilevers. The key concepts of each technique are presented, along with a discussion of their advantages and disadvantages. An understanding of the issues involved in the determination of the stiffness is needed for the proper choice and implementation of any given technique.

Nanotribology and nanomechanics of AFM probe-based data recording technology

With the advent of scanning probe microscopes, probe-based data recording technologies are being developed for ultrahigh areal density recording, where the probe tip is expected to be scanned at velocities up to 100?mm?s?1. In one technique, a conductive atomic force microscope (AFM) tip is scanned over a phase change chalcogenide medium and phase change is accomplished by applying either a high or low magnitude of current which heats the interface. Another technique is ferroelectric data storage, where a conducting AFM tip is scanned over a lead zirconate titanate (PZT) film, a ferroelectric material. Ferroelectric domains can be polarized by applying short voltage pulses between the AFM tip and the bottom electrode layer that exceed the coercive field of the PZT film, resulting in nonvolatile changes in the electronic properties. Tip wear is a serious concern in both data storage methods. The understanding and improvement of tip wear, particularly at the high velocities needed and at high interface temperatures for high data rate recording, is critical to the commercialization of these data storage technologies. This paper presents a review of nanotribological and nanomechanical studies on the materials used in phase change and ferroelectric probe-based recording. Although this work is aimed at probe-based data recording, it is also relevant to the development of robust AFM probes and to the study of nanocontacts in general.

A nanoscale friction investigation during the manipulation of nanoparticles in controlled environments

Future micro/nanodevices will contain very small features such that liquid lubrication is not practical and inherent lubricity is needed. In this study, a nanoscale friction investigation was carried out during the manipulation of Au and SiO(2) nanoparticles on silicon using atomic force microscopy (AFM). Nanoparticle sliding was characterized by quantifying the lateral force associated with the AFM tip twisting as it hits the particle edge. The friction force varies with particle area and humidity, illustrating how meniscus forces on nanoparticles affect friction. A large tip slid on the nanoparticle-coated surface exhibited friction reduction due to nanoparticle sliding and contact area reduction.

Molecularly thick dicationic ionic liquid films for nanolubrication

Ionic liquids (ILs) are attractive as lubricants for micro- and nanoelectromechanical systems due to their superior thermal stability and electrical conductivity compared to conventional lubricants. The adhesion, friction and wear properties of two dicationic ILs based on the imidazolium cation and the triflamide anion were studied and compared to the monocationic IL 1-butyl-3-methyl-1H-imidazolium hexafluorophosphate (BMIM-PF6) using atomic force microscopy. The ionic liquid film removal mechanism was investigated by monitoring the friction force, surface potential, and contact resistance. An IL based on the triflamide anion and two imidazolium cations linked by a pentane chain exhibited the best nanolubrication properties. This is attributed to the presence of hydroxyl groups at its chain ends which can hydrogen bond with the surface, and a hydrophobic linker chain. Another dicationic liquid, with a polyether chain linking the cations, had less desirable adhesion, friction, and wear properties compared ...

Bioadhesion of various proteins on random, diblock and triblock copolymer surfaces and the effect of pH conditions

The adhesive interactions of block copolymers composed of poly(methyl methacrylate) (PMMA)/poly(acrylic acid) (PAA) and poly(methyl methacrylate)/poly(2-hydroxyethyl methacrylate) (PHEMA) with the proteins fibronectin, bovine serum albumin and collagen were studied by atomic force microscopy. Adhesion experiments were performed both at physiological pH and at a slightly more acidic condition (pH 6.2) to model polymer–protein interactions under inflammatory or infectious conditions. The PMMA/PAA block copolymers were found to be more sensitive to the buffer environment than PMMA/PHEMA owing to electrostatic interactions between the ionized acrylate groups and the proteins. It was found that random, diblock and triblock copolymers exhibit distinct adhesion profiles although their chemical compositions are identical. This implies that biomaterial nanomorphology can be used to control protein–polymer interactions and potentially cell adhesion.

