Rapid 3D printing of anatomically accurate and mechanically heterogeneous aortic valve hydrogel scaffolds.

The aortic valve exhibits complex three-dimensional (3D) anatomy and heterogeneity essential for the long-term efficient biomechanical function. These are, however, challenging to mimic in de novo engineered living tissue valve strategies. We present a novel simultaneous 3D printing/photocrosslinking technique for rapidly engineering complex, heterogeneous aortic valve scaffolds. Native anatomic and axisymmetric aortic valve geometries (root wall and tri-leaflets) with 12-22 mm inner diameters (ID) were 3D printed with poly-ethylene glycol-diacrylate (PEG-DA) hydrogels (700 or 8000 MW) supplemented with alginate. 3D printing geometric accuracy was quantified and compared using Micro-CT. Porcine aortic valve interstitial cells (PAVIC) seeded scaffolds were cultured for up to 21 days. Results showed that blended PEG-DA scaffolds could achieve over tenfold range in elastic modulus (5.3±0.9 to 74.6±1.5 kPa). 3D printing times for valve conduits with mechanically contrasting hydrogels were optimized to 14 to 45 min, increasing linearly with conduit diameter. Larger printed valves had greater shape fidelity (93.3±2.6, 85.1±2.0 and 73.3±5.2% for 22, 17 and 12 mm ID porcine valves; 89.1±4.0, 84.1±5.6 and 66.6±5.2% for simplified valves). PAVIC seeded scaffolds maintained near 100% viability over 21 days. These results demonstrate that 3D hydrogel printing with controlled photocrosslinking can rapidly fabricate anatomical heterogeneous valve conduits that support cell engraftment.

A computational environment to simulate complex tendinous topologies.

Static and dynamic manipulation of objects with the fingertips (precision pinch) is essential to the activities of daily living. Despite numerous efforts to study the hand and its pinch function, a comprehensive understanding of biomechanical function and neuromuscular control of the fingers eludes researchers. To make progress in understanding precision pinch we are creating biomechanical models to simulate finger function, neuromuscular control and rehabilitation. An important challenge in creating biomechanical models of the fingers is to simulate the tension distribution in the extensor mechanism--a defining biomechanical feature of the fingers consisting of a tendinous network that wraps over the dorsum of the phalanges. We have created a biomechanical modeling environment that can, among other things, predict tension distribution in the extensor mechanism. Our predictions show that the distribution of tension can be very sensitive to the assumed network topology--the number of elements and their connectivity.

Three generations of automatically designed robots.

The difficulties associated with designing, building, and controlling robots have led their development to a stasis: Applications are limited mostly to repetitive tasks with predefined behavior. Over the last few years we have been trying to address this challenge through an alternative approach: Rather than trying to control an existing machine or create a general-purpose robot, we propose that both the morphology and the controller should evolve at the same time. This process can lead to the automatic design of special-purpose mechanisms and controllers for specific short-term objectives. Here we provide a brief review of three generations of our recent research, which underlies the robots shown on the cover of this issue: Automatically designed static structures, automatically designed and manufactured dynamic electromechanical systems, and modular robots automatically designed through a generative DNA-like encoding.

Clustering irregular shapes using high-order neurons.

This article introduces a method for clustering irregularly shaped data arrangements using high-order neurons. Complex analytical shapes are modeled by replacing the classic synaptic weight of the neuron by high-order tensors in homogeneous coordinates. In the first- and second-order cases, this neuron corresponds to a classic neuron and to an ellipsoidalmetric neuron. We show how high-order shapes can be formulated to follow the maximum-correlation activation principle and permit simple local Hebbian learning. We also demonstrate decomposition of spatial arrangements of data clusters, including very close and partially overlapping clusters, which are difficult to distinguish using classic neurons. Superior results are obtained for the Iris data.

Automatic design and manufacture of robotic lifeforms.

Biological life is in control of its own means of reproduction, which generally involves complex, autocatalysing chemical reactions. But this autonomy of design and manufacture has not yet been realized artificially. Robots are still laboriously designed and constructed by teams of human engineers, usually at considerable expense. Few robots are available because these costs must be absorbed through mass production, which is justified only for toys, weapons and industrial systems such as automatic teller machines. Here we report the results of a combined computational and experimental approach in which simple electromechanical systems are evolved through simulations from basic building blocks (bars, actuators and artificial neurons); the 'fittest' machines (defined by their locomotive ability) are then fabricated robotically using rapid manufacturing technology. We thus achieve autonomy of design and construction using evolution in a 'limited universe' physical simulation coupled to automatic fabrication.

Infrared and laser spectroscopic characterization of aluminium defects in cultured quartz.

Optical absorption at He-Ne and argon laser frequencies has been used to map the distribution of aluminium-hole centres, Al-H(+), in irradiated and vacuum swept quartz. The absorption band associated with Al-h(+) has a strong peak centered at 435 nm and a weaker one at 633 nm. Argon laser lines at 488 and 514.5 nm are close to the wavelength of the stronger peak, while the He-Ne line at 632.8 nm is nearly coincident with that of the weaker peak. Large variations in Al-h(+) absorption for irradiated and vacuum swept crystals correspond to visually observed changes in coloration. The laser method is sensitive and nondestructive, and gives a quantitative measurement adaptable to computerized scanning and recording of data. Measurements of both Al-h(+) and infrared-active aluminium hydroxide centres, Al-OH(-), are used to evaluate the distribution of aluminium-associated defect centres and determine aluminium variations over an entire crystal.

Impurity absorption in CVD ZnSe.

Fourier transmission spectroscopic and laser calorimetric measurements have been used to characterize bulk and surface impurity bands in CVD ZnSe. Temperature dependence studies indicate that, while absorption between 1200 cm-(l) and 1800 cm-(1) is mainly from bulk molecular impurities, surface impurities also contribute and introduce a departure from the expected exponential dependence of multiphonon absorption with frequency above 850 cm-(1).

Residual lattice absorption in semiconducting crystals: frequency and temperature dependence.

Experimental and theoretical investigations of multiphonon absorption in semiconducting solids are conducted. We find that structure persists at lower absorption levels (~10(-2) cm(-1)) and at high temperatures (up to ~550 K) in these materials, in agreement with theoretical predictions. The theoretical analysis indicates that the observed rate of decrease of the absorption with increasing frequency is consistent with available models for the interionic potential and electric moments in these materials. Moreover, the structure in the spectrum is found to be almost exclusively a result of density of states effects, as opposed to k-selection rules.

Temperature dependence of the refractive index of alkaline earth fluorides.

The temperature coefficient of the refractive index dn/dT of CaF(2), SrF(2), and BaF(2) single crystals is measured by a laser interferometric technique at a number of frequencies over temperatures ranging from 20 degrees C to 85 degrees C. Although dn/dT is found to display little dispersion between 0.6328 microm and 3.39 mum, its magnitude shows a slight increase with temperature. A possible origin of the latter effect is discussed.