Cellulose-Based Hydrogel for Personal Hygiene Applications
Personal hygiene product is an inseparable part of urban society. It has given comfort, reliability, and flexibility to sick people, women, and children. The hygiene items containing superabsorbent polymer (hydrogels) for absorbing large amount of body fluids are the attractive inventions of modern science. The hydrogels swell and imbibe body fluids in the presence of hydrophilic functional groups in the polymeric backbone. Current trend of using acrylate-based superabsorbent in hygiene products is creating significant portion of urban garbage. This pile up is not only shrinking land sites but also harming a lot to the environment due to non-degradability of superabsorbent materials existing in the core of hygiene product. In spite of high water-holding capacity of petrochemical-based superabsorbent polymer, it has a hidden curse on nature of non-degradability and health risk. Cellulose is the most abundant biocompatible matter on this earth which basically originated from plants. It is also naturally occurring long chain polymer that plays a vital role in food cycle in animal kingdom. Besides this cellulose, its derivatives have large application in various fields. As cellulose and its etherified and esterified derivatives have attractive physicochemical and mechanical properties, hydrogels synthesized from cellulose and its derivative can be alternative to synthetic superabsorbent polymer. Cellulose-based hydrogels have found application in various fields like agriculture, biomedical, tissue engineering, wound dressing, pharmaceuticals, etc. Among various applications, some products are available in the market, and some are in research level. Due to fast swelling and other extraordinary properties (i.e., biocompatible and biodegradable), cellulosic materials (cellulose-originated hydrogels) can be applied in personal hygiene product so that superabsorbent from nonrenewable materials is partially or completely replaced. In this chapter, history of using superabsorbent in hygiene product, brief discussion on hydrogel synthesis, health and environment risk related to non-cellulosic absorbent materials, suitability of cellulose-based hydrogels over available acrylate hydrogels, and recommendation for development have been discussed.
KeywordsCellulose Hygiene products Diaper Hydrogel Superabsorbent
Cellulose is the most common and omnipresent biopolymer on earth, which is the essential ingredient of plants. Pure cellulose can be in different forms and varied in size, shape, and degree of crystallinity. The common forms are microcrystalline cellulose (MCC), powdered cellulose (PC), and low crystalline powdered cellulose (LCPC). All the forms have different mechanical, physical, and pharmaceutical properties. Cellulosic materials comprise pure cellulose and derivatives of cellulose. Among different derivatives, etherified and esterified derivatives are most useful for industrial purposes. Cellulose and its derivatives can easily be converted in to eco-friendly superabsorbent materials  that can be employed in numerous fields such as hygiene (disposable diapers and feminine care products) [2, 3], agriculture (water retention and pesticide delivery) , biomedical materials (drug carriers, wound dressings, and tissue engineering scaffolds) [5, 6, 7, 8, 9], pollutant adsorbents (heavy metal ions, dyes, and pesticides) [10, 11], biosensors , etc.
From the definition, we can get information that hydrogels are hydrophilic polymer network which can swell on absorbing water without dissolving . Hydrogels attract water due to polar functional groups on the skeletal of macromolecule and inhibit dissolving due to crosslinking. The amount of fluids taken up by the polymer network depends on the structure of the polymer network itself and on the environmental conditions, such as the temperature, pH, and ionic strength of the water solution in contact with the polymer [13, 14]. Hydrogels are of mainly two categories based on their natural or synthetic origin: biopolymer-based and synthetic. Hydrogels prepared at the early stage are non-biodegradable and originated from nonrenewable petroleum based. Among various biopolymers, cellulose is largely available in nature and shows hydrophilic nature and good mechanical properties because of enormous hydroxyl groups. The large number of hydrophilic functional groups in the structure gives possibility to cellulose as promising material for hydrogel preparation.
Any kind of hygiene product follows the common type of mechanism for water absorption. Most of the products are made up of a top layer containing spongy materials, and in most of the cases, it is fluff pulp. The material is capable of absorbing liquid very fast and makes room for further absorption by adsorbent material (i.e., hydrogel). Figure 1b shows the mechanism of absorption. As the adsorbent contains polar groups and water is also polar in nature, for this the water molecule is attracted by hydrogels and forms hydrogen bonds. Finally the hydrogel swells and holds the liquids excreted from the body.
The aim of the chapter is to make general idea about hydrogel and hygiene product to common reader and create source of thinking to the special reader (researchers) to search ways of sustainability in personal care product manufacture as well as making the product biocompatible at the end of use, with the aid of cellulose and its derivatives.
