Microlasers Allow Force Measurements at Microscopic Scale

This is the authors' abstract. We don't add key points for this paper.

Mechanical forces play major roles in almost all aspects of biology. Mechanical force generation guides the development of early embryos as cells divide and reorganize to form an adult organism. The migration of immune cells into tissues to fight infections requires modulations of various forces. Muscular contraction requires the generation and dissipation of forces along multiple axes. Rapid cell division of cancer cells in confined spaces applies force on surrounding cells. Cells are constantly sensing, responding to, and generating mechanical forces. Because of the importance of measuring forces within cells and tissues, over the years many sophisticated methods have been developed to measure the strength of the various forces that modulate biological processes. These methods include measuring the displacement of a substrate (traction force microscopy) [1,2], atomic force microscopy (AFM) [3], optical and magnetic tweezers [4,5], and the injection of small compressible materials [6]. Most of these techniques rely on high-resolution microscopy and are difficult or impossible to implement in dense material. As an alternative method, Dalaka et al. [7] developed a method they call DEFORM. The method uses microdrop laser particles in the size range of 10 μm to 30 μm. The particles are formed from non-toxic oils and fluorescent dyes, so they are not toxic to biological materials. Deformation in the particle shape by applied force results in alteration in the laser emission from the particles. From the known properties of the particles, force measurements can be calculated. Deformations of the particles in the nanometer range can be resolved allowing measurement of sub-nanonewton forces. In theory, deviations from a round shape of the microlasers, such as would occur when surrounding forces exert pressure on the “squishy” microlaser droplets, should alter the emission (mode splitting) in a way that can be measured and mathematically correlated with the change in shape of the deformed particle. If one knew the mechanical properties of the droplet, it should be possible to correlate deformation of the particle with the force needed to generate the deformation. To characterize the properties of microlasers of differing size and composition, the researchers used AFM to determine the surface tension of the differing droplets. Their measurements fit nicely with predicted properties. The researchers then used AFM to apply known forces to deform the particles and observed the alteration in signal output. In this way they calibrated the relationship between force, particle size, and deformation-induced signal changes. They found a predictable and near-linear relationship between applied force and mode splitting. Thus, alteration in the lasing output was correlated with the degree of force applied. Having defined and calibrated the DEFORM system, they then tested DEFORM in a real-world setting. The researchers investigated changes in force generation in a cancer model. Microlaser droplets were added to spheroids of approximately 400 μm diameter formed in culture. The spheroids were formed using a squamous cell carcinoma cell line (WM-SCC-1 cells) mixed with the microlasers. A strong lasing signal was detected throughout the spheroid when the lasers were pumped. The pattern of lasing of individual particles in distinct locations within the spheroid indicated a complex array of forces. Some areas indicated piconewton forces, while other areas exerted forces in the nanonewton range. As a control, cellular movement was inhibited with drugs. Under these conditions, force generation was dramatically reduced. As a second test of their particles, Drosophila (fruit fly) larvae were injected with microlasers and allowed to equilibrate for several days. Single microlasers within the larvae were successfully imaged over days to ascertain force differences during larval development within different regions of the larvae. Muscle tissue is particularly dense and hard to image through the full depth of a sample. However, the authors detected a measurable signal from as deep as 300 μm within the muscle. In contrast, two-photon fluorescence microscopy, known for its ability to image deep into tissue, could only provide useful signal from the outermost bands of muscle in the larvae. The microlasers were remarkably successful for making sensitive force measurements within biological materials and provided information that would be difficult or impossible to attain using other methods. Beyond measuring forces within biological tissues and organs, these microlasers could have a wide variety of uses in both biological and material science applications where identifying and measuring of small, highly localized forces is required.