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.

/pdf/25th-anniversary-article-engineering-hydrogels-for-52t0ojjbic.pdf

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

We present a method to produce tether for the electric solar wind sail (E-sail) by bonding 25µm aluminium wires to a 50µmwire using an industrial semiautomatic ultrasonic wire bonder. We have used this method to create a 1.4 meter long two-wire tether structure with more than 100 wire-to-wire bonds. We tested 38 wire-to-wire bonds by destructive pull test and showed that they meet the 5g pull strength requirement of electric sail.

Method for electric solar wind sail tether production

Guided acoustic waves (GAWs) are sensitive to delaminations between two materials. Certain modes can determine changes in adhesion for well-defined geometries. Considering the sensitivity of GAWs to detect delaminations, there is a paucity of attempts to quantify the adhesion between a hemispherical shell and a surrounding medium. The purpose of this study was to 1) quantify the average adhesion of a hemispherical metal shell to a polymer and 2) demonstrate that it is feasible to localize delaminations in the adhesion. A pair of custom made, needle-based transducers were used to launch GAWs into a 5 cm diameter, 1 mm thick hemispherical metal shell. The experiments were done in two parts: first, the average adhesion of a metal shell to a polymer was quantified by GAW attenuation. The shells were attached to a polymer (ultra-high molecular weight polyethylene) with various degrees of adhesion, ranging from a shell resting freely in a taylor-made polymer cavity to a shell which was adhered to the polymer with excess epoxy. In the second part, an artificial defect was generated in the interface between the metal shell and adhesive by drilling a hole into the polymer (to generate areas with no adhesion). Echoes of GAWs determined that defect localization on a curved surface is feasible. Finally, we determined that noncontacting laser ultrasonics could be used to replace the contacting needle-based transducers.

Non-contact quantification of adhesion between a metal hemispherical shell and a polymer by guided acoustic waves

We are developing a posturographic sleepiness tester that employs wavelet and fractal dimension analysis. It detects and characterizes transients in postural sway signals. We measured 6 subjects' postural steadiness (eyes open/closed) with a Nintendo Wii Fit ® balance board every hour during 10 h. During the next two weeks, we performed three additional measurements with different time awake (TA) for each subject. We analyzed the results with 5 literature-based continuous sway measures and with 8 transient-based sway measures. In the eyes open situation 95% of the estimated TA's were inside ±4.8 h and ±2.8 h, respectively. These results show that the transient analysis has potential to detect daytime changes in postural steadiness and that it can be used to enhance posturographic TA estimation during daytime and possibly during sustained wakefulness.

Transient analysis to enhance Wii Fit sleepiness tester

Electric solar wind sail in.scpace propulsion status report

The mechanical properties of single fibers are important when estimating the behavior of fibrous materials under stress. Appropriate tests are tedious and alter the fibers structurally and mechanically. To avoid this problem we developed a non-destructive ultrasonic method to determine the stiffness of 4–5 mm long, and 30 ± 10 µm thick, wood fibers during RH cycling (90% – 30% – 90 % R H , at 25 °C). The stiffness estimate was obtained by measuring the ultrasonic propagation velocity and fiber density. The stiffness of larch fibres was 75 – 170 GPa during the humidity cycling. The maximum stiffness increased by 3.7 % per cycle during 99 cycles. Our results correspond to earlier results from pull tests of plant fibers. We believe these to be the first reported ultrasonic stiffness measurements on individual plant fibers.

Edward Hæggström

Papers

Method for electric solar wind sail tether production

Non-contact quantification of adhesion between a metal hemispherical shell and a polymer by guided acoustic waves

Transient analysis to enhance Wii Fit sleepiness tester

Electric solar wind sail in.scpace propulsion status report

Ultrasonic stiffness measurements of single plant fibers during humidity cycling