Abstract— The pollen tube is a fast growing cellular protrusion that plays a key role in the reproductive process of flowering plants. It serves as an important model for studying cellular morphogenesis, anisotropic growth mechanisms, and cellular signaling in the plant sciences. The anisotropic growth of pollen tubes is driven by a finely tuned control of the intracellular turgor pressure and the extensibility of the cell wall. To decipher this internal feedback loop and mathematically model the growth process, a quantitative understanding of the mechanical properties of the cell wall is crucial, in addition to biochemical investigations. We report an integrated microfluidic-MEMS force sensor system that allows for high-throughput optical and mechanical investigations of pollen tubes. The system permits large-scale germination, growth, and optical phenotyping of pollen tubes empowering rapid micro-indentation measurements on these cells. I. INTRODUCTION Pollen is ubiquitous in our natural environment. It is commonly known as a seasonal allergen (hay fever) and as a nutrition source for bees. The research community has used the presence, spatio-temporal distribution, and species- specific morphology of pollen in the environment as forensic material for criminal investigations, as fossil records to reconstruct the vegetational history for paleoclimatology, and as bio-sensors for environmental pollution monitoring [1]. Most importantly, pollen is the carrier of genetic information in plants. In cross-pollinated plant species, either the wind, insects, birds, or other pollinators transport the pollen to a receptive stigma. The pollen then germinates and generates an elongated protrusion, called the pollen tube, which carries the sperm cells and delivers them to the female gametophytes located inside the ovules in the flower. This fertilization process is schematically illustrated in Fig.1a,b using Lilium longiflorum, or Easter lily, a major floricultural crop and research plant model. Easter lily is a self-pollinating species The work was supported by the Research and Technology Development project MecanX funded by SystemsX.ch, the Swiss initiative for systems biology. N. Shamsudhin*, B. Atakan, N. Laeubli, C. Hu and B.J. Nelson are with the Multi-Scale Robotics Laboratory, Institute of Robotics and Intelligent Systems, ETH Zurich, CH-8092 Zürich, Switzerland. H. Vogler and U. Grossniklaus are with the Department of Plant and Microbial Biology and the Zurich-Basel Plant Science Center, University of Zurich, Zollikerstrasse 107, CH-8008 Zürich, Switzerland. A. Sebastian is with IBM Research – Zurich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland. (*e-mail: snaveen@ethz.ch) with so-called perfect, bisexual flowers. The style is typically 120 mm long, and the pollen tube must traverse this distance to deliver the sperm cells to the embryo sac. This means that the lily pollen tube, which is approximately 17 m in diameter, must grow several thousand times in body length within a few hours to achieve fertilization. These cellular growth speeds are unrivalled in the natural world. Unraveling the mechanisms of anisotropic, tip-polarized growth and morphogenesis of pollen tubes requires, in addition to biochemical and microscopical investigations, a thorough understanding of the mechanical forces involved in its growth: From the species-specific adhesive forces of the pollen on the stigma [2], over the stylar forces experienced as it grows through the pistil [3], to the resistive force of the ovular tissue before fertilization. To counter these externally encountered resistive forces, in its expansive tip growth, the pollen tubes maintains within its volume a fine regulation of the high turgor pressure and modulates the cell wall stiffness. Unlike animal cells, plant cells have a relatively rigid cell wall that encloses the plasma membrane. Probing the spatio- temporal mechanical variations of the cell wall through micro-indentation, and deciphering the resultant cellular response to these stimuli, can provide clues to understanding the underlying cellular processes of growth [4], [5]. The micrometer size, the rapid three-dimensional growth, entanglement, and non-adhesion of pollen tubes grown in vitro (Fig 1c) hindered previous attempts at high-throughput optical and mechanical characterization [6], [7]. One approach to tackle this problem was to automatize the process using computer-vision techniques, which could continuously monitor and track the pollen tube of interest for micro-indentation [8]. This approach requires a complicated hardware-software interface and relies on pollen tubes that adhere to the surface and do not entangle. Another approach based on microfluidics, used whole-body fluidic loading to estimate the elastic modulus of the pollen tube cell wall [9]. This method suffered from low-throughput and lack of quantitative force calibration. Our contribution to opto-mechanical investigations of this fast tip-growing cell is two-fold. We describe the design and development of a microfluidic platform to germinate, grow, and guide pollen tubes, which make long-term live cell imaging and high-throughput optical phenotyping possible. In addition, the microfluidic chip has an open-channel architecture that allows the integration of a sub-micrometer tipped, individually calibrated MEMS force sensor for rapid mechanical characterization of the pollen tubes. Probing the micromechanics of the fastest growing plant cell – the pollen tube Naveen Shamsudhin, Huseyin Baris Atakan, Nino Läubli, Hannes Vogler, Chengzhi Hu, Abu Sebastian, Ueli Grossniklaus and Bradley J. Nelson 978-1-4577-0220-4/16/$31.00 ©2016 IEEE 461