The general greying of the populations in the U.S., Europe, and Japan in particular, is now widely acknowledged. This recognition means that there will be an increasing need for new products to help manage the challenges of aging that seniors face. One of these is urinary incontinence. This condition constitutes a large and growing market opportunity that is completely technically different from those already addressed for young children. Foremost among the differences, is the fact that the consumers are often mature adults who can express dissatisfactions vocally, without hesitation. The odor of the urine of older persons collected in an incontinence product can be distinctly disagreeable and persistent. This can be especially embarrassing on social occasions when changing to a fresh item is impractical.

But to what can the odor be attributed? Surprisingly, the scientific literature seems to be totally silent on the topic of the chemical composition of senior urine. While the urine of NASA astronauts has been meticulously analyzed, the variety of chemical components identified therein are from obviously healthy and relatively young individuals. None of the compounds characterized shed any light on the nature of the odorous substances in senior urine nor indeed on its often golden hue, reminiscent of a quality lager. Soon the urine becomes a murky brown, a cloudiness develops and the unpleasant odor intensifies. Subsequently a white waxy substance separates. These changes in appearance are probably due to the action of bacteria that were present in the urethra. Thus, it should be realized that the composition and smell of the urine collected in any senior incontinence product will change with time. 

For a substance to have a smell, it must be volatile enough to reach the nasal receptors and create a sensory response therein.  Volatile substances can be acidic, basic or neutral. For example, butyric acid is responsible for the unpleasant odor of rancid butter. The basic cadaverine and putrescine, are aliphatic diamines and are causative of the odor of dead animals that originate from the amino acids, arginine and lysine by bacterial decarboxylation. Some neutral compounds, exemplified by methyl nonyl ketone, have peculiar odors that can evoke a strong reaction. All of these types of chemical molecules can be converted into nonvolatile derivatives, and these by definition must be odorless. 

When senior urine is excreted it is immediately absorbed by the cellulosic fiber component of the senior incontinence product. Any chemical reagents to then render the smelly ingredients odor-free thereafter should logically also be placed within the fibers because that is where the urine is now located.

Fortunately, the cellulosic fibers have an internal microporous structure where the odor-reducing reagents can be placed so that subsequent fallout and dusting before use can be completely avoided. The total internal volume of the micropores in wet pulp fibers is approximately 2 ml per gram of dry fibers. Thus, when commercially produced powders, such as crosslinked sodium polyacrylates, are simply placed in the spaces between the cellulosic fibers of the absorbent product, the manufacturing machinery must be enclosed to prevent the superabsorbent powder from flying everywhere throughout the plant. Augmentation of the particle size of the powder can reduce, but not eliminate, these difficulties. Moreover, the value of the ratio of surface area to particle volume is thereby diminished and so the extent of gel blocking is magnified. 

Clearly, in the not-too-distant future the replacement of superabsorbents between fibers with superabsorbents inside the cellulosic fibers can be anticipated. This may affect how the odor-reducing  chemistry operates since maximum superabsorbency  will always be desirable.

Because the micropores in cellulosic fibers range in size from six to 300 A, any substances placed therein including superabsorbents are nanosized. Consequently, the surface area available for reaction with odors or water is maximized so that amount of reagent or superabsorbent required in the senior incontinence product is thereby minimized.

Any acidic smelly substance can be converted into a nonsmelly nonvolatile calcium, strontium or barium salt by reaction with an alkaline earth hydroxide or carbonate. Strontium salts are particularly attractive because of ease of handling and complete lack of toxicity. Furthermore, abundant deposits are located in Mexico. The potential use of the more common sodium or potassium counterparts can be discounted because the corresponding salts are always water soluble and could adversely affect the performance of the superabsorbents as a result of their contribution to the osmolality of the urine.  While calcium carbonate can be created within the pores of the cellulosic fibers by the interaction of sodium carbonate and calcium chloride solutions inside the fibers the formation of strontium or barium carbonate is more easily secured without the formation of by-product sodium chloride.

Unlike calcium, the hydroxides of both strontium and barium are very soluble in hot water. Application of either of these hot solutions to cellulosic fibers first swells and then reacts to form the strontium or barium cellulosates. Any free hydroxide solutions within the fiber micropores on cooling produce nanocrystals that rapidly absorb carbon dioxide from the atmosphere to form the corresponding insoluble carbonates, ready to trap acidic odors and release carbon dioxide.

In contrast, the entrapment of any smelly components of the urine that are basic in nature requires the addition of an acidic substance that probably should best be polymeric. Such is already partly present in most senior incontinence products by virtue of the free carboxylic acid content of the superabsorbent particles.  If this is insufficient, the addition of a linear film-forming polyacid, such as the commercially available carboxymethycellulose or alginic acid, has been shown in bonding studies to provide acidic coatings strongly adherent to cellulosic fibers.

With odorous compounds that are neither acidic nor basic, other approaches to capture must be devised. The Senior Incontinence Product group of the Japanese company, Unicharm, has recently taken the lead here with a 2013 US Patent reporting that aldehydes are major contributors to the odor of senior urine. The method claimed for imparting nonvolatility to the smelly aldehydes was the facile reaction of the carbonyl groups thereof with primary amines to form imines. The listing of suitable amines included chitosan, a natural, film-forming aminopolysaccharide, now commercially manufactured worldwide from shrimp shells. Many years ago, studies of the properties of chitosan showed it to be an outstanding bonding coating for cellulosic fibers.  Chitosan also has established antimicrobial and wound-healing properties.

From all of the foregoing, it is apparent that the technology now exists to utilize the microporous structure of the pulp component of future senior incontinence products in combination with simple chemistry to reduce urine odors and to enhance absorbency.

About the author
G. GRAHAM ALLAN is now Professor of Chemical Engineering and Professor of Fiber & Polymer Science at the University of Washington in Seattle (Phone 425-486-1649; Email; [email protected]). He has authored some 300 articles and book chapters and has been awarded 72 patents on a variety of subjects. Currently, his teaching emphasizes Creativity & Innovation courses in both online and in-person versions. Previously, he taught at both the Universities of Paisley and Strathclyde in Scotland before taking up sequential positions as research scientist with the DuPont and Weyerhaeuser Companies in Delaware and Washington respectively.