Probing the interaction of a membrane receptor with a surface-attached ligand using whole cells on acoustic biosensors

Recent Advances in Fluorescence Recovery after Photobleaching for Decoupling Transport and Kinetics of Biomacromolecules in Cellular Physiology

Among the new molecular tools available to scientists and engineers, some of the most useful include fluorescently tagged biomolecules. Tools, such as green fluorescence protein (GFP), have been applied to perform semi-quantitative studies on biological signal transduction and cellular structural dynamics involved in the physiology of healthy and disease states. Such studies focus on drug pharmacokinetics, receptor-mediated endocytosis, nuclear mechanobiology, viral infections, and cancer metastasis. In 1976, fluorescence recovery after photobleaching (FRAP), which involves the monitoring of fluorescence emission recovery within a photobleached spot, was developed. FRAP allowed investigators to probe two-dimensional (2D) diffusion of fluorescently-labelled biomolecules. Since then, FRAP has been refined through the advancements of optics, charged-coupled-device (CCD) cameras, confocal microscopes, and molecular probes. FRAP is now a highly quantitative tool used for transport and kinetic studies in the cytosol, organelles, and membrane of a cell. In this work, the authors intend to provide a review of recent advances in FRAP. The authors include epifluorescence spot FRAP, total internal reflection (TIR)/FRAP, and confocal microscope-based FRAP. The underlying mathematical models are also described. Finally, our understanding of coupled transport and kinetics as determined by FRAP will be discussed and the potential for future advances suggested.

Development of a Love-Wave Biosensor Based on an Analytical Model

The present work deals with the development of a Love-wave biosensor for the diagnosis of the modification of cell viscosity. The relevant device performance such as insertion loss, attenuation, phase velocity, and sensitivity needs to be analysed as a function of the device structure and also regarding the effect of the liquid loading. In this study, we used an analytical model based on the equation of motions for a Love wave propagating in a three-layer structure. We show that the effect of the viscous coupling leads to insertion losses and a phase shift that impact the acoustic ratio. A comparison between experimental and theoretical results showed a good agreement between the behaviours as it was observed for the phase shift vs. the insertion loss with a limited difference in values (3.11/3.09—experimental/simulation for the sensitivity to the viscosity for different insertion losses) due to the assumptions made on the model used.

Love Wave Sensor with High Penetration Depth for Potential Application in Cell Monitoring

Love wave (L-SAW) sensors have been used to probe cell monolayers, but their application to detect changes beyond the focal adhesion points on cell monolayers, as viscosity changes on the cytoskeleton, has not been explored. In this work we present for the first time a Love wave sensor with tuned penetration depth and sensitivity to potentially detect mechanical changes beyond focal adhesion points of cell monolayers. We designed and fabricated a Love wave sensor operating at 30 MHz with sensitivity to detect viscous changes between 0.89 and 3.3 cP. The Love wave sensor was modeled using an acoustic transmission line model, whereas the response of interdigital transducers (IDTs) was modeled with the Campbell’s cross-field circuit model. Our design uses a substrate with a high electromechanical coupling coefficient (LiNbO₃ 36Y-X), and an 8-µm polymeric guiding layer (SU-8). The design aims to overcome the high insertion losses of viscous liquid environments, and the loss of sensitivity due to the low frequency. The fabricated sensor was tested in a fluidic chamber glued directly to the SU-8 guiding layer. Our experiments with liquids of viscosity similar to those expected in cell monolayers showed a measurable sensor response. In addition, experimentation with SaOs-2 cells within a culture medium showed measurable responses. These results can be of interest for the development of novel cell-based biosensors, and novel characterization tools for cell monolayers.

Shear-horizontal surface acoustic wave sensor for non-invasive monitoring of dynamic cell spreading and attachment in wound healing assays

A Love-wave based biosensor is introduced for analyzing a standardized wound healing assay by observing cell growth and quantifying cell detachment processes. Utilizing the piezoelectric material LiTaO3 36° XY-cut with a thin SiO2-cover layer, shear horizontal surface acoustic waves (SAW) are excited and detected by a set of Interdigital Transducers. Epithelial cells, being cultivated on the substrate and invading the sensors delay line cause a phase shift in the transmitted SAW signal. This phase shift correlates exactly with the surface coverage of the invading cells. After wound healing, emerging fluctuations in the phase shift signal provide information about the cell growth in a confluent cell layer. Additionally, the signal slope allows to quantify the cell detachment process induced by apoptosis, necrosis or cell lysis substances, respectively. Furthermore, culture conditions like temperature or osmolality can be simultaneously monitored by SAW. Based on a theoretical approach and using FEM simulations, we identified the acoustoelectric interaction as the main reason for the phase shift in various frequency- and time-dependent studies. Our model is validated by experimental data and allows predicting the phase change caused by variations in the cell-substrate distance or the volume ratio of the nucleus and the complete cell.

