Pharmaceutical Gel Spreadability Testing: Step-by-Step Texture Analyzer Protocol

The pharmaceutical gel spreadability test is an instrumented method that measures the work required to force a pharmaceutical gel to flow between two matched conical surfaces, producing a quantitative spreadability energy value in N·mm that correlates directly with patient-perceived ease of application. Conducted on a texture analyzer such as the KHT TA-30 using the Ortan 45°/90° spreadability rig, this test provides the primary instrumental readout for topical drug product characterisation under USP <1724> Semi-Solid Drug Products — Performance Tests.

A complete spreadability test takes under one minute per replicate, uses only 5–10 g of sample, and delivers a force-distance curve from which peak force, area-under-curve (spreadability energy) and firmness can all be extracted simultaneously. Spreadability data now appears routinely in ANDA and MAA submissions for topical generics, in topical bioequivalence (Q3 microstructure) data packages, and in ICH Q1 stability programmes.

What Is Spreadability and Why It Matters for Topical Drug Products

Spreadability is formally defined as the ease with which a semi-solid formulation distributes across a substrate under a defined shear geometry. In the pharmaceutical context it is the single most important texture attribute for patient acceptance: a gel or cream that is difficult to spread produces uneven drug distribution, patchy therapy, unpleasant application sensation, and ultimately poor patient compliance.

The physical principle of the texture analyzer method is straightforward. Two matched cones — a female cone (cavity) containing the product and a male cone (probe) of the same included angle — are driven together at controlled speed. As the male cone descends, product is forced outward along the conical gap. The force rises steeply at first (yield onset), passes through a peak, and plateaus or declines as the gap narrows. The area under the force-distance curve is the mechanical work performed on the product in N·mm — the spreadability energy. This parameter is the pharmacopoeial-style value most often cited in topical drug specifications.

Spreadability is also strongly temperature-dependent — gels typically soften 10–25% per 5 °C rise. For reproducible work, samples must be conditioned at 25.0 ± 0.5 °C; for skin-contact simulation, 32 °C Peltier-controlled fixtures are available.

Spreadability Test Equipment: Spreadability Rig vs. Cone Geometry

The spreadability rig is a purpose-built fixture consisting of a female cone (sample holder) and a male cone (probe). Two standard included angles are in common pharmaceutical use. Both cone angles trace back to the original Ortan research programme and are referenced in USP <1724> as acceptable geometries for spreadability characterisation.

• 90° cone (flat-sided, shallow): the default geometry for most pharmaceutical gels, hydrogels and soft creams. The shallow angle produces a broad, low-peak force curve sensitive to small differences in product consistency. Force ranges 0.1–5 N are typical.
• 45° cone (steep-sided, deep): used for firmer semi-solids such as medicated petrolatum ointments and zinc oxide pastes. The steep angle produces higher peak forces (2–20 N) and is less prone to side leakage with stiff products.
• Fixture materials: stainless steel 316L cones are preferred over aluminium for products containing organic solvents. Cones should be polished to Ra < 0.4 µm to ensure reproducible surface shear conditions.
• A flat-probe TPA test does not simulate the spreading geometry and does not correlate reliably with patient-perceived ease of application. The spreadability rig and TPA are complementary, not interchangeable.

KHT TA-30 Setup: Speed, Distance, Pre-Test & Trigger Conditions

Before the first test, four instrument parameters must be set: load cell, test speed, travel distance, and trigger force. The following values are the KHT TA-30 defaults for the pharmaceutical gel spreadability test, aligned with USP <1724> supportive guidance.

Key Results: Spreadability Energy, Peak Force & Area Under Curve

A well-executed gel spreadability test produces a characteristic force-distance curve with three resolvable regions: an initial yield rise, a rounded peak, and a gradual plateau. The software automatically extracts the following parameters.

Documentation, Specification Setting & GMP Batch Record Integration

Every test run on the KHT TA-30 is automatically recorded in a 21 CFR Part 11-compliant audit trail: user login ID, date and time, instrument serial number, method ID, load cell and fixture IDs, all raw force-distance data at 500 Hz, all derived parameters, and any user-initiated modifications with justifications. The record is cryptographically signed and cannot be modified without generating a new audit entry.

A release specification for a pharmaceutical gel product typically includes: peak force (mean within range of X ± Y N), spreadability energy (mean within range of A ± B N·mm), minimum 5 replicates, and %RSD ≤ 12% acceptance criteria. Specification limits are set during method validation using three consecutive batches — the mean ± 3 standard deviations form the initial acceptance window. The KHT TA-30 ships with pre-built ICH Q2 validation protocols and the Part 11 software module as standard — not as a paid upgrade.

Step-by-Step Protocol

The numbered procedure below is the KHT TA-30 pharmacopoeial-style protocol for the pharmaceutical gel spreadability test, suitable for inclusion as an SOP in a GMP laboratory.

• Sample preparation: Weigh 6.0 ± 0.1 g of conditioned product (25.0 ± 0.5 °C, 4-hour conditioning minimum) and transfer to the 90° female cone, filling to the marked line. Press gently with a spatula to eliminate entrained air.
• Instrument preparation: Power on the KHT TA-30 and log in. Install the 5 kg load cell. Mount the 90° male cone on the load cell using the M6 threaded interface and torque to 2.0 N·m.
• Method recall: From the method library, select "Gel Spreadability 90° — USP <1724> Aligned". Verify method parameters (speed 3.0 mm/s, travel 23 mm, trigger 0.05 N, acquisition 500 Hz).
• Tare: With the male cone installed but not in contact with sample, press "Tare". The load cell zero is recorded in the audit trail.
• Positioning: Place the loaded female cone on the instrument platform so that the male cone is approximately centred 5–10 mm above the product surface. Use the centring jig to ensure coaxial alignment.
• Test initiation: Press "Start". The instrument descends at 2.0 mm/s until the 0.05 N trigger is detected, then switches to 3.0 mm/s test speed.
• Test execution: The male cone descends 23.0 mm into the female cone, then withdraws at 10 mm/s. The full cycle takes approximately 10 s.
• Parameter extraction: The software automatically computes peak force (N), spreadability energy (N·mm), gradient over first 5 mm (N/mm) and, if a withdrawal phase was recorded, work of adhesion (N·mm).
• Replicates: Clean the cones between replicates using the standard solvent protocol, dry thoroughly, and repeat a minimum of five times.
• Reporting: The software computes mean ± SD and %RSD and generates a signed PDF report with the method ID, sample information, all raw curves, the parameter table, and the operator's electronic signature.
• Review and approval: The supervisor applies an independent electronic signature and links the record to the batch documentation in the QMS. All changes and approvals are captured in the 21 CFR Part 11 audit trail.