Rheology of living cells has developed an increasing need for high throughput measurements. Diseases such as cancer heavily remodel the cytoskeleton and impinge on cellular functions. Cells affected by such diseases show altered rheologic responses on many different levels rendering cells' mechanical fingerprints - a potential target for diagnostics. To counteract naturally occurring distributions of properties in samples of living cells and foster the validity of experiments, high numbers of single cell measurements are necessary. Here, we present the "in flow optical stretcher" (IFOS), a concept of non-invasive optical cytometry capable of high throughput rates, while working in a regime of long measurement times and low frequencies. The setup deforms whole cells in a continuous flow by optical forces, bypassing steps of cell positioning that are unavoidable in state-of-the-art optical stretcher devices. A prototype was built using polydimethylsiloxane soft lithography. In a proof of premise experiment, we show that in the IFOS it is possible to deform cells of mammalian origin which have been treated with cytochalasin. All recorded successful experiments took place in less than 2 s each, as opposed to 10-20 s in state-of-the-art optical stretcher devices. Although other microfluidic rheology devices achieve significantly higher throughput rates, they operate in different frequency regimes and probe different mechanical responses. The IFOS still captures viscoelastic properties and active responses of cells while aiming to maximize the throughput at creep times on the order of seconds. It can be assumed that an automatic IFOS reaches a throughput an order of magnitude higher than current devices that are based on optical stretching for cell rheology.
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