Nanotribological Printing: A Nanoscale Additive Manufacturing Method.

Additive manufacturing methods are transforming the way components and devices are fabricated, which in turn is opening up completely new vistas for conceiving and designing products and engineered systems. Small-scale (submicrometer) additive manufacturing methods are largely in their infancy. While a number of methods exist, a particular challenge lies in finding methods that can produce a range of materials while obtaining sufficiently robust mechanical properties. In this paper, we describe a novel nanoscale additive manufacturing technique deemed "Nanotribological Printing" (NTP), which creates structures through tribomechanical and tribochemical surface interactions at the contact between a substrate and an atomic force microscope probe, where material pattern formation is driven by normal and shear contact stresses. The "ink" consists of nanoparticles or molecules dispersed in a carrier fluid surrounding the atomic force microscope (AFM) probe, which are entrained into the contact during sliding. Being stress-driven, patterning only occurs locally within regions which experience contact and sufficiently high stresses. Thus, imaging and measurement to characterize the morphology and properties of the deposited structures can be conducted in situ during the manufacturing process. Moreover, using local mechanical energy as the kinetic driver activating the solidification process, the method is compact and does not require application of a bias voltage or laser exposure and can be performed at ambient temperatures. We demonstrate (1) control of pattern dimensions with sub-100 nm lateral and sub-5 nm thickness control through variations in contact size and applied stress, (2) creation of amorphous, polycrystalline, and nanocomposite structures including sequential multimaterial deposition, and (3) formation of manufactured structures which exhibit mechanical properties approaching those of bulk counterparts. The ability to create nanoscale patterns using standard AFM cantilever probes and operation modes (contact mode scanning in fluid) with commercial AFM instruments, independent of substrate, establishes NTP as a versatile and easily accessible method for nanoscale additive manufacturing.

The extended wedge method: atomic force microscope friction calibration for improved tolerance to instrument misalignments, tip offset, and blunt probes.

One of the major challenges in understanding and controlling friction is the difficulty in bridging the length and time scales of macroscale contacts and those of the single asperity interactions they comprise. While the atomic force microscope (AFM) offers a unique ability to probe tribological surfaces in a wear-free single-asperity contact, instrument calibration challenges have limited the usefulness of this technique for quantitative nanotribological studies. A number of lateral force calibration techniques have been proposed and used, but none has gained universal acceptance due to practical considerations, configuration limitations, or sensitivities to unknowable error sources. This paper describes a simple extension of the classic wedge method of AFM lateral force calibration which: (1) allows simultaneous calibration and measurement on any substrate, thus eliminating prior tip damage and confounding effects of instrument setup adjustments; (2) is insensitive to adhesion, PSD cross-talk, transducer/piezo-tube axis misalignment, and shear-center offset; (3) is applicable to integrated tips and colloidal probes; and (4) is generally applicable to any reciprocating friction coefficient measurement. The method was applied to AFM measurements of polished carbon (99.999% graphite) and single crystal MoS2 to demonstrate the technique. Carbon and single crystal MoS2 had friction coefficients of µ = 0.20 ± 0.04 and µ = 0.006 ± 0.001, respectively, against an integrated Si probe. Against a glass colloidal sphere, MoS2 had a friction coefficient of µ = 0.005 ± 0.001. Generally, the measurement uncertainties ranged from 10%-20% and were driven by the effect of actual frictional variation on the calibration rather than calibration error itself (i.e., due to misalignment, tip-offset, or probe radius).

Compartmental modeling of technetium-99m-labeled teboroxime with dynamic single-photon emission computed tomography: comparison with static thallium-201 in a canine model.

