skip to main content


Title: Correlation of Atomic Force Microscopy Tapping Forces to Mechanical Properties of Lipid Membranes
There is considerable interest in measuring, with nanoscale spatial resolution, the physical properties of lipid membranes because of their role in the physiology of living systems. Due to its ability to nondestructively image surfaces in solution, tapping mode atomic force microscopy (TMAFM) has proven to be a useful technique for imaging lipid membranes. However, further information concerning the mechanical properties of surfaces is contained within the time-resolved tip/sample force interactions. The tapping forces can be recovered by taking the second derivative of the cantilever deflection signal and scaling by the effective mass of the cantilever; this technique is referred to as scanning probe acceleration microscopy. Herein, we describe how the maximum and minimum tapping forces change with surface mechanical properties. Furthermore, we demonstrate how these changes can be used to measure mechanical changes in lipid membranes containing cholesterol.  more » « less
Award ID(s):
1054211
NSF-PAR ID:
10018319
Author(s) / Creator(s):
; ; ; ;
Date Published:
Journal Name:
ASME 2012 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference
Volume:
5
Page Range / eLocation ID:
303-310
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. There is great interest in the application of proximal probe techniques to simultaneously image and measure mechancial properties of surfaces with nanoscale spatial resolution. There have been several innovations in generating time-resolved force interaction between the tip and surface while acquiring a tapping mode AFM image. These tip/sample forces contain information regarding mechanical properties of surfaces in an analogous fashion to a force curve experiment. Here, we demonstrate, via simulation, that the maximum and minimum tapping forces change with respect to the Young’s modulus and adhesiveness of a surface, but the roughness of the surfaces has no effect on the tapping forces. Using these changes in tapping forces, we determine the mechanical changes of a lipid membrane after exposure to a huntingtin exon1 (htt exon1) protein with an expanded polyglutamine (polyQ) domain. Expanded polyQ domains in htt is associated with Huntington’s disease, a genetic neurodegenerative disorder. The htt exon1 protein caused regions of increased surface roughness to appear in the lipid membrane, and these areas were associated with decreased elasticity and adhesion to the AFM probe. 
    more » « less
  2. Johnson, Colin (Ed.)
    Lipid membranes in nature adapt and reconfigure to changes in composition, temperature, humidity, and mechanics. For instance, the oscillating mechanical forces on lung cells and alveoli influence membrane synthesis and structure during breathing. However, despite advances in the understanding of lipid membrane phase behavior and mechanics of tissue, there is a critical knowledge gap regarding the response of lipid membranes to micromechanical forces. Most studies of lipid membrane mechanics use supported lipid bilayer systems missing the structural complexity of pulmonary lipids in alveolar membranes comprising multi-bilayer interconnected stacks. Here, we elucidate the collective response of the major component of pulmonary lipids to strain in the form of multi-bilayer stacks supported on flexible elastomer substrates. We utilize X-ray diffraction, scanning probe microscopy, confocal microscopy, and molecular dynamics simulation to show that lipid multilayered films both in gel and fluid states evolve structurally and mechanically in response to compression at multiple length scales. Specifically, compression leads to increased disorder of lipid alkyl chains comparable to the effect of cholesterol on gel phases as a direct result of the formation of nanoscale undulations in the lipid multilayers, also inducing buckling delamination and enhancing multi-bilayer alignment. We propose this cooperative short- and long-range reconfiguration of lipid multilayered films under compression constitutes a mechanism to accommodate stress and substrate topography. Our work raises fundamental insights regarding the adaptability of complex lipid membranes to mechanical stimuli. This is critical to several technologies requiring mechanically reconfigurable surfaces such as the development of electronic devices interfacing biological materials. 
