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Research Papers: Fractal Engineering and Biomedicine

J. Nanotechnol. Eng. Med. 2016;6(3):031001-031001-6. doi:10.1115/1.4031276.

Ball milling (BM) offers a flexible process for nanomanufacturing of reactive bimetallic multiscale particulates (nanoheaters) for self-heated microjoining engineering materials and biomedical tooling. This paper introduces a mechanics-based process model relating the chaotic dynamics of BM with the random fractal structures of the produced particulates, emphasizing its fundamental concepts, underlying assumptions, and computation methods. To represent Apollonian globular and lamellar structures, the simulation employs warped ellipsoidal (WE) primitives of elasto-plastic strain-hardening materials, with Maxwell–Boltzmann distributions of ball kinetics and thermal transformation of hysteretic plastic, frictional, and residual stored energetics. Interparticle collisions are modeled via modified Hertzian contact impact mechanics, with local plastic deformation yielding welded microjoints and resulting in cluster assembly into particulates. The model tracks the size and diversity of such particulate populations as the process evolves via sequential collision and integration events. The simulation was shown to run in real-time computation speeds on modest hardware, and match successfully the fractal dimension and contour shape of experimental ball-milled Al–Ni particulate micrographs. Thus, the model serves as a base for the design of a feedback control system for continuous BM.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2016;6(3):031002-031002-9. doi:10.1115/1.4032005.

The aim of this study is to analyze the origin of multifractality of surface electromyography (sEMG) signals during dynamic contraction in nonfatigue and fatigue conditions. sEMG signals are recorded from triceps brachii muscles of 22 healthy subjects. The signals are divided into six equal segments on time scale for normalization. The first and sixth segments are considered as the nonfatigue and fatigue conditions, respectively. The source of multifractality can be due to correlation and probability distribution. The original sEMG series are transformed into shuffled and surrogate series. These three series namely, original, shuffled, and surrogate series in the nonfatigue and fatigue conditions are subjected to multifractal detrended fluctuation analysis (MFDFA) and features are extracted. The results indicate that sEMG signals exhibit multifractal behavior. Further investigation revealed that origin of multifractality is primarily due to correlation. The origin of multifractality due to correlation is quantified as 80% in nonfatigue and 86% in fatigue conditions. This method of multifractal analysis may be useful for analyzing the progressive changes in muscle contraction in varied neuromuscular studies.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2016;6(3):031003-031003-5. doi:10.1115/1.4032170.

In this work, data from two-dimensional (2D) images of the human retina were taken as a case study. First, the characteristic data points had been removed using the Douglas–Peucker (DP) method, and subsequently, more data points were added using random fractal interpolation approach, to reconstruct a three-dimensional (3D) model of the blood vessel. By visualizing the result, we can see that all the small blood vessels in the human retina are more visible and detailed. This algorithm of 3D reconstruction has the advantage of being fast with calculation time less than 40 s and also can reduce the 3D image storage level on a disk with a reduction ratio between 78% and 96.65%.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2016;6(3):031004-031004-7. doi:10.1115/1.4032689.

The aim of this study was to investigate the effect of dental caries on the stability of the periodontal system. This study presents a numerical analysis performed with three-dimensional (3D) finite element (FE) method to evaluate stresses in the bone surrounding the tooth with dynamic mastication combined loadings. In this work, we present a comparative study on infected and healthy periodontal systems. The infected tooth was modeled and a caries defect was introduced to the tooth coronal part. The infected tooth was evaluated and equivalent von Mises interface stress values were obtained for comparison with the ones exhibited by the healthy tooth. Our results by 3D FE analysis indicated that maximum stresses occurred primarily at the cervical level of root and alveolar bone. In the cortical bone, the stress value was greater in infected system (21.641 MPa) than in healthy system (15.752 MPa), i.e., a 37.4% increase. However, in the trabecular bone we observed only 1.6% increase in the equivalent stress values for the infected tooth model. Stress concentration at the cervical level may cause abnormal bone remodeling or bone loss, resulting loss of tooth attachment or bone damage. Our findings showed that decayed single-rooted teeth are more vulnerable to apical root resorption than healthy teeth. The numerical method presented in this study not only can aid the elucidation of the biomechanics of teeth infected by caries but also can be implemented to investigate the effectiveness of new advanced restorative materials and protocols.

