Micron sized structures/components are commonly employed in a variety of devices (e.g., biosensors, array devices). At present such devices are based on macroscopic technologies. Future applications of differentiated structures/surfaces are expected to place considerable demands on down-sizing technologies, i.e. enable meso/nanoscopic manipulation. An emerging set of methods known collectively as soft lithography is now being utilised for a large variety of applications including micromolding, microfluidic networks and microcontact printing. In particular stamps and elastomeric elements can be formed by transfer of a pattern to a polymer by a master. The 'master' can be fabricated by a variety of techniques capable of producing well-defined surface topographies. Established lithographic techniques used in the microelectronic industry, such as photolithography, are generally used to fabricate such master templates at the micron scale. A number of polymers can be used to transfer patterns. One of the most widely used polymers for pattern transfer has been polydimethylsiloxane (PDMS). The elastomer is chemically resistant, has a low surface energy and readily conforms to different surface topographies. Obtaining a master is the limiting factor in the production of PDMS replicas. In this study we demonstrate the use of Diamond-Like-Carbon (DLC) as a master template for producing PDMS micro/nano stamps and 3 dimensional PDMS structures. Intricate surface relief patterns were formed on the DLC surface from lithographic techniques by Atomic Force Microscopy (AFM) operated in the electrical conductivity mode. Attributes of the technique include:
-Features with line widths less than 20 nm can be formed on the DLC.
-The radius of curvature at edges can be less than 10 nm.
-The slope of the features is limited by the aspect ratio of the tip.
-Highly complex shapes can be fashioned.
-Feature depth can be controlled by DLC film thickness and/or by the bias voltage applied.
-The master is highly durable.
-The master relief after patterning is extremely flat.
The surface structure and chemistry of polymers affect their functionality for a great range of applications in areas as diverse as biosensors, corrosion protection, semiconductor processing, biofouling, tissue engineering and biomaterials technology. Some of those applications require purposeful tailoring of laterally differentiated regions (e.g., array structures for multi-channel/multi-analyte biosensors and patterning for promotion of selective adhesion of cells/proteins). While such tailoring is currently taking place on the μm-scale, it is likely in the future to progress into the nm-regime. Attachment of biological moieties at surfaces and interfaces has been shown to be highly dependant on local chemistry at the intended site of attachment. Additionally, the local molecular-scale geometry may promote or hinder attachment events, as in the case of biofilms. To date, however, the effect of frictional properties of surfaces for chemical and biomolecular attachment is a much less understood phenomenon. In this study we show controlled patterning of a polymer surface (polydimethylsiloxane (PDMS)) arising from manipulation by Atomic Force Microscopy (AFM). PDMS is a bio-active/selective polymer having a broad range of applications, such as biomedical devices, molecular stamps, hydraulic fluid devices and in soft lithography. The polymer surface has been selectively altered by high speed scanning in order to generate regions on the surface that exhibit differentiated frictional properties. By altering the loading force, scan width, and area of the AFM probe-to-polymer contact it is possible to produce a variety of detailed and complex patterns with frictional contrast, including anisotropic frictional gradients on the polymer surface. The controlled manipulation of the polymer surface can be carried out on the micro-, meso- and nano-scale.
It has been shown recently that it is possible to reverse-engineer the folding of complex proteins by an AFM-based force versus distance methodology. In essence, the protein is attached to a functionalised surface, then linked to a functionalized tip of a force-sensing/imposing lever, and finally stretched by withdrawal of the probe. The experiments are technically demanding, and subject to a number of artefacts. Moreover, the unfolding process cannot readily be reversed. A 'reverse’ methodology is possible in principle and is likely to provide greater insight into the folding/unfolding sequence. The protein in question will now be attached to a functionalized superparamagnetic bead in solution. A functionalized probe is then introduced into solution whereupon the protein (and its bead) attaches itself to the tip of the probe, while being tracked optically. The system is then submerged in an inhomogeneous magnetic dipole field, giving rise to a magnetic dipole-dipole interaction with the bead. The resultant force will stretch the protein, while being monitored at a force resolution of ca. 1 pN by the deflection of the lever. The significant aspect of the scheme is that the probe acts as a position-sensitive element as well as being a force-sensing device. The field gradient and its rate of change within the interaction volume can readily be controlled to 1 part in 106, and can be reversed. Thus the unfolding/folding sequence can be retraced for the same molecule. Moreover, information about kinetics will be accessible from varying the rate of change in field strength, and the effects of ambient fluid conditions can be investigated in real time with a flow-through arrangement. The proposed methodology is likely to be more userfriendly as a result of dispensing with the functionalized surface, and by being able to determine optically that a single functionalized bead has arrived at the probe tip. Elements of the experimental arrangement are currently at the design stage.