2 Brief Description on Hygiene Products with Source and Polymeric Composition
The chapter demands some basic information about the types and construction of personal care products for general readers. It is mentioned earlier that disposable diapers, sanitary pads, and adult protective underwear for incontinence are common hygiene items and all the products consist of an absorbent core filled with hydrogels. In addition the products include the top layer of nonwoven or perforated film, a back sheet, and a fastening system. Again according to personal need, the hygiene product also varies in size, shape, and quality. The components are attached in such an organized way to gather the edges of the hygiene product into the proper shape so it fits snugly around a baby’s legs and crotch or to elder persons. When the products fit properly (e.g., disposable diaper and rest of the hygiene products), they give comfort and retain body fluids which pass through the permeable top sheet and absorb into the pad.
Diaper consists of mainly two parts: core and chassis. The core is mainly composed of polypropylene, cellulose, and a superabsorbent polymer, and chassis holds the core and attaches the diaper onto the baby and creates a proper fit around the legs. The top layer of a diaper is important as it remains in direct contact with baby’s skin. The layer is designed such that the liquid passes rapidly toward the core. It is mainly made up of permeable polypropylene (PP) nonwoven. The next layer is made from fluff pulp and hydrogel, and the outer layer which prevents leakage of liquid is mainly made up of polyethylene (PE).
Among feminine hygiene products, sanitary pads and tampons are most frequently used. The common material as absorbent in these products is cellulose fluff, and modern trend is to use hydrogels for absorption which makes the product thinner and more reliable.
Elderly skin is sensitive to injury being thinner and has poor healing capabilities. In addition adult incontinence is not always accompanied by fecal incontinence as in baby diaper. People with different ages and different complexities are the user of the product. Adult diapers also consist of similar materials in construction as baby diaper but the absorption capacity is higher compared to baby diaper.
Hydrogel is an interesting field of research, and before studying its application elaborately, it is logical to get idea about classification of the materials. Many authors tried to classify hydrogels through various perspectives or viewpoints taking into consideration of many factors. In this section, only classification of hydrogels based on source from which the product is synthesized has been discussed.
2.1 Hydrogels Based on Source and Polymeric Composition
Hydrogels are generally originated from two main sources, and they are synthetic (petrochemical based) and natural . Hydrogels synthesized from natural sources can also be divided into two main groups, i.e., polysaccharide-based hydrogels and others are polypeptide (proteins) based. In real practice, natural-based hydrogels also contain synthetic parts. Generally hydrogels are prepared through addition of some synthetic monomers onto the natural substrates, e.g., graft copolymerization of vinyl monomers on polysaccharides.
On the basis of starting materials, hydrogels can also be classified as natural polymer hydrogels, synthetic polymer hydrogels, and combination of the natural and synthetic. Again methods of preparation of hydrogels change to the types of product. Using a number of monomers for the synthesis of hydrogels gives hydrogels with different properties. Classes of hydrogels based on polymeric composition are described below:
2.1.1 Hydrogels from Homopolymer
Homopolymer means that the product is derived from the same monomeric unit. When the hydrogel is derived from single monomer, it is homopolymeric hydrogel, and it is the fundamental structural unit from which the whole chain grows. Depending on the type of monomer and polymer technique, the growing chain may have cross-linked skeletal structure .
2.1.2 Hydrogels of Copolymers
Copolymer results from the combination of two or more different types of monomers. Copolymer-containing hydrogels are formed by two or more different monomer species among them at least one hydrophilic component, arranged in a random, block, or alternating arrangement along the chain of the polymer network .
2.1.3 Hydrogels of Interpenetrating Polymeric Network (IPN)
This is an important class of hydrogels, which is prepared from two independent cross-linked synthetic and/or natural polymer components, arranged in a network form. In semi-IPN hydrogel, there are two components among which one is of cross-linked polymer and the other is a non-cross-linked polymer .
3 Methods of Synthesis of Cellulose-Based Hydrogels
Hydrogels are polymeric substances, and they are generally prepared from one of the two common pathways: (a) polymerization reaction among hydrophilic monomers and (b) renovation or reactivation of existing polymeric substance (natural or synthetic).
There are mainly two original sources from where hydrogels are synthesized, i.e., synthetic or petrochemical based and natural. Natural sources can also be classified into two groups, i.e., polysaccharide-based (mainly cellulosic) and polypeptide-based hydrogels. The synthetic sources are hydrophobic in nature but chemically stronger. After polymerization, hydrogels gain hydrophilic character. The strength of nature is responsible for slow degradation rate as well as provides the durability of the product. Scientists are searching to bring balance through optimal design between the two opposite qualities to make product successful to the consumer . Recently considering the environmental issue, scientists are designing hydrogels from natural sources and also meeting the expected demands of durability and degradability. As the sources contain suitable functional groups or have been functionalized with radically polymerizable groups, they can be converted to eco-friendly hydrogels .
