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.     

Friction of copolymers of methyl α-n-alkylacrylates (alkyl ⩾ C16H33) and n-octadecyl methacrylate with methyl methacrylate

A comparative study was made of the friction and wear properties of homopolymers and copolymers (with methyl methacrylate (MMA)) of methyl α-n-hexadecylacrylate (MHDA), methyl α-n-octadecylacrylate (MODA) and n-octadecyl methacrylate (ODMA). In the first two compounds the long alkyl side chain is attached to the polymer backbone, whereas in the last it is adjacent to the carboxyl group that is attached to the polymer backbone. The friction data obtained for homopolymers and copolymers (thin films on glass or steel) of MHDA, MODA and ODMA were similar. The coefficient of friction μ decreased rapidly at a low concentration of MHDA, MODA or ODMA in the copolymers, then decreased more slowly. At about 50 mol.% μ (copolymer) ≈ μ(homopolymer) and further increases in the concentrations of these monomers in the copolymer led to a slight decrease in μ to the limit of the μ value for the homopolymer (μ ≈ 0.10). The durability of thin films (about 3700–4100 Å thick) of the copolymers increased with the concentration of MHDA, MODA or ODMA up to 21–25 mol.%. However, 100% MHDA and MODA have lower durabilities than some of the copolymers. All homopolymers and copolymers show higher durabilities than poly-MMA and the durabilities of some of the copolymers approach that of poly-ODMA, which has the highest durability in the series of polymers studied. The lower durabilities of poly-MHDA and poly-MODA ($\text{M}\text{?}{}_{\mathrm{n}}$ values 7083 and 1320) compared with that of poly-ODMA ($\text{M}\text{?}{}_{\mathrm{n}}\text{= 1.9 × 10}{}^5$) may be related to their low molecular weights. In all cases the durability of the homopolymers and copolymers was greater on glass than on steel. Available evidence indicates that the decrease in μ and the increase in durability of the copolymers studied is attributable to the long alkyl side chain and not to the polymer backbone. Further, the effect of the long alkyl side chain on μ and on durability is related to its flexibility and capability of orientation during the rubbing process.     

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.     

Measuring the activation energy of thiol desorption using lateral force microscopy

Thermal stability of self-assembled monolayers (SAMs) is important for applications in various surface science applications. As a model material, 16-mercaptohexadecanoic acid (MHA) on template stripped gold surfaces was investigated to determine the effect of temperature on the change of lateral force signal using atomic force microscopy (AFM). Friction force signals were obtained at various temperatures in order to determine whether it was possible to correlate the friction signal with desorption of the thiol molecule from the surface. Samples were heated for up to 10 h ranging from 40 to 80 °C in air and scanned every hour. A kinetic model was introduced to correlate the lateral force signal to the activation energy of desorption of the SAM from gold surface with heating. The activation energy of the detachment using this technique is 25.4 kcal/mol, which is consistent with other more complex techniques.     

Lubrication with Oil-Compatible Polymer Brushes

A polymer-brush-based, surface-modification strategy for friction and wear reduction in hard contact under boundary-lubrication conditions is proposed, specifically for a non-aqueous environment. Surface-initiated atom-transfer radical polymerization was employed for the synthesis of three different oil-compatible, hydrophobic polymer brushes based on alkyl methacrylates. This study presents polymerization kinetics, chemical characterization by means of Fourier-transform infrared spectroscopy and surface morphologies observed in atomic force microscopy. The lubrication properties of the anchored polymers were evaluated macroscopically by means of ball-on-disk methods and on the nanonewton scale by lateral force microscopy and showed significant reduction in friction up to contact pressures as high as 460 MPa. The frictional response of surface-grafted polymers is shown to depend strongly on the compatibility of the polymer with the chosen lubricating fluid.     

Fabrication of functional poly(thiophene) electrode for biosensors

Three-type polymer electrodes such as poly(Th), poly(Th-AP) and poly(Th-AP-TAA) were fabricated, respectively, by electro-oxidative polymerization of thiophene (Th), mixture of Th and 2-aminophenol (AP), and mixture of Th, AP and 3-thiopheneacetic acid (TAA) on the surface of indium tin oxide (ITO) glass by cyclic voltammetry (CV). The polymer electrodes were electrodeposited by cycling the potential between -1.0 and +2.5V in acetonitrile containing 50mM tetrabutylammoniumhexafluorophosphate (TBAF(6)P). The surface morphology of polymer electrodes was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis. The surface morphology of the poly(Th) showed typical roughness and fractal-like growth patterns, and the morphologies of poly(Th-AP) and poly(Th-AP-TAA) were dramatically changed. The water contact angle at the poly(Th-AP-TAA) (23 degrees ) is lower in comparison to poly(Th) (47 degrees ). The functional groups (-OH) and carboxylic acid (-COOH) group play an important role. Horseradish peroxidase was loaded onto poly(Th-AP-TAA) surface and used to test the sensing of H(2)O(2).     

