Comparing the effect of virtual and in-person instruction on students' performance in a design for additive manufacturing learning activity.

The goal of this work is to compare the outcome of a design for additive manufacturing (DfAM) heuristics lesson conducted in a virtual learning environment to the same in an in-person learning environment. Prior work revealed that receiving DfAM heuristics at different points in the design process impacts the quality and novelty of designs produced afterward, but this work may have been limited by the solely virtual format. In this work, an identical experiment was performed in a face-to-face learning environment. Results indicate that neither learning format presents an advantage over the other when it comes to the quality of designs produced during the intervention. Participants across all experimental groups reported an increase in self-efficacy after the intervention, with improved performance on quiz-type questions. However, the novelty and variety of the designs produced by the in-person experimental groups were significantly lower than that of the virtual experimental groups. In addition to validating the effectiveness of virtual instruction as a teaching method, these results also support the authors' hypothesis that the priming effect is stronger in an in-person classroom than in a virtual classroom.

Invited review article: high-speed flexure-guided nanopositioning: mechanical design and control issues.

Recent interest in high-speed scanning probe microscopy for high-throughput applications including video-rate atomic force microscopy and probe-based nanofabrication has sparked attention on the development of high-bandwidth flexure-guided nanopositioning systems (nanopositioners). Such nanopositioners are designed to move samples with sub-nanometer resolution with positioning bandwidth in the kilohertz range. State-of-the-art designs incorporate uniquely designed flexure mechanisms driven by compact and stiff piezoelectric actuators. This paper surveys key advances in mechanical design and control of dynamic effects and nonlinearities, in the context of high-speed nanopositioning. Future challenges and research topics are also discussed.

Bridging the gap between conventional and video-speed scanning probe microscopes.

A major disadvantage of scanning probe microscopy is the slow speed of image acquisition, typically less than one image per minute. This paper describes three techniques that can be used to increase the speed of a conventional scanning probe microscope by greater than one hundred times. This is achieved by the combination of high-speed vertical positioning, sinusoidal scanning, and high-speed image acquisition. These techniques are simple, low-cost, and can be applied to many conventional microscopes without significant modification. Experimental results demonstrate an increased scan rate from 1 to 200 Hz. This reduces the acquisition time for a 200 x 200 resolution image from 3 min to 1s.

The pursuit of a scalable nanofabrication platform for use in material and life science applications.

In this Account, we describe the use of perfluoropolyether (PFPE)-based materials that are able to accurately mold and replicate micro- and nanosized features using traditional techniques such as embossing as well as new techniques that we developed to exploit the exceptional surface characteristics of fluorinated substrates. Because of the unique partial wetting and nonwetting characteristics of PFPEs, we were able to go beyond the usual molding and imprint lithography approaches and have created a technique called PRINT (Particle [or Pattern] Replication In Nonwetting Templates). PRINT is a distinctive "top-down" fabrication technique capable of generating isolated particles, arrays of particles, and arrays of patterned features for a plethora of applications in both nanomedicine and materials science. A particular strength of the PRINT technology is the high-resolution molding of well-defined particles with precise control over size, shape, deformability, and surface chemistry. The level of replication obtained showcases some of the unique characteristics of PFPE molding materials. In particular, these materials arise from very low surface energy precursors with positive spreading coefficients, can be photocured at ambient temperature, and are minimally adhesive, nonswelling, and conformable. These distinctive features enable the molding of materials with unique attributes and nanometer resolution that have unprecedented scientific and technological value. For example, in nanomedicine, the use of PFPE materials with the PRINT technique allows us to design particles in which we can tailor key therapeutic parameters such as bioavailability, biodistribution, target-specific cell penetration, and controlled cargo release. Similarly, in materials science, we can fabricate optical films and lens arrays, replicate complex, naturally occurring objects such as adenovirus particles, and create 2D patterned arrays of inorganic oxides.

Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex.

The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 degrees C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not beta-H elimination).

On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes: a combined computational and experimental study.

