Showing papers by "Brian J. Rodriguez published in 2015"

Macromolecularly crowded in vitro microenvironments accelerate the production of extracellular matrix-rich supramolecular assemblies

Abstract: Therapeutic strategies based on the principles of tissue engineering by self-assembly put forward the notion that functional regeneration can be achieved by utilising the inherent capacity of cells to create highly sophisticated supramolecular assemblies. However, in dilute ex vivo microenvironments, prolonged culture time is required to develop an extracellular matrix-rich implantable device. Herein, we assessed the influence of macromolecular crowding, a biophysical phenomenon that regulates intra- and extra-cellular activities in multicellular organisms, in human corneal fibroblast culture. In the presence of macromolecules, abundant extracellular matrix deposition was evidenced as fast as 48 h in culture, even at low serum concentration. Temperature responsive copolymers allowed the detachment of dense and cohesive supramolecularly assembled living substitutes within 6 days in culture. Morphological, histological, gene and protein analysis assays demonstrated maintenance of tissue-specific function. Macromolecular crowding opens new avenues for a more rational design in engineering of clinically relevant tissue modules in vitro.

Nanoscale Piezoelectric Properties of Self-Assembled Fmoc-FF Peptide Fibrous Networks.

Abstract: Fibrous peptide networks, such as the structural framework of self-assembled fluorenylmethyloxycarbonyl diphenylalanine (Fmoc–FF) nanofibrils, have mechanical properties that could successfully mimic natural tissues, making them promising materials for tissue engineering scaffolds. These nanomaterials have been determined to exhibit shear piezoelectricity using piezoresponse force microscopy, as previously reported for FF nanotubes. Structural analyses of Fmoc–FF nanofibrils suggest that the observed piezoelectric response may result from the noncentrosymmetric nature of an underlying β-sheet topology. The observed piezoelectricity of Fmoc–FF fibrous networks is advantageous for a range of biomedical applications where electrical or mechanical stimuli are required.

Biocompatibility of ferroelectric lithium niobate and the influence of polarization charge on osteoblast proliferation and function.

Abstract: In this work, the influence of substrate surface charge on in vitro osteoblast cell proliferation on ferroelectric lithium niobate (LN) crystal surfaces is investigated. LN has a spontaneous polarization along the z-axis and is thus characterized by positive and negative bound polarization charge at the +z and -z surfaces. Biocompatibility of LN was demonstrated via culturing and fluorescence imaging of MC3T3 osteoblast cells for up to 11 days. The cells showed enhanced proliferation rates and improved osteoblast function through mineral formation on the positively and negatively charged LN surfaces compared to electrostatically neutral x-cut LN and a glass cover slip control. These results highlight the potential of LN as a template for investigating the role of charge on cellular processes.

Accelerated Development of Supramolecular Corneal Stromal-Like Assemblies from Corneal Fibroblasts in the Presence of Macromolecular Crowders.

Abstract: Tissue engineering by self-assembly uses the cells' secretome as a regeneration template and biological factory of trophic factors. Despite the several advantages that have been witnessed in preclinical and clinical settings, the major obstacle for wide acceptance of this technology remains the tardy extracellular matrix formation. In this study, we assessed the influence of macromolecular crowding (MMC)/excluding volume effect, a biophysical phenomenon that accelerates thermodynamic activities and biological processes by several orders of magnitude, in human corneal fibroblast (HCF) culture. Our data indicate that the addition of negatively charged galactose derivative (carrageenan) in HCF culture, even at 0.5% serum, increases by 12-fold tissue-specific matrix deposition, while maintaining physiological cell morphology and protein/gene expression. Gene analysis indicates that a glucose derivative (dextran sulfate) may drive corneal fibroblasts toward a myofibroblast lineage. Collectively, these results ...

Nanomechanics of Cells and Biomaterials Studied by Atomic Force Microscopy.

Abstract: The behavior and mechanical properties of cells are strongly dependent on the biochemical and biomechanical properties of their microenvironment. Thus, understanding the mechanical properties of cells, extracellular matrices, and biomaterials is key to understanding cell function and to develop new materials with tailored mechanical properties for tissue engineering and regenerative medicine applications. Atomic force microscopy (AFM) has emerged as an indispensable technique for measuring the mechanical properties of biomaterials and cells with high spatial resolution and force sensitivity within physiologically relevant environments and timescales in the kPa to GPa elastic modulus range. The growing interest in this field of bionanomechanics has been accompanied by an expanding array of models to describe the complexity of indentation of hierarchical biological samples. Furthermore, the integration of AFM with optical microscopy techniques has further opened the door to a wide range of mechanotransduction studies. In recent years, new multidimensional and multiharmonic AFM approaches for mapping mechanical properties have been developed, which allow the rapid determination of, for example, cell elasticity. This Progress Report provides an introduction and practical guide to making AFM-based nanomechanical measurements of cells and surfaces for tissue engineering applications.

