Gentle cell trapping and release on a microfluidic chip by in situ alginate hydrogel formation

Abstract

Microfluidic devices are increasingly used to perform biological experiments on a single-cell basis. However, long-term stability of cell positions is still an issue. A novel biocompatible method for cell entrapment and release on a microchip is presented. It is based on the controlled formation of an alginate hydrogel by bringing two laminar flows of alginate and calcium ions in the range of 2 mM to 40 mM into contact. The resulting growth of a gel bar is used to enclose and immobilize yeast cells. Adding ethylenediaminetetraacetic acid (EDTA) to the alginate solution allows for control of the hydrogel growth, and by varying the ratio of Ca(2+) to EDTA concentrations gel growth or gel shrinkage can be induced at will. Trapped cells are released during shrinkage of the gel. The trapping efficiency for different cell speeds is investigated and the properties of gel growth are discussed using a diffusion model. Precise positioning of a single cell is demonstrated. The technique presented allows not only the reversible immobilization of cells under gentle conditions but also offers the potential of long-term cell cultures as shown by on-chip incubation of yeast cells. The procedure may provide a simple and fully biocompatible technique for a multitude of innovative experiments on cells in microsystems.

Microfluidic patterning of alginate hydrogels

“…1͑d͒ of Ref. 31 can be attributed to the ion depletion of EDTA in the alginate flow down the channel.…”

“…Firm attachment of the gel to the glass is indicated from the stable position of the gel bar and a regular and reproducible shape, which is determined by the diffusion controlled reaction between calcium and alginate. 31 When gel adhesion is weak, bending and shifting of the gel bar is observed in the initial phase of gel formation, and leakage flows between gel and channel walls result in an irregular gel shape ͑videos 1-3 of the supporting information͒. 39 The surface treatment assures strong gel adhesion for about five gel productions, before it has to be renewed.…”

“…Gel formation is based on the controlled manipulation of laminar flows of an alginate and a calcium solution on a chip according to a previously published technique. 31 The gelling of alginate is induced by chemical reaction with calcium ͑and other metal ions͒ even with very low calcium concentrations in the absence of UV light or heat. The harmlessness of the employed substances and conditions and the possibility of easily dissolving the gel make this method very suited for the temporary encapsulation of cells, for which it is widely employed.…”

Bridging the Bio-Electronic Interface with Biofabrication

“…In bioMEMS devices, chitosan films have been used for drug delivery, redox and small molecule detection, biocatalysis, and cell studies 20,23,24,25 . Similarly, alginate is widely used as a cell-entrapment matrix and has been explored for reversible fluidic containment of cell populations and in-film immunoanalysis 26,27,28 . Composite films for tissue engineering applications have been fabricated using alginate electrodeposition, with components such as with hydroxyapatite for orthopedic implants 29 .…”

Cell trapping in microfluidic chips

“…Cells can, moreover, be released by dissolving the gel, a typical feature of contactless immobilization. This was recently demonstrated by us with cells entrapped in a calcium alginate hydrogel on a chip [17]. The extent to which dissociated polymer chains rest on the surface of the released cell has not been examined, however.…”