Co-reporter:Jingkai Guo, Tanvi Shroff, ChangKyu Yoon, Jiayu Liu, Joyce C. Breger, David H. Gracias, Thao D. Nguyen
Extreme Mechanics Letters 2017 Volume 16(Volume 16) pp:
Publication Date(Web):1 October 2017
DOI:10.1016/j.eml.2017.08.001
Curved bilayer plates with soft and stiff segments are widely observed in nature, such as plant cell walls, insect exoskeletons and reptile skins. We report an unusual biaxial and bidirectional bending of microfabricated bilayer plates composed of a swellable, photopolymerized poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM-AAc) layer and a regular array of SU-8, a stiff, non-swellable epoxy. Hydrogels such as pNIPAM-AAc exhibit large and reversible swelling in water in response to a temperature change through the lower critical solution temperature (LCST). The stimuli responsive behavior was harnessed in the composite structure containing materials with mismatched swelling and elastic properties to produce actuation and mechanical motion. The structure undergoes reversible bending along two different axes in response to a temperature cycle through the LCST. Cooling the patterned bilayer structure leads to bending upwards about one axis, while heating leads to bending downwards about a different axis, 90o from the first. To understand the mechanism of this biaxial bending behavior, we developed a finite element model of the patterned bilayer structure. A constitutive model that combined the hyperelastic and swelling behavior was used to describe the thermoresponsive hydrogel. The model was applied to investigate the effects of geometric factors of the patterned bilayer on the bending behavior of the composite structure.
Co-reporter:Dan E. Midgett, Mary E. Pease, Joan L. Jefferys, Mohak Patel, ... Thao D. Nguyen
Acta Biomaterialia 2017 Volume 53(Volume 53) pp:
Publication Date(Web):15 April 2017
DOI:10.1016/j.actbio.2016.12.054
The objective of this study was to measure the pressure-induced deformation response of the human lamina cribrosa (LC) and analyze for variations with age and anatomical region. The posterior scleral cup of 8 eyes from 6 human donors was mounted onto a custom inflation chamber. A laser-scanning microscope was used for second harmonic generation (SHG) imaging of the collagen structure in the posterior volume of the LC at pressures from 5 mmHg to 45 mmHg. The SHG volumes were analyzed by the Fast-Fourier Iterative Digital Volume Correlation (DVC) algorithm for the three dimensional (3D) displacement field. The components of the Green–Lagrange strain tensor and the in-plane principal and maximum shear strains were evaluated from the DVC displacement field for the central and peripheral regions of the LC and the nasal, temporal, inferior, and superior quadrants surrounding the central retinal artery and vein. Among the major findings were that older age was associated with lower strains, the maximum shear strain was larger in the peripheral than central region, and the maximum principal strain was lower in the nasal quadrant. The elliptical shape of the LC was also predictive of the biaxial strain ratio. Age-related and structure-related variations in the pressure-induced strains of the LC may contribute to the susceptibility and severity of optic nerve damage in glaucoma, and regional variations may explain the progression of axonal damage and tissue remodeling observed in the LC in glaucoma.Statement of SignificanceGlaucoma causes vision loss through progressive damage of the retinal ganglion axons at the lamina cribrosa (LC), the connective tissue structure that supports the axons as they leave the eye. Mechanical characterization of the LC is challenging because of the complex 3D shape and inaccessibility of the tissue. We present a new method using digital volume correlation to map the 3D displacement and strain fields in the LC under inflation. We report for the first time significant regional variations in the strains that are consistent with the pattern of optic nerve damage in early glaucoma. Thus regional strain variations may be predictive of the progression of axonal damage in glaucoma.Download high-res image (152KB)Download full-size image
Co-reporter:Rui Xiao, Jingkai Guo, David L. Safranski and Thao D. Nguyen
Soft Matter 2015 vol. 11(Issue 20) pp:3977-3985
Publication Date(Web):09 Apr 2015
DOI:10.1039/C5SM00543D
Thermally-activated temperature memory and multiple shape memory effects have been observed in amorphous polymers with a broad glass transition. In this work, we demonstrate that the same shape recovery behaviors can also be achieved through solvent absorption. We investigate the recovery behaviors of programmed Nafion membranes in various solvents and compare the solvent-driven and temperature-driven shape recovery response. The results show that the programming temperature and solvent type have a corresponding strong influence on the shape recovery behavior. Specifically, lower programming temperatures induce faster initial recovery rates and larger recovery, which is known as the temperature memory effect. The temperature memory effect can be used to achieve multi-staged and multiple shape recovery of specimens programmed at different temperatures. Different solvents can also induce different shape recovery, analogous to the temperature memory effect, and can also provide a mechanism for multi-staged and multiple shape memory recovery.