Hybrid Sensor Device for Simultaneous Surface Plasmon Resonance and Surface Acoustic Wave Measurements

Surface plasmon resonance (SPR) and Love wave (LW) surface acoustic wave (SAW) sensors have been established as reliable biosensing technologies for label-free, real-time monitoring of biomolecular interactions. This work reports the development of a combined SPR/LW-SAW platform to facilitate simultaneous optical and acoustic measurements for the investigation of biomolecules binding on a single surface. The system’s output provides recordings of two acoustic parameters, phase and amplitude of a Love wave, synchronized with SPR readings. We present the design and manufacturing of a novel experimental set-up employing, in addition to the SPR/LW-SAW device, a 3D-printed plastic holder combined with a PDMS microfluidic cell so that the platform can be used in a flow-through mode. The system was evaluated in a systematic study of the optical and acoustic responses for different surface perturbations, i.e., rigid mass loading (Au deposition), pure viscous loading (glycerol and sucrose solutions) and protein adsorption (BSA). Our results provide the theoretical and experimental basis for future application of the combined system to other biochemical and biophysical studies.

A study of Love wave acoustic biosensors monitoring the adhesion process of tendon stem cells (TSCs)

The Love wave biosensor is considered to be one of the most promising probing methods in biomedical research and diagnosis, and has been applied to detect the mechano-biological behaviour of cells attached to the surface of the device. More efforts should be devoted to basic theoretical research and relevant device performance analysis that may contribute to the further developments of Love wave sensors. In this study, a 36º YX-LiTaO₃-based Love wave sensor with a parylene-C wave guiding layer was adopted as a cell-based biosensor to monitor the adhesion process of tendon stem/progenitor cells (TSCs), a newly discovered cell type in tendons. A theoretical model is proposed to describe the Love wave propagation, in which the adherent cells are considered as a uniform viscoelastic layer. The effects of viscoelastic cell layer and wave guiding layer on the propagation velocity υ and propagation loss (PL) are investigated. The numerical results indicate that adherent cell layers of different storage or loss shear modulus in certain ranges can induce pronounced and characteristic variations in υ and PL, revealing the potential of Love wave sensors to provide useful quantitative measures on cellular mechanical properties. The sensor response to the adhesion of TSCs exhibits high consistency with experimental observations, which demonstrates the Love wave biosensor as a very promising sensor platform for investigating cellular activities under multiple physiological conditions.

A Fluidic Interface with High Flow Uniformity for Reusable Large Area Resonant Biosensors

Resonant biosensors are known for their high accuracy and high level of miniaturization. However, their fabrication costs prevent them from being used as disposable sensors and their effective commercial success will depend on their ability to be reused repeatedly. Accordingly, all the parts of the sensor in contact with the fluid need to tolerate the regenerative process which uses different chemicals (H₃PO₄, H₂SO₄ based baths) without degrading the characteristics of the sensor. In this paper, we propose a fluidic interface that can meet these requirements, and control the liquid flow uniformity at the surface of the vibrating area. We study different inlet and outlet channel configurations, estimating their performance using numerical simulations based on finite element method (FEM). The interfaces were fabricated using wet chemical etching on Si, which has all the desirable characteristics for a reusable biosensor circuit. Using a glass cover, we could observe the circulation of liquid near the active surface, and by using micro-particle image velocimetry (μPIV) on large surface area we could verify experimentally the effectiveness of the different designs and compare with simulation results.

Label-free Detection of Protein Released during Platelet Activation by CNT-Enhanced Love Mode SAW Sensors

Platelet-rich plasma (PRP) has been applied in a series of clinical treatments. PRP contains high-concentrated platelets, which, when activated, could secret a variety of growth factors and cytokines, to promote and/or enhance healing of injured tissues. Non-activated platelets suspension could be prepared by an isolation method of centrifugation and washing currently. However, it is not clear whether platelets, if any, are already activated during this process and there is no simple method to monitor their activation accordingly. Shear-Horizontal Surface Acoustic Wave sensors (SH-SAW, Love Mode) are promising in fundamental biology as well as biomedical engineering, detecting cell behaviors in liquid in a non-invasive, simple and quantitative manner. In this study, Love mode sensors are adopted for the label-free detection of protein secreted by platelets. Carbon nanotube (CNT) is reported as an advisable platform of both non-specific protein adsorption and specific protein binding. For further improvement of Love mode sensor performance, novel CNT -coated parylene-C film is prepared on its surface as both the acoustic-wave-guiding layer and bio-interface layer. The S21 loss curves of Love mode sensors were recorded and the corresponding resonance frequencies were extracted. The results showed that the CNT-enhanced sensor possessed an increased resonance frequency shift when compared to normal sensor with single parylene-C film under identical collagen concentrations. Then, the modified sensor is used for label-free detection of protein released by various concentrations of platelets. The results revealed high sensitivity and consistency, indicating the potential of CNT-enhanced Love mode sensors in cell-based applications.