Pincer complexes of the type ((R)PCP)IrH(2), where ((R)PCP)Ir is [eta(3)-2,6-(R(2)PCH(2))(2)C(6)H(3)]Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H(2). We calculate (DFT/B3LYP) that associative (A) reactions of ((Me)PCP)IrH(2) with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(eta(2)-H(2)) intermediates. A dissociative (D) pathway proceeds via initial loss of H(2), followed by C-H addition to ((Me)PCP)Ir. Although a slightly higher energy barrier (DeltaE(+ +)) is computed for the D pathway, the calculated free-energy barrier (DeltaG(+ +)) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has DeltaG(o)(+ +) = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150 degrees C and P(H)2 = 1.0 x 10(-7) atm), DeltaG(+ +) for C-H addition to ((Me)PCP)Ir (plus a molecule of free H(2)) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H(2) from ((Me)PCP)IrH(2) with DeltaG(D)(+ +) approximately DeltaH(D)(+ +) = 27.2 kcal/mol. For CyH, the calculated DeltaG(o)(+ +) for C-H addition to ((Me)PCP)IrH(2) on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H(2) from the resulting "seven-coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (DeltaH approximately 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H(2) elimination (DeltaG(A)(+ +)) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to ((R)PCP)IrH(2) is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C(6)D(6) > mesitylene-d(12) > n-decane-d(22) >> cyclohexane-d(12). Because H/D exchange in cyclohexane-d(12) solution is not observed even after 1 week at 180 degrees C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.

Combined computational and experimental study of substituent effects on the thermodynamics of H(2), CO, arene, and alkane addition to iridium.

The thermodynamics of small-molecule (H(2), arene, alkane, and CO) addition to pincer-ligated iridium complexes of several different configurations (three-coordinate d(8), four-coordinate d(8), and five-coordinate d(6)) have been investigated by computational and experimental means. The substituent para to the iridium (Y) has been varied in complexes containing the (Y-PCP)Ir unit (Y-PCP = eta(3)-1,3,5-C(6)H(2)[CH(2)PR(2)](2)Y; R = methyl for computations; R = tert-butyl for experiments); substituent effects have been studied for the addition of H(2), C-H, and CO to the complexes (Y-PCP)Ir, (Y-PCP)Ir(CO), and (Y-PCP)Ir(H)(2). Para substituents on arenes undergoing C-H bond addition to (PCP)Ir or to (PCP)Ir(CO) have also been varied computationally and experimentally. In general, increasing electron donation by the substituent Y in the 16-electron complexes, (Y-PCP)Ir(CO) or (Y-PCP)Ir(H)(2), disfavors addition of H-H or C-H bonds, in contradiction to the idea of such additions being oxidative. Addition of CO to the same 16-electron complexes is also disfavored by increased electron donation from Y. By contrast, addition of H-H and C-H bonds or CO to the three-coordinate parent species (Y-PCP)Ir is favored by increased electron donation. In general, the effects of varying Y are markedly similar for H(2), C-H, and CO addition. The trends can be fully rationalized in terms of simple molecular orbital interactions but not in terms of concepts related to oxidation, such as charge-transfer or electronegativity differences.

Structure and H(2)-Loss Energies of OsHX(H(2))(CO)L(2) Complexes (L = P(t-Bu)(2)Me, P(i-Pr)(3); X = Cl, I, H): Attempted Correlation of (1)J(H-D), T(1min), and DeltaG().

1J(H-D), T(1min) and k(1) for H(2) dissociation from OsHX(H(2))(CO)L(2) have been measured for X = Cl, I, H (L = P(t-Bu)(2)Me or P(i-Pr)(3)), as well as for OsCl(2)(H(2))(CO)(P(i-Pr)(3))(2). For comparison, new data (including previously unobserved coupling constants) have been reported for W(HD)(CO)(3)(P(i-Pr)(3))(2). A comprehensive consideration of T(1min) data for over 20 dihydrogen complexes containing only 1-2 phosphines cis to H(2), together with a consideration of the shortest "conceivable" H-H distance for H(2) bound to a d(4) or d(6) metal, is used to argue that the "fast spinning" model is not appropriate for determining r(H-H) in such complexes. Regarding OsHX(H(2))(CO)L(2), the stronger electron-donor (lighter) halide, when cis to H(2), facilitates loss of H(2). The complete absence of pi-donor ability when X = H renders H(2) loss most difficult. However, a pi-donor trans to H(2) also makes H(2) loss unobservable. Within the series of isoelectronic, structurally analogous Os complexes, a longer H-H bond shows a larger DeltaG() for H(2) loss. However, this correlation does not continue to W(H(2))(CO)(3)(P(i-Pr)(3))(2), which has r(H-H) comparable to that of OsH(halide)(H(2))(CO)(P(i-Pr)(3))(2), but a significantly higher DeltaG(). This may originate from lack of a pi-donor ligand to compensate as H(2) leaves W.

Somatosensory evoked potentials and noxious stimulation in patients with intractable, noncancer pain syndromes.