Co-reporter:Rui Xiao, Jingkai Guo and Thao D. Nguyen
RSC Advances 2015 vol. 5(Issue 1) pp:416-423
Publication Date(Web):13 Nov 2014
DOI:10.1039/C4RA11412D
Amorphous polymers achieve a shape memory effect from the tremendous change of chain mobility in the glass transition. The span of the glass transition region has a direct influence on the shape recovery behavior. In this paper, we investigated the shape memory behavior of Nafion, which has an extremely broad glass transition region. We measured the influence of the shape memory programming temperature on the recovery response and multiple shape memory effect. We applied a finite deformation, nonlinear viscoelastic model with a discrete spectrum of relaxation times to describe the shape memory behavior of the material. The parameters of the relaxation spectrum were determined from the master curve of the relaxation modulus. The model was implemented for finite element analysis and applied to design a multiple switchable pattern transformation of a Nafion membrane.
Co-reporter:Rajneesh Bhardwaj;Kimberly Ziegler
Biomechanics and Modeling in Mechanobiology 2014 Volume 13( Issue 1) pp:123-140
Publication Date(Web):2014/01/01
DOI:10.1007/s10237-013-0490-3
Ocular injuries from blast have increased in recent wars, but the injury mechanism associated with the primary blast wave is unknown. We employ a three-dimensional fluid–structure interaction computational model to understand the stresses and deformations incurred by the globe due to blast overpressure. Our numerical results demonstrate that the blast wave reflections off the facial features around the eye increase the pressure loading on and around the eye. The blast wave produces asymmetric loading on the eye, which causes globe distortion. The deformation response of the globe under blast loading was evaluated, and regions of high stresses and strains inside the globe were identified. Our numerical results show that the blast loading results in globe distortion and large deviatoric stresses in the sclera. These large deviatoric stresses may be indicator for the risk of interfacial failure between the tissues of the sclera and the orbit.
Co-reporter:Theresa K. Tonge, Lorre S. Atlan, Liming M. Voo, Thao D. Nguyen
Acta Biomaterialia 2013 Volume 9(Issue 4) pp:5913-5925
Publication Date(Web):April 2013
DOI:10.1016/j.actbio.2012.11.035
Abstract
The nonlinear anisotropic properties of human skin tissue were investigated using bulge testing. Full-field displacement data were obtained during testing of human skin tissues procured from the lower back of post-mortem human subjects using 3-D digital image correlation. To measure anisotropy, the dominant fiber direction of the tissue was determined from the deformed geometry of the specimen. Local strains and stress resultants were calculated along both the dominant fiber direction and the perpendicular direction. Variation in anisotropy and stiffness was observed between specimens. The use of stress resultants rather than the membrane stress approximation accounted for bending effects, which are significant for a thick nonlinear tissue. Of the six specimens tested, it was observed that specimens from older donors exhibited a stiffer and more isotropic response than those from younger donors. It was seen that the mechanical response of the tissue was negligibly impacted by preconditioning or the ambient humidity. The methods presented in this work for skin tissue are sufficiently general to be applied to other planar tissues, such as pericardium, gastrointestinal tissue, and fetal membranes. The stress resultant–stretch relations will be used in a companion paper to obtain material parameters for a nonlinear anisotropic hyperelastic model.