Kelvin Probe Force Microscopy in liquid using Electrochemical Force Microscopy

Abstract: Conventional closed loop-Kelvin probe force microscopy (KPFM) has emerged as a powerful technique for probing electric and transport phenomena at the solid–gas interface. The extension of KPFM capabilities to probe electrostatic and electrochemical phenomena at the solid–liquid interface is of interest for a broad range of applications from energy storage to biological systems. However, the operation of KPFM implicitly relies on the presence of a linear lossless dielectric in the probe–sample gap, a condition which is violated for ionically-active liquids (e.g., when diffuse charge dynamics are present). Here, electrostatic and electrochemical measurements are demonstrated in ionically-active (polar isopropanol, milli-Q water and aqueous NaCl) and ionically-inactive (non-polar decane) liquids by electrochemical force microscopy (EcFM), a multidimensional (i.e., bias- and time-resolved) spectroscopy method. In the absence of mobile charges (ambient and non-polar liquids), KPFM and EcFM are both feasible, yielding comparable contact potential difference (CPD) values. In ionically-active liquids, KPFM is not possible and EcFM can be used to measure the dynamic CPD and a rich spectrum of information pertaining to charge screening, ion diffusion, and electrochemical processes (e.g., Faradaic reactions). EcFM measurements conducted in isopropanol and milli-Q water over Au and highly ordered pyrolytic graphite electrodes demonstrate both sample- and solvent-dependent features. Finally, the feasibility of using EcFM as a local force-based mapping technique of material-dependent electrostatic and electrochemical response is investigated. The resultant high dimensional dataset is visualized using a purely statistical approach that does not require a priori physical models, allowing for qualitative mapping of electrostatic and electrochemical material properties at the solid–liquid interface.

Intramolecular Cyclization Dominating Homopolymerization of Multivinyl Monomers toward Single-Chain Cyclized/Knotted Polymeric Nanoparticles

Abstract: Polymerization of multifunctional monomers could produce polymers with different functionalities and novel macromolecular architectures However, the ability to control the homopolymerization of multivinyl monomers (MVMs) has always been a challenge Here we demonstrate that the homopolymerization of acrylate based MVMs can be kinetically controlled via Cu0-mediated controlled/living radical polymerization in the presence of additional CuII, which enables the efficient promotion of intramolecular cyclization and suppression of intermolecular cross-linking The gelation is effectively delayed over ca 40% monomer conversion in the concentrated polymerization system ([M] = 409 wt %), which is far higher than the Flory–Stockmayer theory predicts Moreover, closer inspection of the synthesized polymers reveals that single-chain cyclized/knotted polymeric nanoparticles (SCKNPs) are formed due to the nature of one-pot in situ intramolecular reaction and self-cyclization of the propagating polymer chains This

Quantitative 3D-KPFM imaging with simultaneous electrostatic force and force gradient detection

Abstract: Kelvin probe force microscopy (KPFM) is a powerful characterization technique for imaging local electrochemical and electrostatic potential distributions and has been applied across a broad range of materials and devices. Proper interpretation of the local KPFM data can be complicated, however, by convolution of the true surface potential under the tip with additional contributions due to long range capacitive coupling between the probe (e.g. cantilever, cone, tip apex) and the sample under test. In this work, band excitation (BE)-KPFM is used to negate such effects. In contrast to traditional single frequency KPFM, multifrequency BE-KPFM is shown to afford dual sensitivity to both the electrostatic force and the force gradient detection, analogous to simultaneous amplitude modulated and frequency modulated KPFM imaging. BE-KPFM is demonstrated on a Pt/Au/SiO(x) test structure and electrostatic force gradient detection is found to lead to an improved lateral resolution compared to electrostatic force detection. Finally, a 3D-KPFM imaging technique is developed. Force volume (FV) BE-KPFM allows the tip-sample distance dependence of the electrostatic interactions (force and force gradient) to be recorded at each point across the sample surface. As such, FVBE-KPFM provides a much needed pathway towards complete tip-sample capacitive de-convolution in KPFM measurements and will enable quantitative surface potential measurements with nanoscale resolution.

Band excitation Kelvin probe force microscopy utilizing photothermal excitation

Abstract: A multifrequency open loop Kelvin probe force microscopy (KPFM) approach utilizing photothermal as opposed to electrical excitation is developed. Photothermal band excitation (PthBE)-KPFM is implemented here in a grid mode on a model test sample comprising a metal-insulator junction with local charge-patterned regions. Unlike the previously described open loop BE-KPFM, which relies on capacitive actuation of the cantilever, photothermal actuation is shown to be highly sensitive to the electrostatic force gradient even at biases close to the contact potential difference (CPD). PthBE-KPFM is further shown to provide a more localized measurement of true CPD in comparison to the gold standard ambient KPFM approach, amplitude modulated KPFM. Finally, PthBE-KPFM data contain information relating to local dielectric properties and electronic dissipation between tip and sample unattainable using conventional single frequency KPFM approaches.