Effect of lateral morphology formation of polymer blend towards patterning silane-based SAMs using selective dissolution method

A number of strategies have been developed including soft lithography and photolithography for patterning various surfaces. Here we have discussed a customized strategy for surface patterning of nanosized, silane-based SAMs and monolayer thickness measurement investigated using atomic force microscope (AFM). We have utilized the versatile morphology of a binary polymer blend to generate patterned SAMs over silicon substrate by employing a selective dissolution procedure. This method was confirmed with different organosilanes with varying number of C-atoms and to other polymer blend. The samples were imaged both in tapping mode and pulsed force mode AFM.     

Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation

Generation of high-intensity pulsed ion beam (HIPIB) has been studied experimentally using polyethylene as the anode polymer in magnetically insulated ion diodes (MIDs) with an external magnetic field. The HIPIB is extracted from the anode plasma produced during the surface discharging process on polyethylene under the electrical and magnetic fields in MIDs, i.e., high-voltage surface breakdown (flashover) with bombardments by electrons. The surface morphology and the microstructure of the anode polymer are characterized using scanning electron microscopy and differential scanning calorimetry, respectively. The surface roughening of the anode polymer results from the explosive release of trapped gases or newly formed gases under the high-voltage discharging, leaving fractured surfaces with bubble formation. The polyethylene in the surface layer degrades into low-molecular-weight polymers such as polyethylene wax and paraffin under the discharging process. Both the surface roughness and the fraction of low molecular polymers apparently increase as the discharging times are prolonged for multipulse HIPIB generation. The changes in the surface morphology and the composition of anode polymer lead to a noticeable decrease in the output of ion beam intensity, i.e., ion current density and diode voltage, accompanied with an increase in instability of the parameters with the prolonged discharge times. The diode voltage (or surface breakdown voltage of polymer) mainly depends on the surface morphology (or roughness) of anode polymers, and the ion current density on the composition of anode polymers, which account for the two stages of anode polymer degradation observed experimentally, i.e., stage I which has a steady decrease of the two parameters and stage II which shows a slow decrease, but with an enhanced fluctuation of the two parameters with increasing pulses of HIPIB generation.     

Surface modification of an organosilane self-assembled monolayer on silicon substrates using atomic force microscopy: scanning probe electrochemistry toward nanolithography

Organosilane self-assembled monolayers (SAMs) have been applied to resist materials for nanolithography based on scanning probe microscopy. An organosilane SAM was prepared on Si substrates from a precursor, that is octadecyltrimethoxysilane. Using an atomic force microscope with a conductive probe, current was injected from the probe into the SAM-covered Si substrate so that the SAM was locally degraded at the probe-contacting point. Nanoscale patterns drawn on the SAM was clearly imaged by lateral force microscopy. The patterning could be conducted in air while, in vacuum at the order of 10(-6) Torr, no detectable patterns were fabricated. The presence of adsorbed water at the probe/sample junction was confirmed to be crucial for the patterning of the SAM/Si. Its mechanism was, thus, ascribed to electrochemical reactions of both the SAM and Si with adsorbed water.     

Volume of extracellular polymeric substance coverage of individual Acidithiobacillus ferrooxidans bacterium measured by atomic force microscopy

The Acidithiobacillus ferrooxidans response to stress associated with the drying process is known to be the production of extracellular polymeric substance (EPS) coverage. Here, samples of A. ferrooxidans suspensions grown in 1.8 pH and 3.0 pH and dried on mica and silicon are shown to form a structure of isolated bacteria. Individual bacteria coverage patterns were imaged by atomic force microscopy (AFM) on hydrophobic (silicon) and hydrophilic (mica) substrates. A comparison of images of covered and uncovered bacteria establish the volume of individual EPS coverage. The EPS production for bacteria on hydrophobic substrates shows a substantial decrease (a factor of 30) in the EPS volume per bacterium when compared with the one on hydrophilic substrates. Shape and volume determination of EPS structures on bacteria as a function of hydrophobicity or hydrophilicity of the substrate may help to determine the functions of EPS on bacterial aggregates.     

