Capillary Action and Sample Volume Physics

Capillary Action and Sample Volume Physics

A glucose test strip fills itself. When a drop of blood touches the opening at the strip's edge, a thin channel with water-attracting walls pulls the blood inside while air escapes through an outlet, until the space over the electrodes is full [1]. That space is tiny. The strips we checked list minimum samples of 0.3 to 0.6 microliters (µL, millionths of a liter) [2][3][4], and their meters check that the chamber filled before they report a number.

What pulls the blood in

Capillary action is a liquid moving into a narrow gap on its own, with no pump and no squeezing. In 1921 Edward W. Washburn worked out how far a liquid travels into a thin tube under capillary pressure alone [5]. The distance grows with the square root of time, so a channel fills fastest in its first instant and slows as the liquid column gets longer [5].

His formula also lists the levers a strip designer can pull. Liquid moves in faster when its surface tension is higher, when the tube is wider, and when the contact angle is smaller. The contact angle is the angle a liquid's edge makes against a surface; a small angle means the liquid spreads out and wets it. A more viscous (thicker) liquid moves in more slowly [5]. Here's what surprised us: Washburn checked his equations with mercury, water and other liquids [5] 68 years before Matsushita filed the capillary strip patent described in the next section [1].

How a strip turns a gap into a measuring chamber

Matsushita's patent, filed in the US in 1989, shows the basic build. Carbon electrodes and a reaction layer of enzyme and electron acceptor sit on an insulating base. A spacer and a cover on top leave a thin open space with an inlet at one end and an outlet at the other [1]. When a drop touches the inlet, the sample flows in and pushes the air out through the outlet, which the filing says avoids leftover bubbles. The inner surfaces were made hydrophilic (water-attracting), so a set quantity of sample enters just by contact [1]. We cover how strip electrodes are made in screen printing vs. laser ablation.

The height of that gap is a compromise. The filing prefers 0.05 to 1.0 mm. A wider gap needs more blood and fills less readily by capillary action, while a narrower one raises the electrical resistance between the electrodes and distorts the current [1].

TheraSense, the company behind the first FreeStyle strips and now part of Abbott Diabetes Care [6], was granted a patent in 2001 that covers coating chamber walls with a surfactant, a soap-like wetting agent. It also describes filling part of the chamber with hydrophilic material such as glass beads, which shrinks the volume and wicks fluid in, and sensors that fill from the side or from the tip [7]. A patent shows what a company claimed. It doesn't tell you what's inside any strip on a pharmacy shelf.

Why the chamber's size sets the minimum drop

The minimum sample is the volume of channel that has to be wet before the meter can measure. A Roche patent granted in 2008 does the arithmetic for one example strip: a channel 1.5 mm wide and 0.15 mm deep, with 1.765 mm from the sample port to the electrodes that confirm filling, needs 1.5 × 1.765 × 0.15 cubic millimeters, or about 0.397 µL [8]. (One cubic millimeter is one microliter.) Labeled strips land in the same range. FreeStyle Lite strips need 0.3 µL [2], OneTouch Verio strips 0.4 µL [3] and Accu-Chek Guide strips 0.6 µL [4].

How a strip measures changes why that volume matters. FDA describes FreeStyle Lite strips as coulometric [2]: the meter adds up the total electric charge as the glucose in the chamber reacts, so the sample's volume is part of the answer. Heller and Feldman described the original FreeStyle strip as a printed parallel-plate coulometer holding about 300 nanoliters (0.3 µL) of blood between two facing electrodes 50 micrometers apart, with the volume between the plates accurately controlled [6].

OneTouch Verio and Accu-Chek Guide strips are amperometric, meaning the meter reads the current from the reaction [3][9]. For strips like these, Roche's patent says that making sure the sample covers the measuring electrodes is an important factor in accurate results, and one that has caused problems in capillary-fill strips [8]. We explain the two methods side by side in Coulometry vs. Amperometry in Biosensors.

How the meter knows the strip is full

The patents we read describe three ways to check the fill. TheraSense's filing names uncertainty about whether a small chamber has filled as a source of error, and it describes a fill indicator: an extra indicator electrode, or a second electrode pair, that shows when the chamber has filled [7]. Roche's patent puts two "dose sufficiency" electrodes past the measuring zone, near the far end of the channel. The meter sends a test signal into one and looks for a response above a threshold at the other, which means blood has reached the end; the claim covers blood fronts that are curved either way or flat [8].

LifeScan took a different route in a patent granted in 2005. The boundary between an electrode and the blood behaves like a capacitor, and the filing says its capacitance rises in direct proportion to how much electrode area the sample covers, and so to the sample's volume [10]. If the volume comes out too small, the filing describes compensating for it during the measurement [10].

FDA reviews confirm the behavior, though not the mechanism. For the first OneTouch Verio system, cleared in February 2011, FDA reviewed a study that gave three strip lots 0.25 to 0.45 µL of blood. At 0.4 µL the results were accurate; anything smaller gave an error code [3]. That original Verio meter has since been replaced; as of October 2026, LifeScan lists the Verio Reflect and Verio Flex meters alongside OneTouch Verio test strips [11]. Roche tested the Accu-Chek Guide with 24 meters, and its system displays an error code when it detects an insufficient sample [9].

Topping up a short sample

Only one of the three FDA summaries we read mentions adding more blood. The one for FreeStyle Lite lists a second sample application within 60 seconds [2]. The OneTouch Verio and Accu-Chek Guide summaries we read don't list a second-application window [3][9], so each strip's own instructions are the place to check.

What we couldn't find in any public FDA summary is which fill check a specific strip uses. The patents show what companies filed, and the decision summaries show only that a short fill produces an error instead of a number.