Anatomy of a disposable diaper
The modern disposable diaper contains multiple layers of material, depending on the manufacturer's design, but has four basic zones of functionality. The inner layer or layers in contact with the infant's skin are made of polypropylene, a synthetic material that remains relatively dry while allowing liquid to pass through to the middle or transfer zone. The material in the transfer zone facilitates movement of the liquid away from the genital area to avoid "pooling." Moisture is then "wicked" to the superabsorbent gel material in the diaper core, which is able to absorb more than 80 times its weight of liquid. The fourth or outer layer, made of soft, textured, cloth-like polypropylene, prevents moisture from leaking through to the outer clothes or crib but is porous enough to allow humidity to escape.
Spraker: Dr. Krafchik, can you talk about the inner layer - the layer that's closest to the infant's skin?
Krafchik: Yes. It's intriguing how many qualities that material has to fulfill. First, since it's the layer closest to the baby's skin, it has to be very smooth and soft. That's crucial for reducing friction. The inner lining also has to have wicking capability, so that the urine can pass quickly through to the center absorbent core. The inner layer itself stays dry and thus lessens hydration of the infant's skin.
Spraker: What is the inner liner made of?
Krafchik: The makers of the leading brands use polypropylene in the layer that is in contact with the skin.
Spraker: That's the material used in winter underwear for skiers and runners. It stays relatively dry while passing moisture through so that outdoor exercisers don't get as chilled. In the diaper, it also stays dry while wicking the moisture through.
Krafchik: That's correct.
Spraker: What's the next layer?
Krafchik: The urine goes rapidly through the soft inner layer and next encounters a transfer layer that temporarily holds the urine and moves it toward the superabsorbent core. The transfer material has been designed to be able to move the liquid uphill - in a child who's standing, for example - before it meets the available combination of fluff and superabsorbent material in the next layer. The transfer material allows the liquid to be distributed along the entire surface of the diaper rather than collect in a pool where it would first hit the diaper. It carries liquid away from that loaded area quickly so that the entire diaper is utilized for absorption. Then the superabsorbent material below grabs it and holds it.
Leyden: The original concept was, "Let's make the skin less wet." When the technology of the superabsorbent material came along, someone said, "Let's put it in diapers. Maybe that will make the skin less wet." And it worked. Then it became apparent that a lot of the urine was not getting to the superabsorbent gel inside. So the industry developed ways to channel or attract more liquid into the superabsorbent gel. Each new step contributed to keeping the skin less wet. And that's why the incidence of diaper rash has been going down.
Spraker: In fact, the modern diaper has multiple layers and three to four zones of functionality, which underscores how complex these apparently simple products are. They're really very "high tech" devices.
Krafchik: In addition to these layers, there's also a special elasticized cuff to prevent leakage down the legs. It's generally made of the same material as the inner lining.
Spraker: The addition of this elasticized cuff in 1995 enhanced the leakage prevention properties of the diaper. Now let's move on to the core. Dr. Schuman, can you tell us about that?
Schuman: The core is not homogeneous as you move away from the center. It varies in the amount of cellulose and the amount of superabsorbent material in particular places. About 10 years ago, the industry tried gender-specific diapers. The idea was to put most of the superabsorbent in a target zone, so there was more absorbent material in the crotch of the female diapers and more in the front of the male diaper. Now they've moved away from gender-specific diapers, but there is still more material at both primary urine entry points; that is, there is extra material where the urine's going to be, whether the baby is male or female.
Spraker: So there are two areas that are maximally loaded with superabsorbent material, but the material is also well dispersed, to utilize the other areas of the diaper for absorption and retention. Dr. Leyden, would you finish our discussion of components by describing the outermost layer or "zone"?
Leyden: One of the trends in diaper technology is to make the outermost layer more and more vapor permeable so that it's less humid inside the diaper and the skin surface is therefore less wet.
Spraker: The outer layer or cover in the better grades of disposable diapers no longer feels like plastic. It feels like fabric.
Leyden: Yes. It's polypropylene and feels like cloth. It's still "plastic," but it's very soft, texturized plastic, and it's becoming more porous than the earlier covers in an attempt to reduce the humidity in the diaper area.
Schuman: From the practicing pediatrician's standpoint, there had been one minor drawback to the superabsorbent diapers. Before superabsorbents, we were able to extract urine quite easily from diaper material. That's more difficult to do with the superabsorbents. That's why in hospitals most NICUs still use diapers that don't have superabsorbent material. However, there have been two recent studies in which urinalysis and urine cultures were performed on extracted core material and were shown to be accurate. That's encouraging, because using urine bags is much less convenient. It would be very helpful if we could just send the lab a wet superabsorbent diaper and say, "Test it."