Co-reporter:Theresa K. Tonge, Liming M. Voo, Thao D. Nguyen
Acta Biomaterialia 2013 Volume 9(Issue 4) pp:5926-5942
Publication Date(Web):April 2013
DOI:10.1016/j.actbio.2012.11.034
Abstract
A thin shell method is presented to analyze the results of the bulge test presented in Part I of this paper. The method accounts for the effects of bending, which can be significant for thick tissues inflated from a planar state. We fit two commonly used hyperelastic distributed fiber constitutive models to the stretch–stress resultant data for human skin tissue calculated in Part I from the measured inflation pressure and deformed geometry of the tissue. To validate the method, the resulting parameters were implemented in a specimen-specific finite-element analysis. The method was capable of reproducing the experimentally measured pressure–stretch response of the tissue for a fully integrated distributed fiber model, but not for the pre-integrated distributed fiber models. The parameters obtained for the pre-integrated models significantly underestimated the anisotropic properties of the tissue. The thin shell method presented in this work has been applied to human skin tissues but is sufficiently general to be applied to analyze the inflation response of other planar tissues.
Co-reporter:Rui Xiao and Thao D. Nguyen
Soft Matter 2013 vol. 9(Issue 39) pp:9455-9464
Publication Date(Web):27 Aug 2013
DOI:10.1039/C3SM51210J
We present a constitutive model for the effect of solvent absorption on the thermomechanical properties and shape-memory behavior of amorphous polymers. The absorption of low concentrations of solvent depresses the glass transition temperature by increasing the molecular mobility of the polymer chains. In a shape-memory application, this can lead to premature shape recovery and significant alterations to the time-dependence and temperature-dependence of shape recovery. We implemented the constitutive model for finite element analysis and developed a computational model that considered the time-dependent effect of diffusion to study the solvent-induced shape-memory behavior of a meth(acrylate) copolymer network. The model was validated by comparing to isothermal shape recovery experiments of specimens in air and water at different temperatures.
Co-reporter:Jinwoo Choi, Alicia M. Ortega, Rui Xiao, Christopher M. Yakacki, Thao D. Nguyen
Polymer 2012 Volume 53(Issue 12) pp:2453-2464
Publication Date(Web):25 May 2012
DOI:10.1016/j.polymer.2012.03.066
This paper presents an experimental and modeling study of the effects of physical aging on the shape-memory performance of (meth)acrylate-based networks composed of tert-butyl acrylate (tBA) crosslinked by various concentrations of poly(ethylene glycol dimethacrylate) (PEGDMA). The experiments measured the unconstrained recovery response of samples stored at 20 °C (Tg − 36 °C) for zero to 180 days and evaluated the effects of storage on the strain fixity, activation temperature, and initial recovery rate. A thermoviscoelastic model recently developed for amorphous networks near the Tg was applied to study the influence of structural and viscoelastic relaxation and the aging time and temperature on the recovery response. Results showed that the activation temperature and the initial recovery rate increased with the aging time, producing a sharper initial recovery response. The thermoviscoelastic model predicted that the magnitude of these effects depended on the aging temperature. There was an optimum aging temperature that maximized the initial recovery rate. These results suggest that physical aging can be manipulated to accelerate the recovery performance of shape-memory polymer devices.Graphical abstract
Co-reporter:T. D. Nguyen;B. L. Boyce
Biomechanics and Modeling in Mechanobiology 2011 Volume 10( Issue 3) pp:323-337
Publication Date(Web):2011 June
DOI:10.1007/s10237-010-0237-3
An inverse finite element method was developed to determine the anisotropic properties of bovine cornea from an in vitro inflation experiment. The experiment used digital image correlation (DIC) to measure the three-dimensional surface geometry and displacement field of the cornea at multiple pressures. A finite element model of a bovine cornea was developed using the DIC measured surface geometry of the undeformed specimen. The model was applied to determine five parameters of an anisotropic hyperelastic model that minimized the error between the measured and computed surface displacement field and to investigate the sensitivity of the measured bovine inflation response to variations in the anisotropic properties of the cornea. The results of the parameter optimization revealed that the collagen structure of bovine cornea exhibited a high degree of anisotropy in the limbus region, which agreed with recent histological findings, and a transversely isotropic central region. The parameter study showed that the bovine corneal response to the inflation experiment was sensitive to the shear modulus of the matrix at pressures below the intraocular pressure, the properties of the collagen lamella at higher pressures, and the degree of anisotropy in the limbus region. It was not sensitive to a weak collagen anisotropy in the central region.