Tip-induced domain structures and polarization switching in ferroelectric amino acid glycine

Abstract: Bioorganic ferroelectrics and piezoelectrics are becoming increasingly important in view of their intrinsic compatibility with biological environment and biofunctionality combined with strong piezoelectric effect and a switchable polarization at room temperature. Here, we study tip-induced domain structures and polarization switching in the smallest amino acid β-glycine, representing a broad class of non-centrosymmetric amino acids. We show that β-glycine is indeed a room-temperature ferroelectric and polarization can be switched by applying a bias to non-polar cuts via a conducting tip of atomic force microscope (AFM). Dynamics of these in-plane domains is studied as a function of an applied voltage and pulse duration. The domain shape is dictated by polarization screening at the domain boundaries and mediated by growth defects. Thermodynamic theory is applied to explain the domain propagation induced by the AFM tip. Our findings suggest that the properties of β-glycine are controlled by the charged domain walls which in turn can be manipulated by an external bias.

Thickness, humidity, and polarization dependent ferroelectric switching and conductivity in Mg doped lithium niobate

Abstract: Mg doped lithium niobate (Mg:LN) exhibits several advantages over undoped LN such as resistance to photorefraction, lower coercive fields, and p-type conductivity that is particularly pronounced at domain walls and opens up a range of applications, e.g., in domain wall electronics. Engineering of precise domain patterns necessitates well founded knowledge of switching kinetics, which can differ significantly from that of undoped LN. In this work, the role of humidity and sample composition in polarization reversal has been investigated under application of the same voltage waveform. Control over domain sizes has been achieved by varying the sample thickness and initial polarization as well as atmospheric conditions. In addition, local introduction of proton exchanged phases allows for inhibition of domain nucleation or destabilization, which can be utilized to modify domain patterns. Polarization dependent current flow, attributed to charged domain walls and band bending, demonstrates the rectifying ability of Mg:LN in combination with suitable metal electrodes that allow for further tailoring of conductivity.

The role of nanoporosity on the local piezo and ferroelectric properties of lead titanate thin films

Abstract: Nanoporous and dense ferroelectric PbTiO3 thin films are prepared by a modified sol–gel process. The presence of nanoporosity, with ∼50 nm pore size formed using a block polymer as a structure-directing agent, markedly affects the microstructure, crystallization and ferroelectric properties of the film. The crystallization of the tetragonal phase is enhanced in nanoporous films. It is suggested that the decomposition of the block-copolymer in porous films triggers the crystallization of the perovskite phase at low temperatures via the local increase of temperature. Consequently, nanoporous films with improved tetragonality exhibit enhanced piezoelectric coefficients, switchable polarization and low local coercivity. By providing a means of achieving enhanced properties, nanoporosity may have a broad impact on the applications of ferroelectric thin films.

Interface and thickness dependent domain switching and stability in Mg doped lithium niobate

Abstract: Controlling ferroelectric switching in Mg doped lithium niobate (Mg:LN) is of fundamental importance for optical device and domain wall electronics applications that require precise domain patterns. Stable ferroelectric switching has been previously observed in undoped LN layers above proton exchanged (PE) phases that exhibit reduced polarization, whereas PE layers have been found to inhibit lateral domain growth. Here, Mg doping, which is known to significantly alter ferroelectric switching properties including coercive field and switching currents, is shown to inhibit domain nucleation and stability in Mg:LN above buried PE phases that allow for precise ferroelectric patterning via domain growth control. Furthermore, piezoresponse force microscopy (PFM) and switching spectroscopy PFM reveal that the voltage at which polarization switches from the “up” to the “down” state increases with increasing thickness in pure Mg:LN, whereas the voltage required for stable back switching to the original “up” state d...

Local piezoresponse and polarization switching in nucleobase thymine microcrystals

Abstract: Thymine (2-oxy-4-oxy-5 methyl pyrimidine) is one of the four nucleobases of deoxyribonucleic acid (DNA). In the DNA molecule, thymine binds to adenine via two hydrogen bonds, thus stabilizing the nucleic acid structure and is involved in pairing and replication. Here, we show that synthetic thymine microcrystals grown from the solution exhibit local piezoelectricity and apparent ferroelectricity, as evidenced by nanoscale electromechanical measurements via Piezoresponse Force Microscopy. Our experimental results demonstrate significant electromechanical activity and polarization switchability of thymine, thus opening a pathway for piezoelectric and ferroelectric-based applications of thymine and, perhaps, of other DNA nucleobase materials. The results are supported by molecular modeling of polarization switching under an external electric field.

Electrochemical force microscopy

Abstract: A system and method for electrochemical force microscopy are provided. The system and method are based on a multidimensional detection scheme that is sensitive to forces experienced by a biased electrode in a solution. The multidimensional approach allows separation of fast processes, such as double layer charging, and charge relaxation, and slow processes, such as diffusion and faradaic reactions, as well as capturing the bias dependence of the response. The time-resolved and bias measurements can also allow probing both linear (small bias range) and non-linear (large bias range) electrochemical regimes and potentially the de-convolution of charge dynamics and diffusion processes from steric effects and electrochemical reactivity.

Porosity… Advantage or Disadvantage?

Abstract: Nowadays, due to the increasing demand on portable electronic devices with ultrahigh density data storage and lower power consumption, the miniaturization of ferroelectrics has become an important issue. This progressive development requires the morphology control of the materials and the understanding of the effects of size reduction on the properties at the nanoscale, namely ferroelectric properties.