Stephan Koehler

I’m the senior scientist in the Weitzlab, a new position geared towards helping lab members with their research projects and coordinating work.  I’m enjoy this position because it affords me the opportunity to interact with talented researchers working on exciting and challenging problems across multiple disciplines. Having a more traditional soft condensed matter background (e.g., granular media, foams, low-Reynolds number flows), working here has involved me in many fields of study that are new to me, such as biofilms, biomechanics, microfluidics, mechanics and formulations of rubber, genetic mutations, enhanced oil recovery, chaotic flow of emulsions in micro channels and motility of epithelial cells.

One of my recent projects focuses on understanding the growth of biofilms. I have been working with a graduate student, two postdocs and several professors trying to understand how environmental stressors trigger phenotypic changes. More specifically we are studying the growth patterns of fluorescently triple-labeled bacillus subtilis strains on agar plates. We track both the spatiotemporal evolution of phenotype expression, size and wrinkle patterns in response to different nutrition levels. How the bacteria collectively organize and coordinate to adapt to different environments, develop resistance to antibiotics and disperse to more favorable conditions is interesting in terms of evolutionary biology and critical to resolving practical issues such as infections and industrial fouling.

Another study with a graduate student and postdoc focuses on understanding the mechanical properties of cross-linked rubber with fillers. We observe a surprising similarity between the rheology of these rubbers and soft glassy materials such as hydrogel microspheres dispersed in water. Having established this link, we focused on colloidal systems to model rubbers and aid in improving current formulations for our industrial sponsor, Michelin.

One of the most exciting series of projects that I’ve worked on is aimed at developing nano-liter drop-based microfluidic bio-assays and applying them to biological problems requiring single-cell or single-template isolation. This includes error-free single-template PCR amplification whereby a generation of chimeras is eliminated by placing a single template in a single droplet and performing PCR within these isolated droplets. Another problem is understanding infectivity of cells by viruses through the isolation of single cells within droplets containing viruses and tracking infections. Together with New England Biolabs, we have developed an in vitro assay for protein-protein binding whereby libraries of proteins are generated within single drop libraries and their binding properties are measured. These drop-based techniques have generated a lot of interest because they hold promise to increase throughput by orders of magnitude while commensurately decreasing overhead and cost.

In addition to using drops for bio-assays, I’ve been working on projects involving drops for cargo transport. We have shown that drops can encapsulate many different fluids and particles for targeted triggered release, such as delivery of drugs or surfactants at desired locations. A recent publication shows how double-emulsion drops turn into polymerosome vesicles that can be loaded with drugs whose release can be adjusted by varying the polymeric formulation.

One of my favorite aspects of this role is helping visiting students understand and publish their work. I’ve collaborated with a French doctoral student in a study of evaporation from porous media. We found that evaporation is set by the shape of the water meniscus closest to the surrounding atmosphere, which we were able to calculate in the case of a simplified model system. In another study, a new type of water filter utilizing a perforated graphene sheet is developed. This filter can selectively remove large molecules with the goal of improving the selectivity to allow for water desalination. In other research, we’ve investigated how dipping a dandelion seed into water results in a small trapped water droplet; amazingly dipping in oil also results in trapped oil droplets too!

Recently I’ve started working on visually identifying cells using image segmentation and tracking their motion. Together with Jeff Fredberg’s group in the department of environmental health, we have been investigating the collective motion of epithelial cells using paradigms borrowed from granular flows and glassy materials. The motion of cells is important in embryonic development, wound healing and cancer cell proliferation, which makes this research very promising for possible applications in healthcare.

The above was just a brief summary of some of my projects. I’d be more than happy to discuss my work and offer my consultation on other research projects. Please come find me in the GM 517 group office.