Kilpatrick, J. I.
Kilpatrick, J. I.
Kilpatrick, J. I.
Now showing 1 - 10 of 12
- PublicationNanomechanics of Cells and Biomaterials Studied by Atomic Force Microscopy(Wiley, 2015-07-22)
; ;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. Atomic force microscopy is an indispensable tool for nanomechanical measurements of cells, cell microenvironments, and biomaterials. The mechanical properties of cells and their function are influenced by the elasticity of the extracellular matrix. Thus, understanding the nanomechanical properties is key for tissue engineering applications. 457Scopus© Citations 31
- PublicationFrequency modulation atomic force microscopy in ambient environments utilizing robust feedback tuning(American Institute of Physics, 2009-02-02)
; ; ;Frequency modulation atomic force microscopy (FM-AFM) is rapidly evolving as the technique of choice in the pursuit of high resolution imaging of biological samples in ambient environments. The enhanced stability afforded by this dynamic AFM mode combined with quantitative analysis enables the study of complex biological systems, at the nanoscale, in their native physiological environment. The operational bandwidth and accuracy of constant amplitude FM-AFM in low Q environments is heavily dependent on the cantilever dynamics and the performance of the demodulation and feedback loops employed to oscillate the cantilever at its resonant frequency with a constant amplitude. Often researchers use ad hoc feedback gains or instrument default values that can result in an inability to quantify experimental data. Poor choice of gains or exceeding the operational bandwidth can result in imaging artifacts and damage to the tip and/or sample. To alleviate this situation we present here a methodology to determine feedback gains for the amplitude and frequency loops that are specific to the cantilever and its environment, which can serve as a reasonable "first guess", thus making quantitative FM-AFM in low Q environments more accessible to the nonexpert. This technique is successfully demonstrated for the low Q systems of air (Q∼40) and water (Q∼1). In addition, we present FM-AFM images of MC3T3-E1 preosteoblast cells acquired using the gains calculated by this methodology demonstrating the effectiveness of this technique. 465Scopus© Citations 28
- PublicationInvestigation of AFM-based machining of ferroelectric thin films at the nanoscale(American Institute of Physics, 2020-01-17)
; ; ; ; ;Atomic force microscopy (AFM) has been utilized for nanomechanical machining of various materials including polymers, metals, and semiconductors. Despite being important candidate materials for a wide range of applications including data storage and actuators, ferroelectric materials have rarely been machined via AFM. AFM-based machining of ferroelectric nanostructures offers advantages over established techniques, such as bottom-up approaches and focused ion beam milling, in select cases where low damage and low-cost modification of already-fabricated thin films are required. Through a systematic investigation of a broad range of AFM parameters, we demonstrate that AFM-based machining provides a low-cost option to rapidly modify local regions of the film, as well as fabricate a range of different nanostructures, including a nanocapacitor array with individually addressable ferroelectric elements. 120Scopus© Citations 12
- PublicationPiezoelectricity in collagen type II fibrils measured by scanning probe microscopy(American Institute of Physics, 2014-08-11)
; ; ; ;The converse piezoelectric effect in collagen type II fibrils, the main collagen constituent in cartilage, was investigated using piezoresponse force microscopy. The fibrils exhibited shear piezoelectric behavior similar to that previously reported in collagen type I fibrils and followed the same cantilever-fibril angle dependence present for type I. A uniform polarization directed from the amine to carboxyl termini, as seen for collagen type I, was observed in all type II fibrils studied. The shear piezoelectric coefficient, d 15, however, for type II was roughly 28–32% of the value measured for type I fibrils. Possible explanations for the reduced piezoelectric coefficient of type II collagen are provided. 452Scopus© Citations 18
- PublicationDual harmonic Kelvin probe force microscopy at the graphene-liquid interface(American Institute of Physics, 2014-04)