Co-reporter:Thao. D. Nguyen;Christopher M. Yakacki;Parth D. Brahmbhatt;Matthew L. Chambers
Advanced Materials 2010 Volume 22( Issue 31) pp:3411-3423
Publication Date(Web):
DOI:10.1002/adma.200904119
Abstract
In this progress report, we review two common approaches to constitutive modeling of thermally activated shape memory polymers, then focus on a recent thermoviscoelastic model that incorporates the time-dependent effects of structural and stress relaxation mechanisms of amorphous networks. An extension of the model is presented that incorporates the effects of multiple discrete structural and stress relaxation processes to more accurately describe the time-dependent behavior. In addition, a procedure is developed to determine the model parameters from standard thermomechanical experiments. The thermoviscoelastic model was applied to simulate the unconstrained recovery response of a family of (meth)acrylate-based networks with different weight fractions of the crosslinking agent. Results showed significant improvement in predicting the temperature-dependent strain recovery response.
Co-reporter:Kristin M. Myers, Baptiste Coudrillier, Brad L. Boyce, Thao D. Nguyen
Acta Biomaterialia 2010 Volume 6(Issue 11) pp:4327-4335
Publication Date(Web):November 2010
DOI:10.1016/j.actbio.2010.06.007
Abstract
An in vitro inflation test method was developed to characterize the mechanical behavior of the bovine posterior sclera. The method used digital image correlation to provide a spatially resolved, full-field deformation map of the surface of the posterior sclera in response to controlled pressurization. A series of experiments were performed in the range of 2–6 kPa (15–45 mmHg) to characterize the load–unload displacement response at various pressure rates and the time-dependent displacement response at different applied pressures. The magnitude of the displacement was largest in the peripapillary region, mainly between the apex and the optic nerve head. Further, the results showed that bovine scleral tissue exhibited nonlinear and viscoelastic behavior characterized by a rate-dependent displacement response, hysteresis during unloading and creep. The creep rate was insensitive to the applied pressure, suggesting that the tissue can be modeled as a quasilinear viscoelastic material in the physiological pressure range of 2–6 kPa.
Co-reporter:Rui Xiao, Thao D. Nguyen
Journal of the Mechanics and Physics of Solids (September 2015) Volume 82() pp:62-81
Publication Date(Web):1 September 2015
DOI:10.1016/j.jmps.2015.05.021
Amorphous polymers lack an organized microstructure, yet they exhibit structural evolution, where physical properties change with time, temperature, and inelastic deformation. To describe the influence of structural evolution on the mechanical behavior of amorphous polymers, we developed a thermomechanical theory that introduces the effective temperature as a thermodynamic state variable representing the nonequilibrium configurational structure. The theory couples the evolution of the effective temperature and internal state variables to describe the temperature-dependent and rate-dependent inelastic response through the glass transition. We applied the theory to model the effect of temperature, strain rate, aging time, and plastic pre-deformation on the uniaxial compression response and enthalpy change with temperature of an acrylate network. The results showed excellent agreement with experiments and demonstrate the ability of the effective temperature theory to explain the complex thermomechanical behavior of amorphous polymers.
