The adhesion of human epidermal keratinocytes to the implant surface is one of the most critical steps during the patient's recovery from implantation of transcutaneous prosthesis. To improve the success rate of transcutaneous prosthetic implants, we explored a new "top-down" approach to promoting this dynamic adhering process through modulation of upstream cell signaling pathways. To examine the feasibility of this novel approach, we first established an in vitro platform that is capable of providing a non-invasive, real-time, quantitative characterization of the keratinocyte-implant interaction. This platform is based on the dissipation monitoring function of the quartz crystal microbalance with dissipation monitoring (QCM-D) in conjunction with the open-module setup of the QCM-D. We then employed this platform to assess the effects of various pathways-specific modulators on the adhering process of keratinocytes. We demonstrated that this "top-down" approach is as effective in enhancing the adhesion of keratinocytes as the conventional "bottom-up" approach that relies on modifying the substrate surface with the adhesion protein such as fibronectin. We envision that this new "top-down" approach combined with the QCM-D-based in vitro platform will help facilitate the future development of new therapies for enhancing osseointegration and promoting wound healing.

Cells rely on their signaling network to coordinate various downstream effectors in regulation of essential cellular functions, including adhesion, migration, proliferation, and differentiation. Cell signaling network consists of signaling pathways that often interact in many different ways. Gaining a better understanding of how multiple signaling pathways are coordinated is critical to the success of development of medical diagnosis and therapeutic treatment. We have developed a noninvasive, quantitative approach with the use of the quartz crystal microbalance with dissipation monitoring (QCM-D) for in vitro, real-time examination of the G protein coupled receptor (GPCR)-mediated cell signaling. With this approach, we have been able to dissect the multiplicity of GPCR-mediated cell signaling and obtain mechanistic insight into the dynamic coordination and coupling of Gαq, Gαs, and Gαi pathways.

We previously reported the finding of a linear correlation between the change of energy dissipation (AD) of adhered cells measured with the quartz crystal microbalance with dissipation monitoring (QCM-D) and the level of focal adhesions of the cells. To account for this correlation, we have developed a theoretical framework for assessing the AD-response of adhered cells. We rationalized that the mechanical energy of an oscillating QCM-D sensor coupled with a cell monolayer is dissipated through three main processes: the interfacial friction through the dynamic restructuring (formation and rupture) of cell extracellular matrix (ECM) bonds, the interfacial viscous damping by the liquid trapped between the QCM-D sensor and the basal membrane of the cell layer, and the intracellular viscous damping through the viscous slip between the cytoplasm and stress fibers as well as among stress fibers themselves. Our modeling study shows that the interfacial viscous damping by the trapped liquid is the primary process for energy dissipation during the early stage of the cell adhesion, whereas the dynamic restructuring of cell ECM bonds becomes more prevalent during the later stage of the cell adhesion. Our modeling study also establishes a positive linear correlation between the AD-response and the level of cell adhesion quantified with the number of cell ECM bonds, which corroborates our previous experimental finding. This correlation with a wide well-defined linear dynamic range provides a much needed theoretical validation of the dissipation monitoring function of the QCM-D as a powerful quantitative analytical tool for cell study.

Cell adhesion is an essential aspect of cellular behavior. Finding innovative methods to probe the adhesion of cells in their native state can greatly advance the understanding of control and regulation of cellular behavior and their impact on human health. The quartz crystal microbalance (QCM) is a label-free, biosensing system that has, in the past fifty years, evolved from a simple acoustic based mass sensor to a powerful bioanalytical tool. Its unique capability of monitoring the cell-substrate interaction non-invasively in real time has led to the emergence of its applications in areas that are relevant to fundamental cell biology and medical research. This review is intended to provide readers an overview of the use of the QCM for examination of cell-substrate adhesion. It also describes how this innovative approach can be extended to the study of other aspects of cellular behavior, such as cell morphology, cell mechanics, cell motility, cell signaling, all of which can potentially be applied to medical diagnosis and/or pharmaceutical development. In this review a major emphasis is placed on informing readers about some of the most important practical aspects of the QCM-based cell study including data acquisition and analysis, the substrate surface manipulation, and cell manipulation.

