# How Disposable Diapers Keep Skin Dry Using Porous Plastic Sheets > A 1970 patent by Procter and Gamble describing a specialized plastic top layer for diapers that allows liquid to pass through while keeping the baby's skin feeling dry. - **Patent:** US 3489148 - **Original title:** Topsheet for disposable diapers - **Owner:** Procter and Gamble Co - **Granted:** 1970 - **Status:** Public domain (expired) - **Times cited:** 133 - **Field:** consumer_electronics, mechanical, materials ## What it does This patent describes a hydrophobic, porous plastic film used as the inner layer of a disposable diaper. The material is designed with specific hole patterns that allow urine to quickly pass through into an absorbent core. Because the material is hydrophobic, or water-repelling, it prevents the liquid from flowing back to the surface, effectively separating the moisture from the baby's skin. This mechanism was a fundamental shift from using cloth or non-porous materials that would trap wetness against the skin. ## What it does NOT cover - Does not cover the absorbent core material itself, only the top layer in contact with the skin. - Does not cover non-plastic or fabric-based top layers that rely on absorption rather than fluid transport through holes. - Does not cover diapers that do not use a hydrophobic material to prevent liquid back-flow. ## The clever bit The innovation lies in using a hydrophobic material with specifically sized apertures that exploit surface tension to let liquid move in one direction only, effectively creating a one-way valve for moisture. ## Real-world examples 1. Pampers disposable diapers 2. Generic disposable diaper top sheets ## Why it matters This invention was a cornerstone in the development of the modern disposable diaper industry. By solving the problem of 'wet-back'—where moisture returns to the skin surface—it enabled the mass-market success of brands like Pampers. It transformed diapers from a simple absorbent pad into a sophisticated engineered product. ## Frequently asked questions ### What does How Disposable Diapers Keep Skin Dry Using Porous Plastic Sheets cover? A 1970 patent by Procter and Gamble describing a specialized plastic top layer for diapers that allows liquid to pass through while keeping the baby's skin feeling dry. ### Who owns patent US 3489148? Procter and Gamble Co owns this patent, granted in 1970. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 3489148 cited by? This patent has been cited by 133 later patents that build on its ideas. ### What problem does this patent solve? This invention was a cornerstone in the development of the modern disposable diaper industry. By solving the problem of 'wet-back'—where moisture returns to the skin surface—it enabled the mass-market success of brands like Pampers. It transformed diapers from a simple absorbent pad into a sophisticated engineered product. ### What does this patent NOT cover? Does not cover the absorbent core material itself, only the top layer in contact with the skin. **Full plain-English explainer:** https://patentbrief.org/patent/us/3489148/disposable-diaper-topsheet **Original patent:** https://patents.google.com/patent/US3489148 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How the Modern Waterbed Works](https://patentbrief.org/patent/us/3585356/waterbed-liquid-support) — A 1971 patent describing a liquid-filled, heated furniture support designed to cradle human bodies without letting them touch the bottom of the container. - [How Soft Contact Lenses Were Invented Using Hydrogels](https://patentbrief.org/patent/us/3220960/soft-contact-lens-hydrogel) — This patent describes the chemical recipe for soft, water-absorbing plastic materials that form the basis of modern soft contact lenses. - [Making Strong, Porous PTFE: The Gore-Tex Process](https://patentbrief.org/patent/us/3953566/gore-tex-expanded-ptfe) — This patent describes a specific process for rapidly stretching a highly crystalline form of PTFE plastic to create a strong, porous material with a unique internal structure, forming the basis for products like Gore-Tex. - [How the First Modern Water Slide Was Designed](https://patentbrief.org/patent/us/2982547/slip-n-slide-carrier) — A 1960 patent for a water-based amusement structure featuring a sloped surface and a water supply system to create a sliding experience. - [How Bubble Wrap Is Manufactured](https://patentbrief.org/patent/us/3142599/bubble-wrap-cushioning) — A 1959 manufacturing process that creates cushioning material by trapping air between two layers of plastic film.