Oxidative nano-writing by a conducting AFM tip to Si substrates has so far been ascribed to an anodic electrochemical process. However, there is evidence that thermal effects may play a role. More recently it has been demonstrated that conducting diamond-like carbon (DLC) films can be nano-machined by a biased conducting tip; the process is evidently due to local thermally activated oxidation and formation of CO2. Thus the prevailing description in terms of an athermal anodic oxidative mechanism will need to be revisited and possibly revised. The physico-chemical state of the tip is known to affect the rate and efficiency of the process in some unspecified manner. It is plausible to ascribe variations in the I-V characteristics and thermal transport to tip effects that are operational at the point of contact. However, so far the focus has been on the surface, and little attention has been given to a detailed and relevant characterization of the tip and its efficacy as a manipulative probe. Tips were prepared with a range of known initial conditions (i.e. doped Si plus H-termination, Au-coating, or native oxide). I-V characteristics were correlated against oxidative efficiency versus clean H-terminated Si, Si plus native oxide, Si plus 2.5 nm thermal oxide and DLC. The outcomes at positive sample bias can variously be described by ohmic transport, by Fowler-Nordheim, and direct tunnelling. Likewise, tip alteration resulting from oxidative surface manipulation has been monitored by subsequent characterization. The evidence favours a thermal mechanism of oxidation arising from a repetitive, but interrupted, deposition of thermal energy at the tip-to-surface junction, where inelastic tunnelling completes the circuit. Thus there is a narrow temperature window of optimum conditions. The tip will deteriorate irreversibly if the peak temperature is too high, while oxidation of the surface will not take place if the peak temperature is too low.
Silicone rubbers have steadily gained importance in industry since their introduction in the 1960’s. Poly(dimethylsiloxane) (PDMS) is a relatively soft and optically clear, two-part elastomer with interesting and, more importantly, useful physical and electrical properties. Some of its common applications include protective coatings (e.g., against moisture, environmental attack, mechanical and thermal shock and vibrations), and encapsulation (e.g., amplifiers, inductive coils, connectors and circuit boards). The polymer has attracted recent interest for applications in soft lithography. The polymer is now routinely used as a patterned micro-stamp for chemical modification of surfaces, in particular Au substrates. Prominent stick-slip effects, surface relaxation and elastic recovery were found to be associated with micro/nano manipulation of the polymer by an AFM-based contact mode methodology. Those effects provide the means to explore in detail the meso-scale tip-to-surface interactions between a tip and a soft surface. The dependence of scan speed, loading force, attack angle and number of scan lines have been investigated.
The demand for polymer-based DNA microarrays will increase because of their cost-effectiveness, biocompatibility and easy processing. However not all polymers are ideal substrates because of different chemical interactions of polymeric substrates with the DNA molecules. Results from AFM analysis of DNA immobilised on polymeric surfaces are evaluated using fractality, Gaussian distribution and lateral force imaging. It has been found that the nanosize defects in the substrate, such as poly-l-lysine, plays an important role in the growth of DNA aggregates in a vertical direction, whereas the covalent binding of DNA molecules on NHS-functionalised cyclo-olefin copolymer leads to the lateral growth of DNA aggregates.
Biomolecules in a confined solution environment may be subject to electrostatic forces with a range up to 100 nm, while the van der Waals interaction will account for shorter-range forces. The response of two model poly(amino acids) - poly-L-lysine and poly-L-glutamic acid - has been investigated for a number of model surfaces at pH 6 - including silica/Si-oxide. The model amino acids were adsorbed, or covalently coupled, to colloidal probes consisting of a microsphere attached to a force-sensing lever. The methodology was based on sensing of an interaction between the probe and a flat surface through carrying out force versus distance analysis with an atomic force microscope. The results were analysed within the framework of the conventional DLVO theory. The outcomes illustrate both repulsive and attractive long-range interactions that will hinder, or promote, colloidal biospecies in solution from entering the region of short-range force-fields at the physical interface. Accordingly the results have implications for the efficacy of methods and devices that seek to exploit the properties of micro/nano-fluidic systems. Large 'snap-on' distances were observed for some systems and were ascribed to compression of the 'soft' functionalized layers. Those observations and measurements of adhesion provided insights into conformation of the adsorbed species and strength of attachment.