; ; ; ; ;Kelvin probe force microscopy (KPFM) is a powerful technique for the determination of the contact potential difference (CPD) between an atomic force microscope tip and a sample under ambient and vacuum conditions. However, for many energy storage and conversion systems, including graphene-based electrochemical capacitors, understanding electrochemical phenomena at the solid¿liquid interface is paramount. Despite the vast potential to provide fundamental insight for energy storage materials at the nanoscale, KPFM has found limited applicability in liquid environments to date. Here, using dual harmonic (DH)-KPFM, we demonstrate CPD imaging of graphene in liquid. We find good agreement with measurements performed in air, highlighting the potential of DH-KPFM to probe electrochemistry at the graphene¿liquid interface. 519Scopus© Citations 46
- PublicationHigh viscosity environments: an unexpected route to obtain true atomic resolution with atomic force microscopy(IOP Publishing, 2014-04-09)
; ; ; ;Atomic force microscopy (AFM) is widely used in liquid environments, where true atomic resolution at the solid–liquid interface can now be routinely achieved. It is generally expected that AFM operation in more viscous environments results in an increased noise contribution from the thermal motion of the cantilever, thereby reducing the signal-to-noise ratio (SNR). Thus, viscous fluids such as ionic and organic liquids have been generally avoided for high-resolution AFM studies despite their relevance to, e.g. energy applications. Here, we investigate the thermal noise limitations of dynamic AFM operation in both low and high viscosity environments theoretically, deriving expressions for the amplitude, phase and frequency noise resulting from the thermal motion of the cantilever, thereby defining the performance limits of amplitude modulation, phase modulation and frequency modulation AFM. We show that the assumption of a reduced SNR in viscous environments is not inherent to the technique and demonstrate that SNR values comparable to ultra-high vacuum systems can be obtained in high viscosity environments under certain conditions. Finally, we have obtained true atomic resolution images of highly ordered pyrolytic graphite and mica surfaces, thus revealing the potential of high-resolution imaging in high viscosity environments. 348Scopus© Citations 7
- PublicationDual harmonic Kelvin probe force microscopy for surface potential measurements of ferroelectrics(IEEE, 2012-07)
; ; ; ; ; ;In this work, we implemented dual harmonic Kelvin probe force microscopy (DH-KPFM) for surface potential mapping of ferroelectric thin films, namely bismuth ferrite (BFO) and strontium barium niobate (SBN). We applied DH and conventional KPFM to charge-patterned BFO and found agreement between recorded relative surface potential values between domains, demonstrating that DH-KPFM can be used for quantitative mapping of relative surface potentials. We used piezoresponse force microscopy (PFM) to determine whether polarization switching had occurred. From the PFM data, we found that BFO was poled successfully, and that the measured surface potential was consistent with the sign of the bound polarization charge. For SBN, a thin surface layer was evident in the topography after the application of DC bias, suggesting an electrochemical reaction had taken place between the tip and the sample. We used DH-KPFM to simultaneously map the surface potential and changes in the dielectric properties resulting from this surface layer. The results presented herein demonstrate that DH-KPFM can be used for electric characterization of voltagesensitive materials, and we anticipate that DH-KFPM will become a useful tool for non-intrusive electrical characterization of materials. 558Scopus© Citations 14
- PublicationNanoscale Piezoelectric Properties of Self-Assembled Fmoc-FF Peptide Fibrous Networks(American Chemical Society, 2015-05-21)
; ; ; ; ; ; ;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. 357Scopus© Citations 50
- PublicationPiezoelectric Tensor of Collagen Fibrils Determined at the Nanoscale(ACS, 2017-05-03)
; ; ; ; ; ;Piezoelectric properties of rat tail tendons, sectioned at angles of 0, 59, and 90° relative to the plane orthogonal to the major axis, were measured using piezoresponse force microscopy. The piezoelectric tensor at the length scale of an individual fibril was determined from angle-dependent in-plane and out-of-plane piezoelectric measurements. The longitudinal piezoelectric coefficient for individual fibrils at the nanoscale was found to be roughly an order of magnitude greater than that reported for macroscopic measurements of tendon, the low response of which stems from the presence of oppositely oriented fibrils, as confirmed here. 730Scopus© Citations 51
- PublicationKelvin Probe Force Microscopy in liquid using Electrochemical Force Microscopy(Beilstein-Institut, 2015-01-19)
; ; ; ;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 notrequire a priori physical models, allowing for qualitative mapping of electrostatic and electrochemical material properties at the solid-liquid interface. 392Scopus© Citations 34