Papers by Horacio Espinosa
Key Engineering Materials, Sep 1, 2017
A continuum model for GO membranes is developed in this study. The model is built representing th... more A continuum model for GO membranes is developed in this study. The model is built representing the membrane as a two-dimensional, heterogeneous, two-phase continuum and the constitutive behavior of each phase (graphitic or oxidized) is built based on DFTB simulations of representative patches. A hyper-elastic continuum model is employed for the graphene areas, while a continuum damage model is more adequate for representing the behavior of oxidized regions. A finite element implementation for GO membranes subjected to degradation and failure is then implemented and, to avoid localization instabilities and spurious mesh sensitivity, a simple crack band model is adopted. The developed implementation is then used to investigate the existence of GO nano-representative volume elements.
Journal of Micromechanics and Microengineering, Jan 5, 2006
This paper addresses the design and optimization of thermal actuators employed in a novel MEMS-ba... more This paper addresses the design and optimization of thermal actuators employed in a novel MEMS-based material testing system. The testing system is designed to measure the mechanical properties of a variety of materials/structures from thin films to one-dimensional structures, e.g. carbon nanotubes (CNTs) and nanowires (NWs). It includes a thermal actuator and a capacitive load sensor with a specimen in-between. The thermal actuator consists of a number of V-shaped beams anchored at both ends. It is capable of generating tens of milli-Newton force and a few micrometers displacement depending on the beams' angle and their number. Analytical expressions of the actuator thermomechanical response are derived and discussed. From these expressions, a number of design criteria are drawn and used to optimize the device response. The analytical predictions are compared with both finite element multiphysics analysis (FEA) and experiments. To demonstrate the actuator performance, polysilicon freestanding specimens cofabricated with the testing system are tested.
Nanowires are among the most exciting one-dimensional nanomaterials because of their unique prope... more Nanowires are among the most exciting one-dimensional nanomaterials because of their unique properties, which result primarily from their chemical composition and large surface area to volume ratio. These properties make them ideal building blocks for the development of next generation electronics, opto-electronics, and sensor systems. In this article, we focus on the unique mechanical properties of nanowires, which emerge from surface atoms having different electron densities and fewer bonding neighbors than atoms lying within the nanowire bulk. In this respect, atomistic simulations have revealed a plethora of novel surface-driven mechanical behavior and properties, including both increases and decreases in elastic stiffness, phase transformations, shape memory, and pseudoelastic effects. This article reviews such atomistic simulations, as well as experimental data of these phenomena, while assessing future challenges and directions.
Nature Communications, Feb 7, 2017
Advanced NanoBiomed Research
Soft biological tissues are natural biomaterials with structures that have evolved to perform phy... more Soft biological tissues are natural biomaterials with structures that have evolved to perform physiological functions, for example, conferring elasticity while preserving the mechanical integrity of arteries. Furthermore, the mechanical properties of the tissue extracellular matrix (ECM) significantly affect cell behavior and organ function. ECM mechanical properties are strongly affected by collagen ultrastructure, and perturbations in collagen networks can cause tissue mechanical failure. It is thus crucial to understand the ultrastructural mechanical properties of soft tissues. Herein, the ultrastructural and nanomechanical properties of arterial tissues are reported. Specifically, maps of aorta tissue stiffness in its three constitutive layers, namely tunica intima, media, and adventitia, are reported. Atomic force microscopy (AFM) with large and ultrasharp tips is used to explore tissue stiffness at two scales. Quasistatic tensile tests are further conducted to understand a pot...
Scuola di Dottorato 'Pitagora' Scienze Ingegneristiche, Dottorato di Ricerca in Ingegneri... more Scuola di Dottorato 'Pitagora' Scienze Ingegneristiche, Dottorato di Ricerca in Ingegneria Meccanica Ciclo XXV Ciclo, a.a. 2011-2012
National Science Foundation (U.S.). Science and Technology Center Emergent Behaviors of Interated... more National Science Foundation (U.S.). Science and Technology Center Emergent Behaviors of Interated Cellular Systems (EBICS) (Grant CBET-0939511)
npj Computational Materials, 2021
This investigation presents a generally applicable framework for parameterizing interatomic poten... more This investigation presents a generally applicable framework for parameterizing interatomic potentials to accurately capture large deformation pathways. It incorporates a multi-objective genetic algorithm, training and screening property sets, and correlation and principal component analyses. The framework enables iterative definition of properties in the training and screening sets, guided by correlation relationships between properties, aiming to achieve optimal parametrizations for properties of interest. Specifically, the performance of increasingly complex potentials, Buckingham, Stillinger-Weber, Tersoff, and modified reactive empirical bond-order potentials are compared. Using MoSe2as a case study, we demonstrate good reproducibility of training/screening properties and superior transferability. For MoSe2, the best performance is achieved using the Tersoff potential, which is ascribed to its apparent higher flexibility embedded in its functional form. These results should fac...
