A disposable underpad is a layered laminate rather than a single sheet. It is built from a liquid-permeable body-facing layer, an acquisition and distribution layer, an absorbent core that holds the fluid, a tissue wrap that keeps the core together, and a liquid-impermeable backsheet. Wet-back is fluid that travels back toward the skin when the pad is compressed; construction prevents it by locking liquid into the core and by letting flow move one way.
That layer stack is what makes one pad different from another even when the two look identical on the shelf. The absorbency figure gets the attention, but the core and the barrier are what decide whether a resident stays dry between changes.
What layers make up a disposable underpad?
The layers are arranged by function, from the surface that touches the skin to the surface that touches the bed or chair. A published underpad patent from Medline Industries describes the same sequence: "a liquid permeable patient facing layer and a liquid impermeable base layer. An absorbent layer is positioned above the base layer and below the patient facing layer, and a tissue layer is positioned above the absorbent layer and below the patient facing layer."
| Layer | Job | Typical material |
|---|---|---|
| Body-facing sheet | Lets fluid pass through quickly and stays comfortable against skin | Hydrophilic nonwoven |
| Acquisition and distribution layer | Spreads a sudden gush across the core so it is not concentrated in one spot | Nonwoven or tissue |
| Absorbent core | Holds the fluid and locks it away | Fluff pulp blended with a superabsorbent polymer |
| Tissue wrap | Keeps the core from shifting or escaping | Tissue sheet |
| Backsheet | Stops fluid reaching the bed or chair | Liquid-impermeable film, for example polypropylene |
The superabsorbent polymer is the part that holds fluid under pressure rather than simply soaking it up. The same patent describes the core polymer as "a super-absorbent polymer known in the art", and states that "in a particular embodiment the super-absorbent polymer comprises sodium polyacrylate." Sodium polyacrylate is the chemistry behind the "gel" that forms when the core soaks, and it is what allows a thin pad to hold more than its own fluff pulp would.
What is wet-back, and how is it measured?
Wet-back, also called re-wet, is the amount of fluid that leaves the core and returns to the body-facing layer when the pad is pressed against the skin. It is measured separately from absorbency. The ISO 11948-1 method determines the absorption capacity of the absorbent core of body-worn urine-absorbing aids by weighing the aid dry, soaking it, draining it and weighing it again, which measures how much fluid the core can take up. Wet-back is a different measurement: fluid is loaded onto the pad and a weighted stack of dry filter paper is pressed against it, and the mass the paper gains is the amount that travelled back to the surface.
| Measurement | What it asks | How it is taken |
|---|---|---|
| Absorption capacity (ISO 11948-1) | How much fluid the absorbent core can hold | Weigh dry, soak, drain, weigh again |
| Wet-back or re-wet | How much fluid returns to the surface under pressure | Load fluid, press weighted dry filter paper, weigh the gain |
| Leakage under pressure | How much fluid escapes when the pad is compressed | Apply a defined load and measure liquid released |
Keeping those three apart matters, because a pad can score well on absorbency and still feel wet.
Why does the backsheet matter?
The backsheet is the layer that makes the pad a barrier rather than a sponge. The Medline patent describes it as the fifth layer, "not liquid permeable", intended to capture the liquid absorbed by the pad and not allow it to pass through to the surface beneath, and gives polypropylene as an example of a material that does not allow the passage of liquid. A pad with a strong core and a weak backsheet will still let fluid reach the bed, which is why the barrier layer is specified rather than assumed.
How does construction affect absorbency?
Two pads with the same nominal capacity can behave differently depending on how the fluid is distributed and held. A well-built acquisition layer spreads a gush before it reaches the core, so the polymer has time to absorb it rather than letting it pool in one place. The one-way design keeps the fluid from returning. The Medline patent states the principle directly: the body-facing layer is "permeable to liquids such that the liquids can pass through the surface of the first layer toward the second layer", but "the liquids cannot substantially reverse direction and move back toward the surface of the first layer." That one-way behaviour is the construction goal, and it is what the term wet-back prevention describes in practice. The underpad range lists the layer count and construction for each format, and facilities standardising a specification across sites can start from the wholesale and institutional account page.
Care homes that also reprocess instruments keep those records alongside their supply records, and the printable sterilization log sheet and label generators build the load, service and malfunction fields without an account.
Have a question about your own facility? Send it in and you will get a written answer specific to your setup, with the regulation or standard it is based on cited. Ask your compliance question.
Related reading
- What does heavy absorbency actually mean on an adult incontinence pad?
- How much does a bed pad reduce laundry costs in a care home?
- Which absorbent incontinence products should a long-term care home stock?
Frequently Asked Questions
What is inside a disposable underpad?
A body-facing nonwoven layer, an acquisition and distribution layer, an absorbent core of fluff pulp blended with a superabsorbent polymer, a tissue wrap, and a liquid-impermeable backsheet.
What makes an underpad leak-proof?
The backsheet, which does not allow liquid to pass through to the surface beneath, together with a core that holds fluid under pressure rather than releasing it back to the surface.
How is wet-back different from absorbency?
Absorbency is how much fluid the core can hold, while wet-back is how much fluid returns to the body-facing layer under pressure. A pad can be highly absorbent and still feel wet if its core does not hold fluid against compression.
What is the superabsorbent polymer in a pad core?
It is a polymer that absorbs and retains fluid under pressure. The Medline underpad patent gives sodium polyacrylate as one example, and describes it as a super-absorbent polymer added to the core.
Does ISO 11948 measure wet-back?
The ISO 11948-1 method measures the absorption capacity of the absorbent core by weighing the pad dry and wet. Wet-back is a separate gravimetric measurement, taken by pressing weighted dry filter paper against a loaded pad and weighing the fluid it picks up.
CliniEco Medical holds Medical Device Establishment Licence MDEL #35334. This article summarises published standards and product engineering literature for information only; it does not replace the product's own instructions or the clinical judgement of the care team for a specific resident.
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