Formation of polymer chains via chemical reaction.
Assistance of ionizing radiation to generate main-chain free radicals propagate reaction to enlarge chain and recombine as cross-link junctions.
Instead of chemical, physical interactions such as entanglements, electrostatics, and crystallite formation are used to form polymeric chain.
Each of the polymerization technique has some positive achievement for the finished product as well as darken contribution. Bulk polymerization is the simplest technique which contributes high rate and degree of polymerization. Hydrogels from the bulk polymerization is homogeneous, glassy, and transparent but very hard. The glassy matrix swells in water to become soft and flexible.
An important class of polymerization technique for hydrogel synthesis is suspension polymerization. The product obtained by this technique in the form of powder or microspheres (beads) for this grinding is not required. The process is advantageous than bulk polymerization due to used solvent system where water-in-oil (W/O) system was used instead of common oil-in-water (O/W). As the process is thermodynamically unstable, there requires continuous stirring and addition of a low hydrophilic–lipophilic-balance (HLB) suspending agent .
In some application, graft copolymerization is a useful tool to improve hydrogel quality. Grafting onto a strong support is an old concept but helpful in property enhancement of weak structure. The technique involves formation of free radicals of support onto which monomers will combine through covalent bonding .
Besides discussing different polymerization techniques, one of the main objectives is to describe briefly the factors affecting synthesis of cellulose-based hydrogels. It is possible to obtain cellulose-based hydrogels from cellulose and its derivatives. The inherent fact of cellulose-based hydrogels has also been disclosed in earlier part of the chapter that this type of hydrogels contains a cellulosic part and another synthetic part. Synthesis technique consists of physical or chemical stabilization of aqueous solution of cellulosics (cellulose and its derivatives). For this the hydrogels containing both cellulose and synthetic part are termed as composite hydrogels which also gain specific properties . Cellulose-based hydrogels stabilized physically are synthesized from aqueous solution of methyl and/or hydroxypropylmethyl cellulose , and they are thermoreversible in nature. In this case, gel formation mechanism includes addition of hydrophobic macromolecules containing methoxy groups. At low temperature, hydration of polymer chains takes place in solution which propagates entanglement of chains. Further increase in temperature makes it possible to dehydrate from entangled chain which also gears up hydrophobic association of chains, and finally polymeric network of hydrogels is formed.
The degree of substitution of cellulose ethers as well as addition of salts controls the solgel temperature of the system. Higher degree of substitution of cellulose derivative results to more hydrophobic character of polymer chain which is necessary for hydrogel network formation. Salt concentration also gives similar result. It was found that hydrogel for biomedical application requires specific properties which can be obtained from hydrogels with specific formulation. It was also reported that by adjusting degree of substitution and salt concentration and keeping temperature at 37 °C, obtained hydrogel is suitable for biomedical application [25, 26].
It is found that physically cross-linked (i.e., high-energy radiation) hydrogels are stiff and stable and have flow properties; the same result of crosslinking can be obtained in case of using suitable chemical agent that gives stable cellulose-based networks. In both cases, degree of crosslinking (the number of crosslinking sites per unit volume of the polymer network) is an important factor which affects the diffusive, mechanical, and degradation properties of the hydrogel. The degree of crosslinking can be controlled in a number of ways, and chemical modification of cellulose backbone is the most popular and common of them. Few published articles report that some chemically modified cellulose derivatives are used for hydrogel preparation and showed potentiality in biomedical application .
Another remarkable factor which is essential to think before designing a hydrogel is the biocompatibility of crosslinking agents. The most widely practiced crosslinkers for cellulose are epichlorohydrin, aldehydes and aldehyde-based reagents, urea derivatives, carbodiimides, and multifunctional carboxylic acids. Be aware about toxicity of them like aldehydes. Recently it has been reported that cellulose-based hydrogels cross-linked with citric acid are biocompatible as well as show good swelling.
Due to consideration of environmental and safety aspect, scientists show more interest to radiation crosslinking of polymers, based on gamma radiation or electron beams. Though it is not risk free, but it has more potentiality in biomedical application with controlled dose of radiation [28, 29].