    more » « less
  3. Sub-resonance tapping (SRT) mode of atomic force microscopy (AFM) enables researchers to image surfaces with well-controlled load forces and to collect maps of multiple physical properties of samples. The major bottleneck of this mode is a relatively low scan speed compared to other scanning modes. This paper presents a novel control algorithm that substantially improves the scanning speed over the standard SRT. We propose naming the new modality Trajectory Tracking SRT (TT-SRT). In contrast with the standard SRT control, TT-SRT uses the feedback within every single touch of the sample by the AFM probe. To demonstrate the advantage of TT-SRT, we conduct scans on a variety of samples with differing topologies, roughnesses, and mechanical properties. Each sample region is scanned with both standard SRT and TT-SRT at the same set of speeds. The control gains are tuned before each scan for maximum performance in each mode. Performance is evaluated by selecting a given level of image quality and finding the maximum speed that can be achieved by each algorithm. We find that with increased demand for data quality, the utility of TT-SRT becomes more apparent; for example, the speed of TT-SRT can be ten times faster or more than standard SRT for a reasonable expectation of data quality.

     
    more » « less
  4. Photo-induced force microscopy (PiFM) is a scan probe technique that offers images with spectroscopic contrast at a spatial resolution in the nanometer range. PiFM utilizes the non-propagating, enhanced near field at the apex of a sharp tip to locally induce a polarization in the sample, which in turn produces an additional force acting on the cantilevered tip. This photo-induced force, though in the pN range or less, can be extracted from the oscillation properties of the cantilever, thus enabling the generation of photo-induced force maps. Since its inception in 2010, the PiFM technique has grown into a useful nano-spectrocopic tool that has expanded its reach in terms of imaging capabilities and applications. In this review, we present various technical implementations of the PiFM approach. In addition, we discuss the physical origin of the PiFM signal, highlighting the contributions from dipole–dipole forces as well as forces that derive from photo-thermal processes. 
    more » « less
  5. Jabbari, Esmaiel (Ed.)
    This study presents novel biocompatible Polydimethylsiloxane (PDMS)-based micromechanical tweezers (μTweezers) capable of the stiffness characterization and manipulation of hydrogel-based organoids. The system showed great potential for complementing established mechanical characterization methods such as Atomic Force Microscopy (AFM), parallel plate compression (PPC), and nanoindentation, while significantly reducing the volume of valuable hydrogels used for testing. We achieved a volume reduction of ~0.22 μl/sample using the μTweezers vs. ~157 μl/sample using the PPC, while targeting high-throughput measurement of widely adopted micro-mesoscale (a few hundred μm-1500 μm) 3D cell cultures. The μTweezers applied and measured nano-millinewton forces through cantilever’ deflection with high linearity and tunability for different applications; the assembly is compatible with typical inverted optical microscopes and fit on standard tissue culture Petri dishes, allowing mechanical compression characterization of arrayed 3D hydrogel-based organoids in a high throughput manner. The average achievable output per group was 40 tests per hour, where 20 organoids and 20 reference images in one 35 mm petri dish were tested, illustrating efficient productivity to match the increasing demand on 3D organoids’ applications. The changes in stiffness of collagen I hydrogel organoids in four conditions were measured, with ovarian cancer cells (SKOV3) or without (control). The Young’s modulus of the control group (Control—day 0, E = 407± 146, n = 4) measured by PPC was used as a reference modulus, where the relative elastic compressive modulus of the other groups based on the stiffness measurements was also calculated (control-day 0, E = 407 Pa), (SKOV3-day 0, E = 318 Pa), (control-day 5, E = 528 Pa), and (SKOV3-day 5, E = 376 Pa). The SKOV3-embedded hydrogel-based organoids had more shrinkage and lowered moduli on day 0 and day 5 than controls, consistently, while SKOV3 embedded organoids increased in stiffness in a similar trend to the collagen I control from day 0 to day 5. The proposed method can contribute to the biomedical, biochemical, and regenerative engineering fields, where bulk mechanical characterization is of interest. The μTweezers will also provide attractive design and application concepts to soft membrane-micro 3D robotics, sensors, and actuators. 
    more » « less