Laterally differentiated chemistry and structure of surfaces are commonly employed in a variety of devices/components (e.g., biosensors, array devices). At present such devices are based on macroscopic technologies. Future applications of differentiated surfaces are expected to place considerable demands on down-sizing technologies, i.e. enable meso/nanoscopic manipulation. The atomic force microscope (AFM) has emerged as an ideal platform for manipulation, visualization and characterisation of surface structures on the nano-scale1-14. Controlled AFM-based tip-induced lithography on P(tBuMA) thin film polymer surfaces has been obtained, at line widths down to tens of nanometres and depths in the sub-nanometre range. Parameters giving rise to production of nano-structures can in principle be defined for different polymers (lever-induced out-of-plane loading and in-plane shear forces, linear tip speed, tip shape and chemistry, polymer surface chemistry and mechanical properties). However, those sets of parameters, and their relationship to lithographic outcomes, cannot be derived from the currently accepted models for wear between macroscopic objects in sliding contact.
The scanning probe microscope (SPM) system is emerging as an increasingly important tool for non-intrusive interrogation of live cells in vitro. Its particular merit is that it retains complete functionality in a biocompatible fluid environment and can track the dynamics of cellular processes in real time and real space at nm resolution, as an imaging tool, and with pN force-sensing/imposing resolution, as an interaction tool. However, a live cell is a fragile object; meeting the requirements of non-intrusiveness presents a challenging task. The better-known recent contributions of SPM towards explanatory and predictive descriptions of biomolecular interactions at surfaces and interfaces will also be reviewed, and some recent attempts to reconfigure the SPM platform for demonstration of novel bio-device applications will be described.
The bio-activity of polymer surfaces and interfaces depends on surface structure and chemistry, and on the size scale of lateral differentiation. The Atomic Force Microscope (AFM), in its various operational modes, is emerging as an important tool in sub-μm-scale characterization. As-received polyimide surfaces were masked with a TEM grid and irradiated for time periods ranging from 90 to 540 s. Topographical imaging revealed shrinkage of the irradiated regions, presumably arising from loss of species and/or densification. Tip-to-surface adhesion was measured in the Force-vs-distance (F-d) mode. Irradiation was found to cause increased adhesion, arising from increased hydrophilicity and thus greater capillary interaction. Lateral force maps showed that there was an associated increase in friction. This difference is due in part to the additional attractive interaction of the thicker layer of adsorbed moisture, and a contribution from change in surface chemistry. Irradiated surfaces were seeded with live human fibroblasts. One surface was seeded without any additional treatment, whilst another was exposed to poly-L-lysine prior to the seeding. After incubation and growth the surfaces were imaged in PBS (Phosphate Buffered Saline) solution. Growth and strong attachment was evident on both surfaces with no preference to either the irradiated or unirradiated regions.
The atomic force microscope (AFM) allows investigation of the properties of surfaces and interfaces at atomic scale resolution. However, several different operational modes, (imaging, force versus distance and lateral force modes), need to be deployed in order to gain insight into the structure, tribological and mechanical properties. A new method, based on a variation of the force versus distance mode, has been developed. In essence, a coupling of the deformational modes of the probe is exploited whereby the tip is induced to undergo lateral travel in response to application of an out-of-plane force (and thus normal bending of the force-sensing lever). The lateral travel induces in-plane forces that are then measurable as a consequence of stimulation of the 'buckling' deformational mode of the lever. Due to the lever geometry, the technique offers an increase in resolution of an order of magnitude over existing AFM methods for measurement of atomic scale stick-slip events. In addition, the method allows measurement of the lateral deformation of the sample as well as scanner calibration. Outcomes will be demonstrated for atomically flat surfaces such as WTe2 and highly oriented pyrolytic graphite.
Poly-tert-butyl methacrylate-co-methyl methacrylate thin film surfaces were patterned and subjected to surface treatment by UV radiation and NaOH exposure, in order to tailor hydrophilic/hydrophobic conditions. The polymer has potential applications as elements of advanced biosensors and other bio-active devices. The topographies and surface chemistries on the micro- and meso-scales of thin films have been characterized by scanning force microscopy operated in the normal contact imaging mode as well as the lateral force and force versus distance modes.
The differences in hydrophobicity produced by the e-beam patterning exposure of the surface of poly[(tert-butyl methacrylate)-co-(methyl methacrylate)] -a common e-beam and deep-UV resist, resulted in the selective attachment of heavy meromyosin on hydrophobic, unexposed surfaces. The movement of the actin filaments on myosin-rich and myosin- poor surfaces was statistically characterized in terms of velocity, acceleration, and angle of movement. The actin filaments have a smooth motion on myosin-rich surfaces and an uneven motion on myosin-poor surfaces. Interestingly, an excess of myosin sites has a slowing, albeit mild, effect on the motion of the action filaments. It was also found that the myosin-rich/myosin-poor boundary has an alignment- enforcement effect, especially for the filaments approaching the border from the myosin-rich side. Based on these results we discuss the feasibility of building purposefully designed molecular motor arrays and the testing of the hypothesis regarding the functioning of the biomolecular dynamic biodevice.
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