Commentary by Dr. Valentin Fuster

Research Papers: Engineering Cell Microenvironments Using Novel Hydrogels

J. Nanotechnol. Eng. Med. 2016;6(3):031005-031005-9. doi:10.1115/1.4032602.

A novel chitosan scaffold with micro- and nano-hybrid structures was proposed in this study. The hemispheric array of the barrier layer of an anodic aluminum oxide (AAO) film was used as the substrate. Microelectromechanical systems and nickel electroforming techniques were integrated for fabricating chitosan scaffolds with different micro/nanohybrid structures. Nerve cells were then cultured on the conduits. It was demonstrated that the scaffold with pure microstructures can guide the nerve cells to grow along the ridges of the microstructure and some cells to grow across the groove in between two ridges of the microstructure. It was also shown that the scaffold with microscale ridges and nanopatterns on the groove between two ridges can more effectively guide the cells to grow along the ridges, thus enhancing the proliferation of nerve cells.

Commentary by Dr. Valentin Fuster

Technical Brief: Engineering Cell Microenvironments Using Novel Hydrogels

J. Nanotechnol. Eng. Med. 2016;6(3):034501-034501-5. doi:10.1115/1.4031898.

Fabrication of cellular spheroids is critical for creating functional tissue units and investigating the mechanism of tumorigenesis, development, and intercellular and cell–matrix interactions in vitro. Herein, we developed a novel, simple, and facile method for cell spheroid fabrication by using polyacrylamide/gelatin methacrylate (PA/GelMA) hydrogel composites. Arrays of Michigan Cancer Foundation-7 (MCF-7) breast cancer cell spheroids can be easily formed by tuning the GelMA composition. The shape and size of cell spheroids can be also well controlled by regulating cell seeding density and culturing time. All these results suggested that this simple and facile platform can serve as a useful tool to generate three-dimensional (3D) cell spheroids and can be integrated within high-throughput drug screening platforms, which will be of great help in engineering functional tissue models and regenerative medicines.

Commentary by Dr. Valentin Fuster

Technical Brief: Fractal Engineering and Biomedicine

J. Nanotechnol. Eng. Med. 2016;6(3):034502-034502-7. doi:10.1115/1.4032224.

To study the effect of damping due to branching in trees and fractal structures, a harmonic analysis was performed on a finite element model using commercially available software. The model represented a three-dimensional (3D) fractal treelike structure, with properties based on oak wood and with several branch configurations. As branches were added to the model using a recursive algorithm, the effects of damping due to branching became apparent: the first natural frequency amplitude decreased, the first peak widened, and the natural frequency decreased, whereas higher frequency oscillations remained mostly unaltered. To explain this nonlinear effect observable in the spectra of branched structures, an analytical interpretation of the damping was proposed. The analytical model pointed out the dependency of Cartesian damping from the Coriolis forces and their derivative with respect to the angular velocity of each branch. The results provide some insight on the control of chaotic systems. Adding branches can be an effective way to dampen slender structures but is most effective for large deformation of the structure.

Topics: Damping , Fractals , Vibration
Commentary by Dr. Valentin Fuster

Additional Research Paper

J. Nanotechnol. Eng. Med. 2016;6(3):034601-034601-24. doi:10.1115/1.4032759.

A reliable prediction of three-dimensional (3D) protein structures from sequence data remains a big challenge due to both theoretical and computational difficulties. We have previously shown that our kinetostatic compliance method (KCM) implemented into the Protofold package can overcome some of the key difficulties faced by other de novo structure prediction methods, such as the very small time steps required by the molecular dynamics (MD) approaches or the very large number of samples needed by the Monte Carlo (MC) sampling techniques. In this paper, we improve the free energy formulation used in Protofold by including the typically underrated entropic effects, imparted due to differences in hydrophobicity of the chemical groups, which dominate the folding of most water-soluble proteins. In addition to the model enhancement, we revisit the numerical implementation by redesigning the algorithms and introducing efficient data structures that reduce the expected complexity from quadratic to linear. Moreover, we develop and optimize parallel implementations of the algorithms on both central and graphics processing units (CPU/GPU) achieving speed-ups up to two orders of magnitude on the GPU. Our simulations are consistent with the general behavior observed in the folding process in aqueous solvent, confirming the effectiveness of model improvements. We report on the folding process at multiple levels, namely, the formation of secondary structural elements and tertiary interactions between secondary elements or across larger domains. We also observe significant enhancements in running times that make the folding simulation tractable for large molecules.

Commentary by Dr. Valentin Fuster

Discussion

J. Nanotechnol. Eng. Med. 2015;6(3):035501-035501-2. doi:10.1115/1.4032016.

Nwosu et al. [1] presented a review and parametric investigation into nanofluid viscosity models. The purpose of the present discussion is to draw the attention of readers that Nwosu et al. [1] did not refer to some nanofluid viscosity equations, which are available in literature. Different examples for the missing nanofluid viscosity equations are given below.

Topics: Viscosity , Nanofluids
Commentary by Dr. Valentin Fuster

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