Soft contact lenses are usually characterised under laboratory conditions that are easy to control and easy to repeat. Room temperature. Standard saline. Straightforward loading protocols. Over time, these choices have become the default rather than a deliberate simplification. Mechanical properties measured during soft contact lens mechanical testing are often treated as fixed values, then carried forward into lens design and modelling.
The difficulty is that the eye is not a laboratory.
Once worn, a soft contact lens sits on a warm, hydrated, mechanically active surface. Corneal surface temperatures are typically closer to 33–35 °C, and lenses equilibrate quickly. For hydrated polymer networks, that temperature shift alone can alter material response. This raises a practical question: how representative is room-temperature mechanical testing of on-eye lens behaviour?
Well, that’s the question that Towler et al. (research team out of the UK, Taiwan, and the US) attempted to answer in their recent Processes publication.
Figure 1. Experimental and modelling workflow used to study soft contact lens mechanics under physiological conditions, from material hydration and mechanical testing to hyperelastic model fitting and finite element simulation of on-eye deformation.
Reproduced from Towler et al., Processes (2025), CC BY 4.0.
Room-Temperature Soft Contact Lens Mechanical Testing Falls Short
Room-temperature testing persists largely because it works. It is reproducible, familiar, and compatible with long-standing standards. For many biomaterials, that baseline is sufficient. Soft contact lenses, however, operate in a narrow mechanical window. Small changes in stiffness can influence how a thin lens conforms to the cornea and responds to blinking.
Temperature is often treated as a secondary variable, if it is addressed at all. Mechanical properties are commonly reported as intrinsic constants, even though polymer mobility and water distribution are temperature dependent. Hydration complicates this further. Lenses are tested wet, but usually in saline baths held well below ocular surface temperature.
This gap between laboratory characterisation and physiological conditions is not unique to soft contact lens mechanical testing. Similar limitations appear across soft biomaterials, as discussed previously in broader overviews of mechanical testing practice: Mechanical testing of biomaterials for non-engineers.
The present study approaches this issue directly by repeating familiar tests under physiological temperature testing conditions, rather than introducing new test concepts.
Silicone Hydrogel Mechanics Under Warm, Hydrated Conditions
Definitive 74 and Unisil as contrasting material cases
Two latheable silicone hydrogel materials were examined: Definitive 74 and Unisil. The materials differ in water content and baseline stiffness, placing them at different points along the compliance spectrum. Neither material is experimental, which makes them useful for examining how testing conditions, rather than formulation, influence observed behaviour.
Temperature-dependent modulus and nonlinear response
Mechanical tests performed at 24 °C and 35 °C showed that both materials became more compliant at higher temperature. The absolute change in modulus was modest, but consistent. More notably, the stress–strain response shifted shape, particularly at low strains where lenses spend most of their on-eye life.
These observations align with broader trends seen in hydrated polymer systems that respond to environmental stimuli: Shape-changing hydrogels in response to stimuli.
Why Contact Lens Compression Testing Matters More Than Tensile Data Alone
Uniaxial tensile testing of contact lenses and its limits
Uniaxial tensile testing remains the most common way to report lens stiffness. It is useful for understanding handling and durability off the eye. However, tensile loading does not reflect the dominant forces experienced during wear.
Eyelid pressure and compressive loading
On the eye, lenses experience cyclic compressive loads from blinking. Uniaxial compression testing more closely resembles this environment, particularly when samples are tested fully hydrated. Comparing stiffness values across loading modes highlights why a single modulus value can be misleading (a point explored previously when comparing different approaches to stiffness measurement).
Figure 2. Combined tensile and compressive stress–strain responses of Definitive 74 and Unisil silicone hydrogel materials measured at room temperature (24 °C) and physiological temperature (35 °C), showing nonlinear behaviour and temperature-dependent compliance.
Reproduced from Towler et al., Processes (2025), CC BY 4.0.