Nanoscale characterization of gold nanoparticles created by in situ reduction at a polymeric surface

Transmission Electron Microscopy is used as a quantitative method to measure the shapes, sizes and volumes of gold nanoparticles created at a polymeric surface by three different in situ synthesis methods. The atomic number contrast (Z‐contrast) imaging technique reveals nanoparticles which are formed on the surface of the polymer. However, with certain reducing agents, the gold nanoparticles are additionally found up to 20 nm below the polymer surface. In addition, plan‐view high‐angle annular dark‐field scanning transmission electron microscopy images were statistically analyzed on one sample to measure the volume, height and effective diameter of the gold nanoparticles and their size distributions. Depth analysis from high‐angle annular dark‐field scanning transmission electron microscopy micrographs also gives information on the dominant shape of the nanoparticles.     

A promising thermal pressing used in fabricating microlens array

This paper describes a simple and inexpensive technique for designing and fabricating polygon microlens arrays using a thermal pressing process. Polygon microlens array molds were fabricated using lithography and an electroforming process. The microlens patterns were designed on a photomask and transferred to a substrate through photoresist patterning. Electroforming technology was used to convert the photoresist microlens patterns into metallic molds. A hot pressing machine was then used to produce the microlens array in a polycarbonate (PC) substrate. The experimental variables were compression pressure, temperature, and the pressing time. The surface roughness of the produced microlens array was measured using atomic force microscopy (AFM). The average microlens radii of curvature ranged from 315 to 420 μm and the average sag heights were from 2.98 to 4.03 μm, respectively. The experimental result showed that this fabrication process is useful for microlens array production.     

Low-voltage electron microscopy of polymer and organic molecular thin films

We have demonstrated the capabilities of a novel low-voltage electron microscope (LVEM) for imaging polymer and organic molecular thin films. The LVEM can operate in transmission electron microscopy, scanning transmission electron microscopy, scanning electron microscopy, and electron diffraction modes. The microscope operates at a nominal accelerating voltage of 5 kV and fits on a tabletop. A detailed discussion of the electron-sample interaction processes is presented, and the mean free path for total electron scattering was calculated to be 15 nm for organic samples at 5 kV. The total end point dose for the destruction of crystallinity at 5 kV was estimated at 5 x 10(-4) and 3.5 x 10(-2) C/cm2 for polyethylene and pentacene, respectively. These values are significantly lower than those measured at voltages greater than 100 kV. A defocus series of colloidal gold particles allowed us to estimate the experimental contrast transfer function of the microscope. Images taken of several organic materials have shown high contrast for low atomic number elements and a resolution of 2.5 nm. The materials studied here include thin films of the organic semiconductor pentacene, triblock copolymer films, single-molecule dendrimers, electrospun polymer fibers and gold nanoparticles.     

Pattern abrasion of ultra-high molecular weight polyethylene: Microstructure reconstruction of worn surface

The friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against bearing steel (AISI 52100) in a ring-on-block contact mode under the lubrication of aqueous solution of 3.5% NaCl was evaluated. The worn polymer surfaces were analyzed by means of three dimensional profiling, atomic force microscopy, Polarized Raman microanalysis, field emission scanning electron microscopy, and nanoindentation testing. It was found that unusual wavelike abrasion patterns were formed on the worn surface of UHMWPE under properly selected sliding conditions. In the presence of plowing effect, the molecular chains of UHMWPE and short-rod like microcrystalline grains of abrasion pattern were both further oriented along the plowing direction and became tiny and dense owing to microstructure reconstruction. Resultant microstructurally reconstructed worn surface of UHMWPE had a higher nanoindentation hardness and modulus as well as increased wear resistance.     

AFM imaging in solution of protein-DNA complexes formed on DNA anchored to a gold surface

A chemical procedure for anchoring DNA molecules to gold surfaces was used to facilitate the imaging of DNA and DNA-protein complexes in buffer solution by tapping mode atomic force microscopy (TMAFM). For preparing flat gold surfaces, a novel approach was employed by evaporating small amounts of gold onto freshly cleaved mica to give flat films that were stable under aqueous buffer conditions. The thickness of the investigated films ranged from 1 to 10 nm. For typical films of 4-6 nm, which were stable under aqueous buffer conditions, the root mean square (RMS) roughness ranged between 0.25 and 0.5 nm, as measured by atomic force microscopy (AFM). This roughness is comparable to that of obtained by the template stripped gold (TSG) technique, which is widely used in scanning probe microscopy but involves more preparation steps. In order to visualize DNA and DNA-protein complexes by TMAFM, the DNA was chemisorbed to the gold surface through a linker carrying a terminal thiol group at the 5'-end of each of the DNA strands. The modified DNA fragments were bound to the gold films and imaged in buffer solution, while unmodified DNA could not be visualized. Since the DNA was not dried during the process, it can be assumed that its native conformation was retained. This mode of anchoring did not prevent interaction with proteins, as confirmed by the observation that the topology of a complex formed by adding the protein to a surface-anchored DNA was the same as that obtained by anchoring a pre-formed complex to the gold surface. We attribute this observation to the fact that the DNA is anchored to the gold surfaces only through its ends, therefore the DNA-support interaction is minimized but imaging is still possible.     