Investigations of the evoked potentials (EPs) to noxious laser stimulation have indicated consistent strong linear relationships between subjective response (R), stimulus intensity (S), and EP amplitude (A). Thirty patients with chronic intractable benign pain syndromes (CIBPS) were tested to determine whether their patterns differed from previous studies with normal volunteers. Nearly half of the CIBPS patients were found to be relatively insensitive to acute pain stimuli. A large number were also found to show negative relationships between S and A. These differences from control subjects were considered of potential importance in their implications concerning the nature of chronic pain and its differences from the acute pain process.

Peripheral fiber correlates to noxious thermal stimulation in humans.

Minimal conduction velocities of peripheral nerves contributing to acute thermal pain sensation in human volunteer subjects were calculated. Purely thermal stimulation was administered by a low power laser beam directed at the subjects' fingers, and subjective pain responses correlated with a peak in the event-related brain potential (ERBP). These cerebral responses were found to preclude C fiber peripheral activity from this phenomenon.

Single trial analysis of evoked potentials to noxious thermal stimulation in man.

Thermal (laser) evoked responses were obtained from 13 male volunteers. A single trial analysis technique with a latency adjusting adaptive filter was used to analyze evoked response amplitudes. Significant and substantial within-subject linear correlations were found between the magnitude (A) of the primary waveform (RMS muV of the P200--N300-P400 complex ) and subjective pain response (R) as well as stimulus intensity (S). Since subjective pain response was strongly correlated with stimulus intensity, the partial correlation coefficients were calculated for R vs. A with S controlled, and S vs. A with R controlled, for each subject. The partial correlations revealed a much stronger relationship between subjective response and the evoked response amplitude, suggesting that the primary complex may measure neural events in the pain perception process rather than transduction and transmission of the stimulus event.

What is a pain center?

The authors present the concept that a multi-disciplinary interdepartmental pain center should include: 1) an initial out-patient pain clinic, 2) an inpatient pain service for diagnosis of pain problems and treatment of patients with intractable pain, including pain from cancer, 3) a psychotherapeutically-oriented chronic benign Pain Unit, 4) facilities for clinical and basic pain research, and 5) affiliations for an adequate teaching program.

The role of cutaneous mechanoreceptors in thermal sensation and pain.

The functional behavior of slowly adapting cutaneous mechanoreceptors under combined thermal-mechanical stimulation was investigated by single-unit recordings from the lumbar dorsal roots of the cat. Increased sensitivity to bimodal stimulation was observed in 24 of the 28 units studied, employing stimulus-response functional behavior as the basis for judgment. Low-threshold receptors generally did not exhibit increased spontaneous firing as accompaniment to heightened sensitivity, while such enhanced basal activity was usually observed in moderate-threshold units in addition to increased reactivity. Information theory calculations performed on these stimulus-response data revealed that 15 of the 24 "heat-sensitive" receptors were additionally characterized by an enhanced ability to transmit neural information under bimodal stimulation. These results were interpreted as supporting pattern theories of pain as opposed to the concept of receptor specificity. No contradiction was observed, however, between the present results and those of other investigators. The present conclusions derive from new criteria for thermal reactivity based upon innovative stimulus conditions.

Needle scope attached to stereotactic frame for inspection of cisterna magna during percutaneous radiofrequency trigeminal tractotomy.

Use of a fiber optic needle scope attached to the Todd-Wells stereotactic unit has facilitated electrode placement during percutaneous trigeminal tractotomy.

Neurophysiology of pain-peripheral aspects. Speculation concerning the possibility of a unitary peripheral cutaneous input system for pressure, hot, cold and tissue damage.

The gate theory of pain is criticized at three levels: (1) at the dorsal horn "gate", where pre-synaptic inhibition in the primary afferent endings may go beyond mere reduction of synaptic power at the afferent endings and induce antidromic impulses (dorsal root reflexes) that may modulate peripherally by blocking; (2) central to the "gate", where postsynaptic neuronal repetitive (epileptiform) firing is believed to be an important underlying mechanism in clinical chronic pain syndromes; and, (3) in the periphery, where there is more to input coding than a balance between the ratio of large and smaller fiber inputs. Contrary to the belief of many sensory neurophysiologists, the present authors contend that pattern theory is viable; and that specificity, while important and not to be ignored, should be considered as only a partially evolved refinement superimposed on a basic underlying spatial and temporal patterning of input that probably requires central decoding, which begins in the dorsal horn.

Quantitative measures of the thermal reactivity of cutaneous mechanoreceptors.

Quantitative aspects of the non-specific response of cutaneous mechanoreceptors to two different modalities of stimulation were examined. Parameterization of stimulus temperature reveals the incompleteness of the commonly accepted criterion for thermal sensitivity and suggests the need for a re-examination of current concepts of specific mechanical, thermal and nociceptive sensibility.