UNLABELLED: Di Bella EVR, Ross SG, Kadrmas DJ, et al. Compartmental modeling of technetium-99m-labeled teboroxime with dynamic single-photon emission computed tomography: Comparison with static thallium-201 in a canine model. Invest Radiol 2001;36:178-185. RATIONALE AND OBJECTIVES: A compartmental modeling approach to deriving kinetic parameters from a time series of single-photon emission computed tomography (SPECT) images of technetium-99m-labeled (99mTc-) teboroxime may have value for semiquantitative assessment of myocardial perfusion. This study investigated the value of the kinetic parameters derived from a two-compartment model of 99mTc-teboroxime for measuring myocardial perfusion and compared it with static thallium-201 (201Tl) uptake and microsphere-measured blood flow in dogs. METHODS: Experiments were successfully conducted in 9 of 11 open-chest dogs. During adenosine stress, a single complete set of projections of 201Tl uptake was acquired. 99mTc-teboroxime was then injected during adenosine stress, and a complete set of projections was acquired every 5.7 seconds for 17 minutes. Resting studies were performed on 4 of the animals. All of the projection sets were reconstructed with an iterative algorithm and incorporated corrections for attenuation and the geometric response of the collimators. Regional kinetic parameters (washin and washout) were determined semiautomatically from the time series of reconstructed 99mTc-teboroxime images and registered with microsphere data. Regional washin estimates were compared with 201Tl intensities and myocardial blood flows determined from microspheres. RESULTS: Optimally scaled 99mTc-teboroxime washin parameters and 201Tl uptakes were correlated with microsphere-determined blood flows (r = 0.91, y = 0. 99x + 0.01, and r = 0.92, y = 0.88x + 0.28, respectively). In six of the studies, the left anterior descending coronary artery was occluded, and stress occluded-to-normal (O/N) ratios were calculated. The O/N ratios were 0.32 +/- 0.17 as determined from microspheres injected with 201Tl and 0.38 +/- 0.29 from microspheres injected with 99mTc-teboroxime (P = NS). The O/N ratios were 0.48 +/- 0.16 for static 201Tl uptake and 0.27 +/- 0.21 for 99mTc-teboroxime washin (P < 0.05). CONCLUSIONS: Both 201Tl uptake and 99mTc-teboroxime kinetic parameters were well correlated with flow. The 99mTc-teboroxime washin parameters offer semiquantitative flow values and provide greater defect contrast than can be obtained with 201Tl uptake values.

Static Versus Dynamic Teboroxime Myocardial Perfusion SPECT in Canines.

Tc-99m-teboroxime is a perfusion tracer with high myocardial extraction, fast washin and washout kinetics, and excellent imaging properties. The fast kinetics pose some problems for static imaging, but they also allow for back-to-back stress / rest studies to be performed very quickly. Furthermore, such fast kinetics are ideally suited for dynamic imaging. We have compared static versus dynamic myocardial perfusion SPECT with teboroxime in canines using microsphere-derived flow values as the gold standard. Dynamic data were successfully acquired at rest and under adenosine stress in seven dogs using a fast serial scanning protocol. The data were analyzed in two ways: summing timeframes to create a single, static dataset with consistent projections; and 4D reconstruction and kinetic parameter estimation for a two compartment model. In both cases imaging data (voxel intensity or washin rate parameter) were correlated with flow values measured by microspheres. The static summing procedure that produced the best correlation with flow consisted of summing the projection data acquired from 60 to 180 seconds post-injection. The washin rate parameter was found to provide better correlation with flow than static image intensity in six of seven animals. When the data were pooled over all studies, washin provided significantly better correlation with flow than static imaging (p<0.01). We conclude that dynamic imaging of teboroxime with compartmental modeling provides a better measure of flow than can be obtained from static imaging techniques.

SPECT Imaging of Teboroxime during Myocardial Blood Flow Changes.

Kinetic parameters and static images from dynamic SPECT imaging of (99m)Tc-teboroxime have been shown to reflect blood flow in dogs and in humans at rest and during adenosine stress. When compartment modeling is used, steady-state physiological conditions are assumed. With standard adenosine stress protocols, imaging of teboroxime would likely involve significant changes in flow, even if performed only for five minutes. These flow changes may significantly bias the kinetic parameter estimates. On the other hand, when static imaging is performed, large flow changes during acquisition may improve contrast between normal and occluded regions. Computer simulations were performed to determine the effect of changing flows on kinetic parameter estimation and on static (average tissue uptake) images. Two canine studies were also performed in which adenosine was given with a standard protocol, and then imaging was repeated with adenosine infusion held constant. The simulations predicted biases on the order of 7% for kinetic washin parameter estimation and 18% for the washout parameter. Contrast for static studies was found to depend critically on the time-activity behavior of the distribution as well as on the stress protocol. The differences in washin contrast from the standard and continous adenosine dog studies was slightly larger than predicted from the simulations. Optimal imaging of teboroxime with adenosine using compartment modeling will require non-standard adenosine stress protocols, although sub-optimal imaging may still be useful clinically.