Design and Performance of an Inexpensive Water Activity Meter

Area of Science:

• Pharmaceutical Science
• Analytical Chemistry
• Microbiology

Background:

• Water activity (aw) is critical for microbial growth, influencing product stability and safety.
• Commercial water activity meters are expensive ($10,000+), limiting accessibility for some applications.
• Accurate aw measurement is essential for compounded formulations, especially concerning preservative effectiveness.

Purpose of the Study:

• To construct an affordable and robust water activity meter using readily available components.
• To measure the water activity of liquids commonly used in compounded pharmaceutical formulations.
• To assess the accuracy and precision of the developed low-cost water activity meter.

Main Methods:

• An inexpensive water activity meter was assembled using SHT-85 sensors and an Adafruit Feather HUZZAH microcontroller.
• Standard salt slurries and oral liquid vehicles were tested in airtight containers to determine equilibrium.
• Calibration curves (R2 >0.99) were generated to convert sensor readings to water activity values.

Main Results:

• The constructed water activity meter proved accurate and precise for standards and oral vehicles.
• Oral liquid vehicles exhibited water activity values ranging from 0.62 to 0.99.
• Equilibration times varied by container size, with stirring showing limited impact on smaller volumes.

Conclusions:

• An accurate, precise, and cost-effective water activity meter (approx. $150) was successfully developed.
• Many common oral formulation components exceed the United States Pharmacopeial threshold (0.6) for preservative requirements.
• Pharmacists must exercise caution when diluting preserved vehicles due to potential changes in water activity.