Co-reporter:Thao. D. Nguyen, H. Jerry Qi, Francisco Castro, Kevin N. Long
Journal of the Mechanics and Physics of Solids (September 2008) Volume 56(Issue 9) pp:2792-2814
Publication Date(Web):1 September 2008
DOI:10.1016/j.jmps.2008.04.007
A thermoviscoelastic constitutive model is developed for amorphous shape memory polymers (SMP) based on the hypothesis that structural and stress relaxation are the primary molecular mechanisms of the shape memory effect and its time-dependence. This work represents a new and fundamentally different approach to modeling amorphous SMPs. A principal feature of the constitutive model is the incorporation of the nonlinear Adam–Gibbs model of structural relaxation and a modified Eyring model of viscous flow into a continuum finite–deformation thermoviscoelastic framework. Comparisons with experiments show that the model can reproduce the strain–temperature response, the temperature and strain-rate dependent stress–strain response, and important features of the temperature dependence of the shape memory response. Because the model includes structural relaxation, the shape memory response also exhibits a dependence on the cooling and heating rates.
Co-reporter:Thao D. Nguyen, Jingkai Guo, Rui Xiao
Journal of the Mechanics and Physics of Solids (June 2017) Volume 103() pp:1-2
Publication Date(Web):1 June 2017
DOI:10.1016/j.jmps.2017.02.013
Co-reporter:Rui Xiao, Jinwoo Choi, Nishant Lakhera, Christopher M. Yakacki, Carl P. Frick, Thao D. Nguyen
Journal of the Mechanics and Physics of Solids (July 2013) Volume 61(Issue 7) pp:1612-1635
Publication Date(Web):1 July 2013
DOI:10.1016/j.jmps.2013.02.005
In this paper, a thermomechanical constitutive model was developed for the time-dependent behaviors of the glass transition of amorphous networks. The model used multiple discrete relaxation processes to describe the distribution of relaxation times for stress relaxation, structural relaxation, and stress-activated viscous flow. A non-equilibrium thermodynamic framework based on the fictive temperature was introduced to demonstrate the thermodynamic consistency of the constitutive theory. Experimental and theoretical methods were developed to determine the parameters describing the distribution of stress and structural relaxation times and the dependence of the relaxation times on temperature, structure, and driving stress. The model was applied to study the effects of deformation temperatures and physical aging on the shape-memory behavior of amorphous networks. The model was able to reproduce important features of the partially constrained recovery response observed in experiments. Specifically, the model demonstrated a strain-recovery overshoot for cases programmed below Tg and subjected to a constant mechanical load. This phenomenon was not observed for materials programmed above Tg. Physical aging, in which the material was annealed for an extended period of time below Tg, shifted the activation of strain recovery to higher temperatures and increased significantly the initial recovery rate. For fixed-strain recovery, the model showed a larger overshoot in the stress response for cases programmed below Tg, which was consistent with previous experimental observations. Altogether, this work demonstrates how an understanding of the time-dependent behaviors of the glass transition can be used to tailor the temperature and deformation history of the shape-memory programming process to achieve more complex shape recovery pathways, faster recovery responses, and larger activation stresses.
Co-reporter:Kristin M. Myers, Frances E. Cone, Harry A. Quigley, Scott Gelman, Mary E. Pease, Thao D. Nguyen
Experimental Eye Research (December 2010) Volume 91(Issue 6) pp:866-875
Publication Date(Web):1 December 2010
DOI:10.1016/j.exer.2010.09.009
The purpose of this research was to develop a reliable and repeatable inflation protocol to measure the scleral inflation response of mouse eyes to elevations in intraocular pressure (IOP), comparing the inflation response exhibited by the sclera of younger and older C57BL/6 mice. Whole, enucleated eyes from younger (2 month) and older (11 month) C57BL/6 mice were mounted by the cornea on a custom fixture and inflated according to a load-unload, ramp-hold pressurization regimen via a cannula connected to a saline-filled programmable syringe pump. First, the tissue was submitted to three load-unload cycles from 6 mmHg to 15 mmHg at a rate of 0.25 mmHg/s with ten minutes of recovery between cycles. Next the tissue was submitted to a series of ramp-hold tests to measure the creep behavior at different pressure levels. For each ramp-hold test, the tissue was loaded from 6 mmHg to the set pressure at a rate of 