Glass transitions in nanoscale heated volumes of thin polystyrene films

Glass transitions in confined polystyrene films on a silicon substrate were studied using atomic force microscopy incorporating a thermal tip. Three-dimensional spatial nanoconfinements were achieved by controlling size and boundary conditions of small heated volumes of polymer nanostrands drawn from the polymer surface with the thermal tip, using appropriate loads and temperatures at the tip-polymer contact. Finite element analysis was performed to model mechanical contact and thermal transport, including the effects of contact radius, film thickness, and load on temperature and pressure distributions in the confined volume at the contact. The glass transition temperature (T(g)) was measured by observing the softening of polymers with increasing temperature. The measured surface T(g) exhibited a strong size dependence, while the subsurface T(g) increased with decreasing the distance to the substrate. A large increase in the surface T(g) was observed when the radius of contact was reduced below about 10 nm. The increase in the glass transition temperature at the surface was attributed to the presence of surface and line tension at the nanometer contact, while the enhanced T(g) near the substrate was attributed to the pinning effects that reduces the mobility of the polymer molecules in the film over several hundreds of nanometers away from the polymer-substrate interface.     

Morphological investigations of cells that adhered to the irregular patterned polydimethylsiloxane (PDMS) surface without reagents

Polydimethylsiloxane (PDMS) surface consisting irregular pattern was investigated to develop cell-based biochip using PDMS. PDMS surface was modified with nano- and micro-combined patterns using surface deformation technology. Hydrophobicity of nano-patterned PDMS surface was sustained. Nevertheless it has irregular patterns consisting of micro- and nano-patterns. According to atomic force microscopy (AFM), scanning electron microscopy (SEM) and confocal microscopy results by immunostaining method, human mammary epithelial cells (HMEC) adhered well on irregularly patterned surface without any reagents such as gelatin and collagen, compared to commercial culture dish. It implies PDMS material can be utilized as template for cell-based biochip without any reagents.     

Mechanically engraved mica surface using the atomic force microscope tip facilitates return to a specific sample location

By controlling the interaction between the atomic force microscope tip and mica, patterns of different sizes and shape have been produced on the surface of mica. Using these operator-constructed patterns as a reliable marker, the original scanned sample location can be re-located and imaged again on the same mica surface by atomic force microscopy (AFM). This location technique can be used to find the same object again even if the sample was removed from the AFM instrument or the sample was imaged in a different mode.     

Dynamic friction measurements with an atomic force microscope on polymer surfaces

The combination of scanning friction force microscopy (SFFM) and lock‐in techniques leads to dynamic SFFM (DSFFM) and provides great advantages in friction force studies with sub‐micrometre resolution. In this paper are presented measurements on thin adsorbed organic films on polymers (polymer blend of 75% poly(allylaminehydrochloride) (PAA) and 25% poly(diallyl‐dimethylammonium chloride) (PDDAC)) and on mica (as a reference). The amplitude and phase response as a function of the excitation amplitude can be explained on hard surfaces by a simple static and dynamic friction model. This model allows us further to distinguish static friction forces and kinetic friction forces in a quantitative way. Furthermore, we demonstrate the use of these spectra to determine the correct modulation amplitude of the excitation to achieve the optimal friction contrasts directly. Polymer data suggest that the viscoelastic shear flow under the atomic force microscope (AFM) tip is responsible for the shape of the phase and amplitude spectrum. Lastly, we demonstrate that DSFFM is a useful technique for surface characterisation in situations where SFFM may not be adequate.     

Nanochemical surface analyzer in CMOS technology

We have developed an atomic force microscopy (AFM) cantilever system, fabricated using a standard CMOS process and a few post-processing steps, capable of detecting the difference between hydrophilic and hydrophobic samples for the purpose of nanochemical surface analysis. The fully integrated cantilever comprises a thermal actuator for cantilever deflection and a Wheatstone bridge to sense cantilever bending, thus obviating the need for cumbersome laser detection and external piezoelectric drives. Glass microspheres have been affixed to the cantilevers and, were either modified with a self-assembled monolayer to form hydrophobic tips, or left unmodified for hydrophilic tips. Force-distance curves have been used to measure the force between the functionalized/unfunctionalized tips and hydrophobic/hydrophilic sample surfaces. In an optimization step three different Wheatstone bridge sensors have been designed and characterized; best Wheatstone bridge sensitivity is 8.0 microV/nm with a 713 nm/mW actuator efficiency.