Morphology and protein adsorption characteristics of block copolymer surfaces

Biocompatible polymers are known to act as scaffolds for the regeneration and growth of bone. Block copolymers are of interest as scaffold materials because novel, structurally diverse polymers can be synthesized from biocompatible blocks. Block copolymer nanostructure and surface morphology is easily tunable with synthetic techniques and the diverse nanostructures can be used to affect cell and tissue behaviour. In this paper, we present atomic force microscopy studies on the morphology and corresponding protein adsorption behaviour of a novel class of methyl methacrylate and acrylic acid diblock and triblock copolymers. The topography, phase angle and adhesion maps were obtained to study the morphology. Atomic force microscopy imaging reveals that the diblock and triblock copolymers present distinct nanomorphologies, although their chemical composition is the same. This has implications on the role of nanomorphology in cell–polymer interactions independent of chemical composition. Protein adsorption on a biomaterial surface is critical to understanding its biocompatibility and bovine serum albumin was used to model that behaviour on the block copolymer surfaces. An increase in the adhesive force of the surface was observed to correlate with the adsorption of bovine serum albumin on the block copolymer surfaces investigated.

Synthesis and Morphological Characterization of Block Copolymers for Improved Biomaterials

Biocompatible polymers are known to act as scaffolds for the regeneration and growth of bone. Block copolymers are of interest as scaffold materials because a number of the blocks are biocompatible, and their nanostructure is easily tunable with synthetic techniques. In this paper, we report the synthesis of a novel class of biomaterials from block copolymers containing a hydrophobic block of methyl methacrylate and a hydrophilic block of either acrylic acid, dimethyl acrylamide, or 2-hydroxyethyl methacrylate. The block copolymers were synthesized using a combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and click chemistry. Since the surface morphology is critical for successful cell growth, atomic force microscopy (AFM) studies were conducted for selected block copolymers. The topography, phase angle and friction maps were obtained in dry and physiological buffer environments to study the morphology. Results of AFM imaging identified the presence of polymer domains corresponding to the copolymer components. The distribution of nanoscale features in these block copolymers is comparable to those found on other surfaces that exhibit favorable cell adhesion and growth. In physiological buffer medium, the hydrophilic component of the block copolymer (acrylic acid or hydroxyethyl methacrylate) appears to be present in greater amounts on the surface as a consequence of water absorption and swelling.

Nanotribological and nanomechanical properties of lubricated PZT thin films for ferroelectric data storage applications

Lead zirconate titanate (PZT) is a desirable material for nonvolatile data storage due to its ferroelectric properties. Evaluating the nanoscale mechanical and tribological performance of PZT is crucial in understanding the reliability of this material. To this end, the mechanical properties of the PZT film were characterized by nanoindentation. Nanoscratch studies reveal that the PZT film is removed by a combination of plastic deformation and brittle failure. The adhesion, friction, and wear properties of PZT were evaluated before and after application of two lubricants, namely, the perfluoropolyether Z-TETRAOL and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6). Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated for the first time using atomic-force-microscopy-based surface potential and contact resistance techniques. From this study, ionic liquids were found to exhibit comparable nanotribological properties with Z-TETRAOL.

Nanomechanical and nanotribological characterization of noble metal-coated AFM tips for probe-based ferroelectric data recording

Probe-based data recording is being developed as an alternative technology for ultrahigh areal density. In ferroelectric data storage, a conductive atomic force microscope (AFM) probe with a noble metal coating is placed in contact on lead zirconate titanate (PZT) film, which serves as the ferroelectric material. A crucial mechanical reliability concern is tip wear during contact of the ferroelectric material with the probe. To achieve high wear resistance, the mechanical properties (such as elastic modulus and hardness) of the metal-coated probe should be high. Nanoindentation experiments were performed in order to evaluate the mechanical properties of four commercial noble metal coatings, namely, Pt, Pt-Ni, Au-Ni and Pt-Ir, deposited on AFM probes. The effective hardness and elastic modulus were evaluated, using a contact mechanics model that accounts for the effect of the underlying silicon substrate. The Pt-Ir coating was found to exhibit the highest hardness, highest elastic modulus and lowest creep resistance. Nanoscratch studies reveal that the noble metal coatings are removed primarily by plastic deformation. The Pt-Ir and Pt coatings show the highest and lowest scratch resistance, respectively, which is consistent with results obtained from wear tests of the noble metal-coated AFM probes on a PZT surface.