Research Assistant- PhD Candidate, University of Georgia
Computational and theoretical investigation of growth, instability and gyrification of a brain
My research focuses on the study of physics and characterization of soft materials especially growth and remodeling of soft biological tissues, e.g. Brain, skin, solid tumor, airway, to understand fundamental principles of growth, morphological instability and post-instability pattern selection of living tissues to open new insights into therapy of known disorders and malformations of biological organs. Growth, morphological instabilities and post-instability surface patterns of soft materials such as elastomers, polymeric gels and biological soft tissues are hot topic to a number of research fields including functional material, soft lithography, flexible electronics, and biomedical engineering. Morphological surface instability of soft materials with a wide range applications has attracted great interest between academic people over the past decade. Growth and cortical folding of brain, wrinkling of aged skin, growth and creasing of solid tumor, wrinkling of mucosa in a esophagi, rippling of leaves, buckling of artery, surface wrinkling and creasing of a swelling hydrogel, and wrinkling of gut all are examples of growth and remodeling of soft tissues which undergo large deformation and acquire various morphological instabilities in response to self or environmental stimuli. Although significant progress has been achieved in modeling of growth and morphological instability in soft matter, there remain many interesting problems that worth further investigation. Therefore, much experimental and theoretical effort is needed to explore characteristics of surface patterns as well as understanding the underlying physical mechanisms in different types of materials and tissues. However, despite existing limitations in soft matter study, I hope my research would open new windows towards understanding and treatment various pathological disorders of soft biological tissues.
Abstract: Deciphering mysteries of the structure-function relationship in cortical folding has emerged as the cynosure of recent research on brain. Understanding the mechanism of convolution patterns can provide useful insight into the normal and pathological brain function. However, despite decades of speculation and endeavors the underlying mechanism of the brain folding process remains poorly understood. This paper focuses on the three-dimensional morphological patterns of a developing brain under different tissue specification assumptions via theoretical analyses, computational modeling, and experiment verifications. The living human brain is modeled with a soft structure having outer cortex and inner core to investigate the brain development. Analytical interpretations of differential growth of the brain model provide preliminary insight into the critical growth ratio for instability and crease formation of the developing brain followed by computational modeling as a way to offer clues for brain's postbuckling morphology. Especially, tissue geometry, growth ratio, and material properties of the cortex are explored as the most determinant parameters to control the morphogenesis of a growing brain model. As indicated in results, compressive residual stresses caused by the sufficient growth trigger instability and the brain forms highly convoluted patterns wherein its gyrification degree is specified with the cortex thickness. Morphological patterns of the developing brain predicted from the computational modeling are consistent with our neuroimaging observations, thereby clarifying, in part, the reason of some classical malformation in a developing brain.
Pub.: 16 Oct '15, Pinned: 16 Jun '17
Abstract: Mammalian cerebral cortices are characterized by elaborate convolutions. Radial convolutions exhibit homology across primate species and generally are easily identified in individuals of the same species. In contrast, circumferential convolutions vary across species as well as individuals of the same species. However, systematic study of circumferential convolution patterns is lacking. To address this issue, we utilized structural MRI (sMRI) and diffusion MRI (dMRI) data from primate brains. We quantified cortical thickness and circumferential convolutions on gyral banks in relation to axonal pathways and density along the gray matter/white matter boundaries. Based on these observations, we performed a series of computational simulations. Results demonstrated that the interplay of heterogeneous cortex growth and mechanical forces along axons plays a vital role in the regulation of circumferential convolutions. In contrast, gyral geometry controls the complexity of circumferential convolutions. These findings offer insight into the mystery of circumferential convolutions in primate brains.
Pub.: 09 Mar '17, Pinned: 16 Jun '17
Abstract: As a significant type of cerebral cortical convolution pattern, the gyrus is widely preserved across species. Although many hypotheses have been proposed to study the underlying mechanisms of gyrus formation, it is currently still far from clear which factors contribute to the regulation of consistent gyrus formation. In this paper, we employ a joint analysis scheme of experimental data and computational modeling to investigate the fundamental mechanism of gyrus formation. Experimental data on mature human brains and fetal brains show that thicker cortices are consistently found in gyral regions and gyral cortices have higher growth rates. We hypothesize that gyral convolution patterns might stem from heterogeneous regional growth in the cortex. Our computational simulations show that gyral convex patterns may occur in locations where the cortical plate grows faster than the cortex of the brain. Global differential growth can only produce a random gyrification pattern, but it cannot guarantee gyrus formation at certain locations. Based on extensive computational modeling and simulations, it is suggested that a special area in the cerebral cortex with a relatively faster growth speed could consistently engender gyri.
Pub.: 18 Nov '16, Pinned: 16 Jun '17
Abstract: Precisely controlling the morphology in thin film coatings has emerged as an important tool used to tune surface properties in a wide variety of applications. Previously, a method is reported to fabricate nanoscale surface creases with a high degree of control over crease size and shape using microcontact printing to perform post-polymerization modification on reactive polymer brush surfaces. In this work, this approach has been expanded to manipulate crease morphology in reactive thin films, using only a drop of a reactive, viscous polymer, and crease formation has been investigated with a combination of experimental observations and computational validations. The effects of various rate constants of the reactive polymer brush scaffold, hydrostatic pressure within the droplet of reactive polymer, diffusion profile, and the evolution of the creased morphologies with reaction time are examined in order to better understand crease formation in ultrathin films.
Pub.: 20 Apr '17, Pinned: 16 Jun '17
Abstract: Cortical folding, characterized by convex gyri and concave sulci, has an intrinsic relationship to the brain's functional organization. Understanding the mechanism of the brain's convoluted patterns can provide useful clues into normal and pathological brain function. In this paper, the cortical folding phenomenon is interpreted both analytically and computationally, and, in some cases, the findings are validated with experimental observations. The living human brain is modeled as a soft structure with a growing outer cortex and inner core to investigate its developmental mechanism. Analytical interpretations of differential growth of the brain model provide preliminary insight into critical growth ratios for instability and crease formation of the developing brain. Since the analytical approach cannot predict the evolution of cortical complex convolution after instability, non-linear finite element models are employed to study the crease formation and secondary morphological folds of the developing brain. Results demonstrate that the growth ratio of the cortex to core of the brain, the initial thickness, and material properties of both cortex and core have great impacts on the morphological patterns of the developing brain. Lastly, we discuss why cortical folding is highly correlated and consistent by presenting an intriguing gyri-sulci formation comparison.
Pub.: 26 Sep '15, Pinned: 16 Jun '17
Abstract: Author(s): Mir Jalil Razavi, Ramana Pidaparti, and Xianqiao WangSurface and interfacial creases induced by biological growth are common types of instability in soft biological tissues. This study focuses on the criteria for the onset of surface and interfacial creases as well as their morphological evolution in a growing bilayer soft tube within a confined envir…[Phys. Rev. E 94, 022405] Published Tue Aug 09, 2016
Pub.: 09 Aug '16, Pinned: 16 Jun '17