Cell adhesion is essential in cell communication, cell regulation, and the development and maintenance of tissues. The mechanical interactions between a cell and its extracellular matrix (ECM) can influence and control the function and the behavior of cells. Gaining a better understanding about the mechanical interaction of cell adhesion can provide advances in biomaterial for implants, potential drug treatments for improvement of cell adhesion of implants, and fundamental understanding of signaling pathways related to cell adhesion. We have developed a non-invasive real time method using the quartz crystal microbalance with dissipation monitoring (QCM-D) to quantitatively monitor such cellular processes using the dissipation factor ΔD. In this study, we have used this method to examine the adhesion of human epidermal keratinocytes to the QCM-D sensor surface coated with titanium, a common material for medical implant. The key results from this study have validated this method as an in-vitro approach for examining cell-implant interaction.

The rapid growing bio-refining industry is hindered by the slow reaction rate of cellobiohydrolase catalyzed hydrolysis. Analysis of the mechanism of enzyme interaction in the cellobiohydrolase catalyzed hydrolysis is difficult because of the variety factors affecting the enzymatic reaction rate. In this study, we evaluated the processive hydrolysis parameters in the molecular level using a video rate atomic force microscopy (AFM). Both dissociate and association rates (koff and kon) have been calculated by an experimental data fitted Michaelis-Menten model and real-time observation experimental data. The difference in these two association rates suggested that the cellulose substrate governs the hydrolysis rate and that can deepen the understanding of the mechanisms of processive hydrolysis.

Epidermal growth factor (EGF) is a growth factor that binds a cell surface receptor to an EGF receptor, to promote cell growth and proliferation. The results of quartz crystal microbalance with dissipation (QCM-D) continuous monitoring for 100 min after stimulation with EGF of dosages confirmed that the energy dissipation decreases over time. Fatomic force microscopy (AFM) performs nanoindentation on cell membranes with less than 500 nm deformation in a single-cell analysis fashion revealing cell cytoplasm properties, while QCM-D monitors the cell–extracellular matrix interfacial layer where focal adhesion dominates the energy dissipation. Therefore, AFM and QCM-D work in a complementary manner in the characterization of the changes in viscoelastic properties of the cell. Cell signaling is one of the fundamental processes that control cell fate. Knowledge of the changes in mechanical properties will provide with more insight into the dynamics of cell signaling.

In the past two decades, quartz crystal microbalance (QCM) has evolved from a simple mass sensor to a powerful bioanalytical tool that is capable of assessing the properties of complex biological materials including cells. This evolution has led to the emergence of applications of the QCM in cell research that are potentially relevant to fundamental cell biology, pharmaceutical development, medical diagnosis and prognosis, environmental analysis, etc. This review highlights some of the major advancements of QCM-based cell research and summarizes some of the technical advantages of the QCM that have impacted these advancements.

Epidermal growth factor receptor (EGFR) plays a major role in cell migration and invasion and is considered to be the primary source of activation of various malignant tumors. To gain insight into how elevated levels of EGFR influence cellular function, particularly cell motility, we used a quartz crystal microbalance with dissipation monitoring (QCM-D) to examine restructuring of focal adhesions in MCF-10A cells induced by epidermal growth factor. Engineered cells that overexpress epidermal growth factor receptor (EGFR) exhibited a very different kinetic profile from wildtype MCF-10A cells that have a lower level of EGFR with a higher rate for the initial disassembly of focal adhesion and a much lower rate for the later reassembly of focal adhesions. It is conceivable that these effects exhibited by EGFR-overexpressing cells may promote the initiation and maintenance of a more favorable adhesion state for cell migration. This study has demonstrated the capability of the dissipation monitoring function of the QCM-D to quantitatively assess kinetic aspects of cellular processes with a high temporal resolution and sensitivity.

