Oral film water activity and moisture content: choosing the right test

Give each moisture measurement a specific question

A buyer receives a pliable oral film sample and a second sample that feels brittle. Asking for a moisture percentage is a reasonable start, but the result needs a defined purpose. Is the team investigating the drying process, a packaging problem, an analytical discrepancy or a change in handling? Those questions can require different observations.

FDA defines water activity as a vapor-pressure ratio under equilibrium conditions. At the same temperature, equilibrium relative humidity expressed as a percentage is 100 times water activity. Thus, an illustrative equilibrium humidity of 60% corresponds to an a_w of 0.60. That arithmetic describes a relationship, not a suitable acceptance limit for oral dissolving strips. FDA technical explanation.

Keep the reporting basis beside the number

A report saying “moisture: 5%” leaves several questions open. Ask whether the result is on an as-received or dry basis, which procedure was used and how the specimen was prepared. An a_w result needs its measurement temperature and a documented endpoint. A percentage and a dimensionless equilibrium result cannot be substituted for each other.

Choose the method before comparing suppliers

ISO 18787:2017 covers principles and requirements for determining water activity in food products and animal feed across a measurement range of 0 to 1. Its public scope describes dew-point and electrical measurement principles. A laboratory still needs to assess whether its chosen method is suitable for the particular finished-film matrix. The standard does not supply an oral film release limit. ISO scope.

For a mass-loss result, the name of the method matters. USP <731> describes loss on drying as volatile matter removed under specified conditions. Accordingly, a drying loss should not automatically be labeled water alone. If a water-specific method such as Karl Fischer titration is proposed, ask the laboratory to address suitability for the sample and its ingredients. USP chapter description.

Preserve the sample history from sachet to instrument

Define which condition the sample is supposed to represent. A strip tested immediately after opening an individual sachet and a strip left on a laboratory bench are different sampling situations. Record the opening time, room conditions, handling sequence and the interval before measurement. Use the agreed method to define acceptable exposure rather than inventing a universal number of seconds.

Use matched specimens for different tests

Reserve separate, identified portions for water activity, moisture determination and mechanical examination where the methods require it. Record the batch, roll or sheet location, sachet construction and sample preparation. If several strips are combined in a sample cup, state that clearly: a pooled result cannot describe variation between every individual strip.

1. Identify the finished-film version and the condition being investigated.
2. Assign samples to the planned tests before opening their protective packaging.
3. Record exposure and conditioning consistently for the comparison groups.
4. Request method details, equipment checks and the reported uncertainty where relevant to the decision.
5. Retain the original observations alongside the numerical report.

A temperature-controlled test also needs a stable sample. Agree how the laboratory checks equilibration and handles results that continue to drift. Repeatedly restarting a measurement until one result looks acceptable weakens the investigation.

Read conflicting results without forcing a conversion

Suppose two development batches return similar drying-loss percentages but different water activities. Keep both findings. The discrepancy is a reason to examine the formulation, material grades, conditioning and methods; it is not evidence that one instrument must be wrong. Request matched retesting only after the sample histories and reporting bases have been reconciled.

Moisture can also affect physical behavior. A 2025 comparative study of solvent-cast and hot-melt-extruded pharmaceutical orodispersible films reported changes in mechanical properties under increased humidity. That finding supports recording humidity during comparisons; it does not establish a storage specification for a different nutraceutical formula. Primary comparative study.

Keep microbial quality as a separate decision

FDA’s technical guide discusses 0.85 in the context of specified food regulations. Copying that number into an oral strip specification would skip the assessment of the actual product. Water activity also cannot stand in for the required microbiological testing, ingredient review or finished-product stability work. Ask the responsible specialists to justify the control strategy for the intended market and formulation.

Make the investigation usable by formulation and packaging teams

Finish with a comparison table that connects every result to a sample identity and a decision. Include the film formula version, raw-material lots, conditioning, method, result, relevant uncertainty, visible condition and package history. Preserve rejected and inconclusive results, with the reason they were not used for the decision.

If the issue appears only after storage, compare samples in their intended individual packaging against an appropriately defined control. Review seal condition and storage records before attributing the change to the film formula. Continue that work with the sachet qualification guide; use the laboratory selection checklist to define the commissioned work.

For an OEM or ODM inquiry, send the question the test should resolve and the existing sample records. “Please measure moisture” can become a usable brief when it states the product version, the comparison and the decision that follows.