Mechanical Testing of Soft Contact Lenses Using the UniVert System
Testing hydrated lenses at physiological temperature
Compression and tensile tests were carried out using a UniVert uniaxial testing system, with samples submerged in a PBS bath and held at controlled temperature throughout loading. This setup allowed soft contact lens mechanical testing to be performed at both room temperature and 35 °C without interrupting hydration.
Low-force resolution for compliant materials
Soft contact lenses deform under very small loads. Adequate force resolution is necessary to resolve subtle changes in stiffness without obscuring temperature effects. The same considerations arise in other soft material studies where viscoelastic behaviour becomes more apparent once testing moves beyond simplified conditions: Testing method: viscoelasticity.
Figure 3. Uniaxial compression testing of hydrated silicone hydrogel contact lens material using a UniVert system, with samples submerged in fluid during loading to maintain physiological hydration.
Reproduced from Towler et al., Processes (2025), CC BY 4.0.
Moving Beyond Linear Elasticity in Soft Contact Lens Materials
Nonlinear stress–strain behaviour
Both materials exhibited nonlinear mechanical behaviour across tensile and compressive loading. Linear elastic models were insufficient to describe the full response.
Hyperelastic material modelling
A first-order Ogden hyperelastic model was used to fit the data. Increasing model order did not meaningfully improve fit quality, suggesting that additional complexity was unnecessary for capturing the observed behaviour.
From Bench Testing to On-Eye Deformation
Finite element modelling of contact lenses on the eye
Experimentally derived material models were incorporated into a finite element simulation of a soft contact lens interacting with a realistic anterior eye model. Intraocular pressure, eyelid pressure, and tear-film surface tension were included.
Figure 4. (a) Initial position of the finite element model of the contact lens and the eye, where different colours indicate different material properties. (b) A complete finite element eye model with the eye in contact with the contact lens, both are ready for the fitting. Different colours show elements of materials, so an element-specific eye model was used. (c) Eye and contact lens models are where the lens is positioned on the eye. The colour bar shows the deformation magnitude in mm. (d) Half cross-sectional view showing the contact lens initial position. (e) Half cross-sectional view showing contact lens final position. Reproduced from Towler et al., Processes (2025), CC BY 4.0.
When Mechanical Compliance Influences Optical Performance
Base curve mismatch and refractive power change
Simulation results showed that refractive power changes increased once lens base curve deviated from corneal radius by more than approximately five percent. Temperature-dependent compliance influenced how quickly that threshold was reached.
Figure 5. Predicted refractive power change of soft contact lenses as a function of base-curve mismatch relative to corneal radius at room temperature and physiological temperature, highlighting increased sensitivity once mismatch exceeds approximately five percent.
Reproduced from Towler et al., Processes (2025), CC BY 4.0.
What Physiological Soft Contact Lens Mechanical Testing Adds to Design
Mechanical testing at physiological temperature does not invalidate room-temperature characterisation, but it reframes how those measurements should be interpreted. Off-eye stiffness values may underestimate on-eye compliance, particularly for high-water silicone hydrogels.
From a design perspective, softer materials may conform more readily, but they are also more sensitive to fit mismatch and deformation once warmed. Slightly stiffer materials still soften on the eye, while retaining greater geometric stability. In practice, soft contact lens mechanical testing that accounts for temperature, hydration, and loading mode offers a more realistic basis for modelling and design decisions than room-temperature testing alone.
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About the CellScale UniVert
The CellScale UniVert is used to perform uniaxial tensile and compression testing on soft, hydrated materials under controlled environmental conditions. It is commonly applied to compliant polymers, hydrogels, and biological tissues where small forces and subtle changes in mechanical response are important.
In this study, the UniVert was used for soft contact lens mechanical testing to measure the tensile and compressive behaviour of silicone hydrogel materials while submerged in fluid and held at both room temperature and physiological temperature. This made it possible to observe how modest thermal changes influence material compliance and to carry those measurements forward into finite element models of on-eye lens deformation.