Cellulase is an interfacial enzyme that catalyzes the hydrolytic degradation of cellulose at the interface between a liquid phase (enzyme) and a solid phase (cellulose substrate). Prior to the hydrolytic cleavage, cellulase utilizes an activity known as enzymatic decrystallization to break up the solid aggregate of cellulose molecules. The activity of enzymatic decrystallization has not been characterized and its mechanism has not been elucidated because very few existing experimental approaches are able to examine interfacial enzymatic activity on solid substrates. Here, we report the development of a novel strategy for the real-time detection of cellulase activities including enzymatic decrystallization and hydrolytic cleavage on cellulose with the use of a nanomechanical sensor in a microcantilever. We present both kinetic and physical evidence to support the decrystallization as a kinetically viable step of cellulose hydrolysis by cellulase. To our knowledge, this is the first use of a nanomechanical sensor to study mechanistic enzymology and heterogeneous enzymatic catalysis that involves a solid substrate. This nanomechanical sensor-based approach will help obtain a comprehensive understanding of cellulase actions on cellulose, which would be essential to the success of the development of new cellulases with enhanced efficiency for biofuels production.

Cell adhesion is an essential biological process for cell survival, differentiation, and migration. There are two types of cell adhesion to the extracellular matrix (ECM) and adhesion to adjacent cells. Cell-substrate adhesion is mediated through a class of receptors known as integrins. There receptors simultaneously connect the ECM on the outside to the actin filaments on the inside of the cells. Another important cell adhesion is cell-cell adhesion; it is an essential component of epithelial morphology, cellular communication and function. Epithelial cells adhere tightly to their neighbors through adhesive structures and these structures are connected to intermediate filaments microfilaments. This association with the cytoskeletal network is necessary for stable cell-cell adhesion and for the communication of the changes in neighboring cells. There are many methods that can be used to measure the changes in cell adhesion, but each has its disadvantages. Many of these methods are based on end-point detection which don't allow real-time detection or are based on the introduction of foreign objects which can affect the response of cells. We have developed a non-invasive real time method using the quartz crystal microbalance with dissipation monitoring (QCM-D) to quantitatively monitor such cellular processes using the dissipation factor ΔD. Previously, we've successfully tracked real time changes in cellular adhesion due to induction of the EGFR pathway using the QCM-D. The QCM-D technique can be a useful application in studying other cellular process such as cell signaling and trafficking and can potentially be a useful in vitro method for drug and biomarker screenings. Here we will take a look at pathways that contribute to changes in cell adhesion through cell-ECM adhesion and cell-cell adhesion.

this technique can be used to probe the dynamic process of the enzymatic decrystallization of cellulose by cellulase. The bending of the microcantilever is likely a result of the change in interaction energy within the cellulose caused by the interaction between cellulase and cellulose (e.g., enzymatic decrystallization), not by the adsorption of cellulase onto cellulose. The innovative microcantilever sensor approach will be used to determine the kinetics of the enzymatic decrystallization by cellulase.

Conventional approaches for assessing changes in cell adhesion often lack of time resolution and require invasive force or nonnative label. To circumvent such problems, we have developed an innovative approach of using quartz crystal microbalance with dissipation monitoring (QCM-D) to track real-time changes in cell adhesion. We have experimentally and computationally established a correlation between time-dependent changes in energy dissipation factor (ΔD) measured from the QCM-D and the level of cell adhesion complex (i.e., focal adhesions). Based on this correlation, we have been able to investigate the epidermal growth factor-induced change in cell adhesion and its regulation. We have also been able to evaluate the effects of various pharmacological interventions of this dynamic change in cell adhesion. The results of our study suggest that this QCM-D-based approach can potentially be exploited for fundamental study of cellular processes such as cell signaling, trafficking, and mechanotransduction, as well as for biomedical research on drug and biomarker screening.