0.25 mmHg/s and held for 30 min, and then the specimens were unloaded to 6 mmHg for 10 min. This sequence was repeated for set pressures of: 10.5, 15, 22.5, 30, 37.5, and 45 mmHg. Scleral displacement was measured using digital image correlation (DIC), and fresh scleral thickness was measured optically for each specimen after testing. For comparison, scleral thickness was measured on untested fresh tissue and epoxy-fixed tissue from age-matched animals. Comparing the apex displacement of the different aged specimens, the sclera of older animals had a statistically significant stiffer response to pressurization than the sclera of younger animals. The stiffness of the pressure-displacement response of the apex measured in the small-strain (6–15 mmHg) and the large-strain (37.5–45 mmHg) regime, respectively, were 287 ± 100 mmHg/mm and 2381 ± 191 mmHg/mm for the older tissue and 193 ± 40 mmHg/mm and 1454 ± 93 mmHg/mm for the younger tissue (Student t-test, p < 0.05). The scleral thickness varied regionally, being thickest in the peripapillary region and thinnest at the equator. Fresh scleral thickness did not differ significantly by age in this group of animals. This study presents a reliable inflation test protocol to measure the mechanical properties of mouse sclera. The inflation methodology was sensitive enough to measure scleral response to changes in IOP elevations between younger and older C57BL/6 mice. Further, the specimen-specific scleral displacement profile and thickness measurements will enable future development of specimen-specific finite element models to analyze the inflation data for material properties.Research highlights► This paper presents the first experimental protocol for in vitro inflation testing of intact mouse sclera and reports the inflation response and fresh-tissue thickness measurements comparing younger (2 month) and older (11 month) C57BL/6 mouse eye specimens. The mechanical response of the sclera to increases in pressure, given by the pressure-displacement curves for both younger and older specimens, was non-linear and time-dependent. We found that the thickness of fresh scleral sections was largest in the peripapillary region, thinnest in the equator, and intermediate at the limbus. In the study presented here, we found a statistically significant difference in the inflation response between younger and older C57BL/6 mouse sclera, with younger tissue more compliant than older specimens from the same mouse strain.
Co-reporter:Rui Xiao, Thao D. Nguyen
Procedia IUTAM (2015) Volume 12() pp:154-161
Publication Date(Web):1 January 2015
DOI:10.1016/j.piutam.2014.12.017
The time-dependent behavior of glass transition can be exploited to achieve shape-memory behavior in amorphous polymers. Modeling the shape-memory effect in polymers requires considering the viscoelastic behavior, structural evolution in response to external stimuli, such as temperature, stress, solvent. Furthermore, the broad spectrum of stress and structural relaxation times are needed to accurately predict the rate-dependent recovery response. In this paper, we review our efforts to develop constitutive models for the glass transition behavior of amorphous polymers to predict the shape-memory behavior under a variety of thermomechanical conditions.
Co-reporter:T.D. Nguyen, R.E. Jones, B.L. Boyce
International Journal of Solids and Structures (15 December 2007) Volume 44(Issues 25–26) pp:8366-8389
Publication Date(Web):15 December 2007
DOI:10.1016/j.ijsolstr.2007.06.020
This paper presents constitutive models for the anisotropic, finite-deformation viscoelastic behavior of soft fiber-reinforced composites. An essential assumption of the models is that both the fiber reinforcements and matrix can exhibit distinct time-dependent behavior. As such, the constitutive formulation attributes a different viscous stretch measure and free energy density to the matrix and fiber phases. Separate flow rules are specified for the matrix and the individual fiber families. The flow rules for the fiber families then are combined to give an anisotropic flow rule for the fiber phase. This is in contrast to many current inelastic models for soft fiber-reinforced composites which specify evolution equations directly at the composite level. The approach presented here allows key model parameters of the composite to be related to the properties of the matrix and fiber constituents and to the fiber arrangement. An efficient algorithm is developed for the implementation of the constitutive models in a finite-element framework, and examples are presented examining the effects of the viscoelastic behavior of the matrix and fiber phases on the time-dependent response of the composite.
