Measuring single-cell growth: from bacteria to cancer cells

Nathan Cermak§, Selim Olcum§ Francisco F. Delgado, Steven C. Wasserman, Kristofor R. Payer, Mark Murakami, Scott M. Knudsen, Robert J. Kimmerling, Mark M. Stevens, Yuki Kikuchi, Arzu Sandikci, Masaaki Ogawa, Vincent Agache, Francois Baleras, David M. Weinstock and Scott R. Manalis

Single cells vary widely in their growth rate, a fundamental behavior that reflects biochemical and biophysical differences between cells and ultimately may govern their relative abundance within a population. Even genetically identical cells may grow at extremely different rates, owing to a combination of intrinsic molecular noise and various behavioral programs. We can not easily observe this variation using population-based growth assays, yet it has important consequences for human health. For example, cancer cells within an individual may vary drastically in growth potential, with subsets capable constant growth and proliferation and others being primarily stationary. Similarly, growth rate variation in bacterial populations can dictate the efficacy of antibiotic treatments, as slow- or non-growing cells tend to be more resistant to antibiotics. We present an approach to precisely and rapidly measure growth rates of many individual cells simultaneously. We introduce a micro-chip that incorporates an array of extremely sensitive mass sensors to weigh individual cells multiple times as they grow while flowing through a long microfluidic channel. Our chip reveals subpopulations of cells with different growth kinetics and enables assessment of cellular responses to antibiotics and antimicrobial peptides within minutes.

See MIT News for the news article.


Multiple-mode mass-sensing

Selim Olcum§Nathan Cermak§, Steven Wasserman, Scott Manalis,

Measuring the physical properties -e.g. mass, density or shape- of nanoscale objects is challenging. Such capabilities are particularly important for nanoparticles in biological fluids like viruses, extracellular vesicles or proteins, since most often their functionality depends on their shape, size, content or density. It is extremely challenging to resolve these particles due to wavelength-dependent diffraction limit.

Here we developed a technique to simultaneously oscillate multiple vibrational modes of a mechanical resonator and monitor its mode frequencies with high precision as nanoparticles quickly flow through the integrated fluidic channel inside the resonator.  As the nanoparticles flow through the channel, each mode frequency responds differently to the particle mass and position along the resonator. This way we can extract the mass and position of multiple nanoparticles in close proximity. The ability to resolve nanoparticles in close proximity in our current resonator suggests the possibility of observing mass tomographies of single nanoparticles, or resolving their shapes in solution when more modes of the resonator can be monitored with improved precision in the future.

Read more or see MIT News for the news article.


Weighing particles at the attogram scale

Selim Olcum§, Nathan Cermak§, S.C. Wasserman, K.S. Christine, H. Atsumi, K.R. Payer, W. Shen, J. Lee, A.M. Belcher, S.N. Bhatia, S.R. Manalis, 
PNAS, 2014.

Naturally occurring and engineered nanoparticles (e.g., exosomes, extracellular vesicles, viruses, protein aggregates, and self-assembled nanostructures) have size- and concentration-dependent functionality, yet existing characterization methods in solution are limited for diameters below ∼50 nm. Here, we developed a mechanical resonator that can directly measure the mass of individual nanoparticles down to 10 nm in solution with single-attogram (one billionth of a billionth of a gram or ~600 kilodaltons) precision, enabling access to previously difficult-to-characterize natural and synthetic nanoparticles. 

In addition to testing our device by weighing gold nanoparticles, we analyzed a type of biological nanoparticles called exosomes. We are now investigating if precise measurement of mass profiles of exosomes (or extracellular vesicles in general) in body fluids can provide clinically relevant information about various diseases.

Read more or see MIT News for the news article or PNAS Journal Club in highlighted article news.


Deep-collapse mode capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUTs) have been introduced as a promising technology for ultrasound imaging and therapeutic ultrasound applications which require high transmitted pressures for increased penetration, high signal-to-noise ratio, and fast heating. However, output power limitation of CMUTs compared with piezoelectrics has been a major drawback. In this work, we show that the output pressure of CMUTs can be significantly increased by deep-collapse operation, which utilizes an electrical pulse excitation much higher than the collapse voltage. We extend the analyses made for CMUTs working in the conventional (uncollapsed) region to the collapsed region and experimentally verify the findings. The static deflection profile of a collapsed membrane is calculated by an analytical approach within 0.6% error when compared with static, electromechanical finite element method (FEM) simulations. The electrical and mechanical restoring forces acting on a collapsed membrane are calculated. It is demonstrated that the stored mechanical energy and the electrical energy increase nonlinearly with increasing pulse amplitude if the membrane has a full-coverage top electrode. Utilizing higher restoring and electrical forces in the deep-collapsed region, we measure 3.5 MPa peak-to-peak pressure centered at 6.8 MHz with a 106% fractional bandwidth at the surface of the transducer with a collapse voltage of 35 V, when the pulse amplitude is 160 V. The experimental results are verified using transient FEM simulations.