When a Bigger Bolus is Better
In microinjection, a bolus refers to a discrete, measured volume of solution delivered into a specific biological target [such as a cell, tissue, organ, or localized anatomical space] in a single injection event. The bolus contains the active substance (for example, dyes, nucleic acids, drugs, or cells) suspended in a carrier fluid, and its volume is typically controlled at the nanoliter to microliter scale using precision microinjection systems.
The size of the bolus is a critical experimental parameter because it directly influences distribution, concentration, and biological response. In many cases, smaller boluses are preferred to minimize mechanical disruption, prevent leakage, and maintain precise localization, especially in single-cell or embryonic microinjection.
However, bigger is better when the experimental goal requires broader tissue coverage, sustained exposure, or overcoming rapid diffusion and clearance. Larger boluses can improve signal detectability in tracer studies, ensure sufficient reagent concentration in dense or vascularized tissues, or enhance uptake when targeting extracellular or compartmentalized spaces. They are also useful when compensating for dilution effects in larger structures or when a threshold concentration is necessary to elicit a measurable physiological response.
Ultimately, optimal bolus size balances delivery efficiency, biological impact, and tissue integrity, and should be tailored to the target system and experimental objective.
First, let's consider volumes.
- A milliliter (mL) is one thousandth the volume of a liter (L) or 10-3L
- A microliter (µL) is one thousandth the volume of a mL (10-6L)
- A nanoliter (nL) is one thousandth the volume of a µL (10-9L)
- A picoliter (pL) is one thousandth the volume of a nL (10-12L)
This is graphically represented at the right. Notice that the mL is one trillion times larger than the picoliter. The table (right) shows that the side of a cube with a volume of 1mL is 1cm long. Likewise, the side of a cube with a volume of 1pL is 10µm long. Just for comparison, it shows that the diameter of a sphere with a volume of 1mL is 1.24cm, and the volume of a sphere with a volume of 1pL is 12.4µm.
With these volume comparisons in mind, lets consider some of the available options for microinjection pumps.
| RANGE | PUMPS | APPLICATIONS | |
| Milliliter Range | 10-3L |
Syringe Pumps Peristaltic Pumps |
General pumping applications |
| Microliter | 10-6L | UMP3 | Tissue/organ/body injection Microperfusion |
| Nanoliter | 10-9L | Nanoliter2010 | Cellular Injection |
| Picoliter | 10-12L | Pneumatic PicoPumps | DNA Injection |
Specifications for WPI pumps may be found in the article "Choosing the Correct Pump."