Chemically grafted fibronectin for use in QCM-D cell studies

Traditionally, fibronectin has been used as a physisorbed surface coating (physFN) in cell culture experiments due to its critical role in cell adhesion. However, because the resulting layer is thick, unstable, and of unpredictable uniformity, this method of fibronectin deposition is unsuitable for some types of research, including quartz crystal microbalance (QCM) experiments involving cells. Here, we present a new method for chemical immobilization of fibronectin onto silicon oxide surfaces, including QCM crystals pre-coated with silicon oxide. We characterize these chemically coated fibronectin surfaces (chemFN) as well as physFN ones using spectroscopic ellipsometry (SE), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and contact angle measurements. A cell culture model demonstrates that cells on chemFN and physFN surfaces exhibit similar viability, structure, adhesion and metabolism. Finally, we perform QCM experiments using cells on both surfaces which demonstrate the superior suitability of chemFN coatings for QCM research, and provide real-time QCM-D data from cells subjected to an actin depolymerizing agent. Overall, our method of chemical immobilization of fibronectin yields great potential for furthering cellular experiments in which thin, stable and uniform coatings are desirable. As QCM research with cells has been rather limited in success thus far, we anticipate that this new technique will particularly benefit this experimental system by availing it to the much broader field of cell mechanics.

Detection of cellular damage by hydrogen peroxide using SV40-T2 cells on shear horizontal surface acoustic wave (SH-SAW) sensor

The rat lung epithelial cell line SV40-T2 was used to develop a cellular biosensing system to assay for environmental toxicants. The novel approach on which this system is based involves direct attachment of cultured rat or human cells onto a cell-adhesive matrix on the device through which shear horizontal surface acoustic waves (SH-SAW) are transmitted using 50 MHz SAW resonator. This novel design enables sensitive monitoring of changes of the electrophysical characteristics of cells, such as their conductivity and relative permittivity. A time-dependent change of phase of SAW and change of insertion loss (change of amplitude) were observed when the cells were treated with 0.5 or 1.0 mM H2O2. The change of insertion loss was biphasic, with an early phase (1-3 h) and a late phase (3-6 h). The late phase coincided with the destruction of cell-cell tight junctions detected by measurement of the transepithelial electrical resistance and paracellular permeability; in contrast, the early phase coincided with the destruction of intracellular actin filaments by H2O2. The early-phase effect of H2O2 on phase shift may be attributable to the change of intracellular permittivity by a change of cellular polarity. Immunofluorescence microscopy showed the disappearance of zonula occludens protein 1 from the region of cell-cell contact. These results suggest the correlation between the change of insertion loss as an SAW parameter and the destruction of tight junctions of the cells on the SH-SAW device in the late phase.

Acoustic sensing of the initial adhesion of chemokine-stimulated cancer cells

Chemokines together with their receptors play important roles in tumor metastasis. Intracellular signals stimulated by chemokines regulate the initial adhesion of cancer cells, which controls the subsequent cell spreading and migration. Until now, the nature of initial cell adhesion has been understood very poorly, since conventional assays are static and could not provide dynamic information. In order to address this issue, we adopt an acoustic sensor, quartz crystal microbalance (QCM), to monitor the attachment of chemokine-stimulated cancer cells in real-time. As a model, the chemokine CXCL12 was used to stimulate three human breast cancer cell lines expressing different levels of its receptor CXCR4, which triggers intracellular signaling pathways that activate integrins across cell membrane. Interaction between cellular integrins and adhesion molecules (CAMs) pre-coated on sensor surfaces were in situ monitored by QCM of which the frequency was sensitive to the mechanical connection of cells to the sensor surface. The ratio of frequency shift under stimulation to that without stimulation indicated the number and strength of integrin-CAM binding stimulated by the chemokine. The cell-surface binding was found to be enhanced by CXCL12, which depends on the CAM type and levels of chemokine and receptor, and was significantly inhibited by a blocker of the chemokine pathway. The binding of integrin with intercellular adhesion molecule was also found to be strong and in good correlated with the chemotactic indexes obtained by the classical Boyden chamber assay. This research suggests that acoustic sensing of initial cell adhesion could provide a dynamic insight into cell interfacial phenomena.

Acoustic sensors as a biophysical tool for probing cell attachment and cell/surface interactions

Acoustic biosensors offer the possibility to analyse cell attachment and spreading. This is due to the offered speed of detection, the real-time non-invasive approach and their high sensitivity not only to mass coupling, but also to viscoelastic changes occurring close to the sensor surface. Quartz crystal microbalance (QCM) and surface acoustic wave (Love-wave) systems have been used to monitor the adhesion of animal cells to various surfaces and record the behaviour of cell layers under various conditions. The sensors detect cells mostly via their sensitivity in viscoelasticity and mechanical properties. Particularly, the QCM sensor detects cytoskeletal rearrangements caused by specific drugs affecting either actin microfilaments or microtubules. The Love-wave sensor directly measures cell/substrate bonds via acoustic damping and provides 2D kinetic and affinity parameters. Other studies have applied the QCM sensor as a diagnostic tool for leukaemia and, potentially, for chemotherapeutic agents. Acoustic sensors have also been used in the evaluation of the cytocompatibility of artificial surfaces and, in general, they have the potential to become powerful tools for even more diverse cellular analysis.