Droplet-Scale Estrogen Assays in Breast Tissue, Blood, and Serum

Extracting Estrogen from Small Samples

Microfluidic devices, in which small samples are manipulated within narrow channels, have been profitably applied to clinical sample analysis and biomedical research. These devices have advantages over more standard analysis techniques because they require only limited amounts of reagents, a boon when reagents are expensive. In addition, microfluidic devices are easy to make and can handle multiple processes and multiple samples on a single platform, enabling automation and scale-up.

In closely related digital microfluidic (DMF) devices, the usual channels through which liquids are pumped or electrically coaxed are replaced by arrays of flat electrodes. When an electrical potential is applied to an electrode, the surface becomes locally charged, such that the droplet moves to that region. When several electrodes are charged, one by one, droplets can be made to move across the surface (and dispense and mix) in a controlled fashion. To be moveable, liquids must be slightly conductive or have dipole moment > 1; fortunately, most liquids fall into this category. This DMF technology has now been harnessed by Mousa et al. for extraction of the steroid estradiol from very small tissue or blood samples for ultimate quantitation.

Steroid hormones, which regulate essential reproductive and homeostatic functions in the human body, are routinely measured clinically in blood but not in tissue because of the large samples that would be required by current methods. Separation of steroids from other biological components is usually necessary for accurate measurements because these components often interfere with the assays. To enable measurement of estrogen in tissues or small samples of blood, Mousa et al. have used DMFs and built a device to extract estradiol from 1-μl samples.

In their method, the biological sample is dried onto one of the electrodes and a lysing solvent is moved by application of electrical potential to the sample and dried. Then, a polar solvent moves to the sample, extracting soluble material, including steroids. The droplet containing the steroid (and other components) is then circulated within a pool of nonpolar solvent; many of the interfering compounds partition into the nonpolar liquid. The polar droplet, still containing the steroid, is moved out of the nonpolar solvent pool and dried on an electrode, ready for quantitation. For routine application, the extraction process will need to be linked to an automated measurement method.

A convenient technique for measuring estrogen in small amounts of breast tissue or tumor, for example, would allow testing whether failure to respond to therapy with selective estrogen receptor modulators such as tamoxifen or aromatase inhibitors is a result of locally high estrogen concentrations in the tumor and whether breast cancer risk can be predicted by local estrogen concentrations. Further, androgen concentrations in the prostate could be measured to check their influences on cancer.