Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

With advances in tissue engineering, the possibility of regenerating injured tissue or failing organs has become a realistic prospect for the first time in medical history. Tissue engineering - the combination of bioactive materials with cells to generate engineered constructs that functionally replace lost and/or damaged tissue - is a major strategy to achieve this goal. One facet of tissue engineering is biofabrication, where three-dimensional tissue-like structures composed of biomaterials and cells in a single manufacturing procedure are generated. Cell-laden hydrogels are commonly used in biofabrication and are termed "bioinks". Hydrogels are particularly attractive for biofabrication as they recapitulate several features of the natural extracellular matrix and allow cell encapsulation in a highly hydrated mechanically supportive three-dimensional environment. Additionally, they allow for efficient and homogeneous cell seeding, can provide biologically-relevant chemical and physical signals, and can be formed in various shapes and biomechanical characteristics. However, despite the progress made in modifying hydrogels for enhanced bioactivation, cell survival and tissue formation, little attention has so far been paid to optimize hydrogels for the physico-chemical demands of the biofabrication process. The resulting lack of hydrogel bioinks have been identified as one major hurdle for a more rapid progress of the field. In this review we summarize and focus on the deposition process, the parameters and demands of hydrogels in biofabrication, with special attention to robotic dispensing as an approach that generates constructs of clinically relevant dimensions. We aim to highlight this current lack of effectual hydrogels within biofabrication and initiate new ideas and developments in the design and tailoring of hydrogels. The successful development of a "printable" hydrogel that supports cell adhesion, migration, and differentiation will significantly advance this exciting and promising approach for tissue engineering.

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25th Anniversary Article: Engineering Hydrogels for Biofabrication

3D Printing promises to produce complex biomedical devices according to computer design using patient-specific anatomical data. Since its initial use as pre-surgical visualization models and tooling molds, 3D Printing has slowly evolved to create one-of-a-kind devices, implants, scaffolds for tissue engineering, diagnostic platforms, and drug delivery systems. Fueled by the recent explosion in public interest and access to affordable printers, there is renewed interest to combine stem cells with custom 3D scaffolds for personalized regenerative medicine. Before 3D Printing can be used routinely for the regeneration of complex tissues (e.g. bone, cartilage, muscles, vessels, nerves in the craniomaxillofacial complex), and complex organs with intricate 3D microarchitecture (e.g. liver, lymphoid organs), several technological limitations must be addressed. In this review, the major materials and technology advances within the last five years for each of the common 3D Printing technologies (Three Dimensional Printing, Fused Deposition Modeling, Selective Laser Sintering, Stereolithography, and 3D Plotting/Direct-Write/Bioprinting) are described. Examples are highlighted to illustrate progress of each technology in tissue engineering, and key limitations are identified to motivate future research and advance this fascinating field of advanced manufacturing.

/pdf/recent-advances-in-3d-printing-of-biomaterials-2tjce7ks8k.pdf

Recent advances in 3D printing of biomaterials

Conventional optical tweezers, formed at the diffraction-limited focus of a laser beam, have become a powerful and flexible tool for manipulating micrometre-sized objects. Extending optical trapping down to the nanometre scale would open unprecedented opportunities in many fields of science, where such nano-optical tweezers would allow the ultra-accurate positioning of single nano-objects. Among the possible strategies, the ability of metallic nanostructures to control light at the subwavelength scale can be exploited to engineer such nano-optical traps. This Review summarizes the recent advances in the emerging field of plasmon-based optical trapping and discusses the details of plasmon tweezers along with their potential applications to bioscience and quantum optics.

Plasmon nano-optical tweezers

A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering

Ultrasonic velocity of the upper gneiss series rocks from the Outokumpu deep drill hole, Fennoscandian shield — Comparing uniaxial to triaxial loading

In this paper, we introduce a new method to fabricate Capacitive Micromachined Ultrasonic Transducers (CMUT) that uses a wafer-bonding technique. The transducer membrane and cavity are defined separately on a Silicon-On-Insulator (SOI) wafer and on a prime quality silicon wafer, respectively. Using silicon direct bonding in a vacuum environment, the two wafers are bonded forming the transducer. This new process offers many advantages over surface micromachining on the fabrication of the transducers with different cavity and membrane configurations. CMUTs with different dimensions have been successfully fabricated and characterized. For the first time, sub-MHz operation is achieved with CMUTs. The test results show that the new process is a promising method to fabricate CMUTs for operation in air and water at different frequency ranges.

New fabrication process for Capacitive Micromachined Ultrasonic Transducers

A few models have earlier been proposed to explain Al oxide breakage during US wirebonding as presented by L. Levine, but no widely accepted theory exists. We propose a model to describe the AlOx breakage mechanism during tangential US excitation at constant pressure in a wirebonder. The model is based on theoretical estimations and experimental measurements. We measure with a laser Doppler vibrometer the relative wire-base displacement and propose that stick-slip and micro-slip behavior is prevalent during ultrasonic bonding. Displacement was measured at rim of the rectangular 14 mum thick and 80 mum width Al wire and at the silicon microchip base. A rectangular shaped wire was used to have probe light good reflection. We combined displacement measurements, detailed SEM analysis of contact interfaces and FEM of the bond structure during the bonding process. We also made a synthesis of the current bonding process knowledge. As a synthesis we propose a two-step model, including early stage scrubbing and later microweld expansion. Validation of the proposed model is discussed. This work and the obtained results are steps towards a fundamental quantitative US bonding theory that is necessary to develop reliable bonding technologies towards finer-pitch and more reliable interconnections.

P1E-5 Understanding Ultrasound-Induced Aluminum Oxide Breakage During Wirebonding

Coded signals are commonly used in communication and radar systems. However, in ultrasonics the use of coded signals is still relatively uncommon. Linear frequency modulated chirps, employed to improve signal to noise ratio (SNR), exhibit autocorrelation sidelobes whose amplitude increase with decreasing time bandwidth product. This is important in pulse-echo ultrasonics where the chirps need to be short. The side lobes can be attenuated by windowing the chirp amplitude with a suitable window function, but this broadens the main lobe which lowers the resolution. We describe an alternative method for chirp generation in the frequency domain. The chirp is generated by applying a quadratic phase shift into a signal with predefined spectrum. The autocorrelation side lobes of these chirps are nearly identical with the original unchirped pulse. A sum of four sine modulated Gaussians were used to provide the initial spectrum for evaluating the frequency domain chirping. This method holds potential for use in NDT pulse-echo measurements and medical ultrasonics where short codes to improve SNR are needed.

Frequency domain low time-bandwidth product chirp synthesis for pulse compression side lobe reduction

Influence of enzyme immobilization and skin-sensor interface on non-invasive glucose determination from interstitial fluid obtained by magnetohydrodynamic extraction.

Edward Hæggström

Papers

Ultrasonic velocity of the upper gneiss series rocks from the Outokumpu deep drill hole, Fennoscandian shield — Comparing uniaxial to triaxial loading

New fabrication process for Capacitive Micromachined Ultrasonic Transducers

P1E-5 Understanding Ultrasound-Induced Aluminum Oxide Breakage During Wirebonding

Frequency domain low time-bandwidth product chirp synthesis for pulse compression side lobe reduction

Influence of enzyme immobilization and skin-sensor interface on non-invasive glucose determination from interstitial fluid obtained by magnetohydrodynamic extraction.