Damage Initiation and Prediction in Composites, Sandwich Structures and Thermal Barrier Coatings, 2001
Dynamic crack propagation in an unidirectional Carbon/Epoxy composite is studied through finite e... more Dynamic crack propagation in an unidirectional Carbon/Epoxy composite is studied through finite element analyses in total Lagrangian co-ordinates. A finite deformation anisotropic visco-plastic model is used to describe the constitutive response of the composite. Crack initiation and propagation is simulated by embedding zero thickness interface element along the possible crack path. An irreversible cohesive law is used to describe the evolution of normal and shear tractions as a function of displacement jumps. The compressive response prior to interface failure is analyzed using contact impenetrability conditions. The failure of the first interface element at the pre-notch tip models crack initiation. Crack propagation is modeled through consecutive failure of interface elements. Dynamic crack propagation phenomena are studied in terms of crack initiation time, crack speed, mode I and mode II displacement jumps and tractions associated with the failure of interface elements, effect...
Small, 2020
Introducing exogenous molecules into cells with high efficiency and dosage control is a crucial s... more Introducing exogenous molecules into cells with high efficiency and dosage control is a crucial step in basic research as well as clinical applications. Here, the capability of the nanofountain probe electroporation (NFP‐E) system to deliver proteins and plasmids in a variety of continuous and primary cell types with appropriate dosage control is reported. It is shown that the NFP‐E can achieve fine control over the relative expression of two cotransfected plasmids. Finally, the dynamics of electropore closure after the pulsing ends with the NFP‐E is investigated. Localized electroporation has recently been utilized to demonstrate the converse process of delivery (sampling), in which a small volume of the cytosol is retrieved during electroporation without causing cell lysis. Single‐cell temporal sampling confers the benefit of monitoring the same cell over time and can provide valuable insights into the mechanisms underlying processes such as stem cell differentiation and disease p...
Acta Biomaterialia, 2021
Nature's masterfully synthesized biological materials take on greater relevance when viewed t... more Nature's masterfully synthesized biological materials take on greater relevance when viewed through the perspective of evolutionary abundance. The fact that beetles (order Coleoptera) account for a quarter of all extant lifeforms on Earth, makes them prime exponents of evolutionary success. In fact, their forewings are acknowledged as key traits to their radiative-adaptive success, which makes beetle elytra a model structure for next-generation bioinspired synthetic materials. In this work, the multiscale morphological and mechanical characteristics of a variety of beetle species from the Cetoniinae subfamily are investigated with the aim of unraveling the underlying principles behind Nature's adaptation of the elytral bauplan to differences in body weight spanning three orders of magnitude. Commensurate with the integral implications of size variation in organisms, a combined material, morphological, and mechanical characterization framework, across spatial scales, was pursued. The investigation revealed the simultaneous presence of size-invariant strategies (chemical compositions, layered-fibrous architectures, graded motifs) as well as size-dependent features (scaling of elytral layers and characteristic dimensions of building blocks), synergistically combined to achieve similar levels of biomechanical functionality (stiffness, energy absorption, strength, deformation and toughening mechanisms) in response to developmental and selection constraints. The integral approach here presented seeks to shed light on Nature's solution to the problem of size variation, which underpins the diversity of beetles and the living world.
Experimental Mechanics, 2021
Journal of the Mechanical Behavior of Biomedical Materials, 2020
Naturally occurring biological materials with stiff fibers embedded in a ductile matrix are commo... more Naturally occurring biological materials with stiff fibers embedded in a ductile matrix are commonly known to achieve excellent balance between stiffness, strength and ductility. In particular, biological composite materials with helicoidal architecture have been shown to exhibit enhanced damage tolerance and increased impact energy absorption. However, the role of fiber reorientation inside the flexible matrix of helicoid composites on their mechanical behaviors have not yet been extensively investigated. In the present work, we introduce a Discontinuous Fiber Helicoid (DFH) composite inspired by both the helicoid microstructure in the cuticle of mantis shrimp and the nacreous architecture of the red abalone shell. We employ 3D printed specimens, analytical models and finite element models to analyze and quantify in-plane fiber reorientation in helicoid architectures with different geometrical features. We also introduce additional architectures, i.e., single unidirectional lamina and mono-balanced architectures, for comparison purposes. Compared with associated mono-balanced architectures, helicoid architectures exhibit less fiber reorientation values and lower values of strain stiffening. The explanation for this difference is addressed in terms of the measured in-plane deformation, due to uniaxial tensile of the laminae, correlated to lamina misorientation with respect to the loading direction and lay-up sequence.
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Papers by Horacio Espinosa