4 History and Present Scenario of Hydrogels Used for Personal Hygiene Product
In 1938 water-absorbent polymer found its success, and that time it was prepared by thermal polymerization of acrylic acid and divinylbenzene in aqueous medium . Later in 1950, Otto Wichterle synthesized first-generation hydrogel from poly (hydroxyethyl methacrylate) (PHEMA) and used it for soft contact lenses preparation. Though the preparation was synthetic, but it was a revolution in ophthalmology . Since the 1970s, diaper technology has been flourishing, and about 1000 patents have been issued for last 25 years on the construction and design of diapers and reached at present condition. Japan started their first commercial production of hydrogel in 1978 for use in feminine napkins, and later in 1980, Germany and France started using it in baby diaper . Japan started manufacturing disposable diaper in 1983 with 4–5 g SAP hydrogel in every single piece of diaper. Within very short time, other countries in Asia, the USA, and Europe also started manufacturing diaper with hydrogels. Introduction of SAP hydrogels in personal hygiene product (i.e., diaper and napkin) was a turning point of the concept of using the items for self-protection and comfort. All the synthesized products were synthetic in nature and lack of degradability; scientists started to prepare hydrogels from cellulosic sources which had good absorption and biocompatible property.
From an online market survey report  in the Asia-Pacific region, China is the biggest market, and the USA is the largest end user and producer of SAP in the North American region. According to that report in 2014, Asia-Pacific was estimated as the largest market for SAP with the largest share of the overall SAP market in terms of value. For that year, North America and Europe together accounted for more than 40% of the global SAP market in terms of value. The amount of SAP was estimated at 2.07 million tons in 2014, and the amount will reach to 3.1 million tons in 2023; the worth of the product will be 11 billion USD.
Again from an online presentation given by a diaper consultant (Carlos Richer, https://www.linkedin.com/in/carlosricher), the number of adult people at the age of 70 will be 563 million in the year 2023, and one in every three requires protection for incontinence. The same report gives the projection that in 2025 the world number of babies will be 328 million and this will require a large number of disposable diapers .
Summary of events in modern hygiene product history
First water-absorbent polymer
Synthesized first-generation hydrogel from poly (hydroxyethyl methacrylate, PHEMA) and used for soft contact lenses
Japan started their first commercial production of hydrogel for use in feminine napkins
Germany and France started using hydrogels in baby diaper
The journey of innovation (high and rapid absorption capacity, biocompatible, cost-effective, etc.)
5 Application of Cellulose-Based Hydrogels
Cellulose and its derivatives are low priced, renewable, and biocompatible and can be applied to manufacture hydrogels where biodegradability is prime important. In this section, application of cellulose-based hydrogels in personal hygiene care and other traditional use of hydrogels as water absorbents to more innovative biomedical applications have been discussed.
5.1 Personal Hygiene Care
Superabsorbent hydrogels are the common ingredients for personal hygiene care products (diaper, sanitary napkins, and adults underwear for incontinence). These products keep individuals dry, safe, and out of embarrassing situation at definite time period. Some articles have been published concerning about the advantages of using superabsorbent hydrogels for personal care products especially in diapers . For the recent couple of years, the business of parents has grown much which makes them dependent on diaper for their babies. Modern SAP-based leak-proof disposable diapers not only keep them dry and prevent skin diseases but also control the risk of fecal contamination in day-care play areas and other places. Now it is a proven truth that SAP-based diapers play a vital role in reducing the risk of illness of children and building a healthy society . The use of SAP materials in personal hygiene product was not an old fashion; it only started in 1982 by Unicharm in Japan which is used first in sanitary napkins and later in baby diaper. It made diaper thinner with less weight and better performance in leakage handling . A statistic revealed the fact that a child till the age of 30 months using disposable diaper creates about 1092 m3 of garbage per year which requires removal and almost 1500 m3 of diapers as garbage requires cleaning from a city of 1 million population. As it was a tedious job, many attempts have been taken to make reuse of the items like disposable diapers, napkins, hospital bedsheets, sanitary towels, and other similar products . There are some common materials in these hygiene products which are biodegradable and others are not. Generally the cellulosic part is recyclable and biodegradable, and the objectives of recycling is to separate the ingredients for further use. Since the task was complex, it made scientists to think alternatively, and making the ingredients biocompatible is only solution. As cellulose-based SAPs are biodegradable, it can be a good alternative solution of the problem. It has been reported that hydrogels synthesized from sodium carboxymethylcellulose (NaCMC) and hydroxyethyl cellulose (HEC) cross-linked with divinyl sulfone (DVS) show good water retention capacity under centrifugal loads . Concerning with environmental impact, cellulose-based hydrogels are very much suitable for hygiene product to make them eco-friendly and decomposable. In this regard, using nontoxic crosslinking agents  will be more fruitful attempt, and introduction of radiation technique will make the process more acceptable .
5.2 Water Conservation Aid in Agriculture
Cellulose-based hydrogels have other interesting and attractive applications. One of the promising applications of hydrogels is in agriculture. Sources of underground water are shrinking, and scientists are trying to consume rainwater by holding sprayed water for a long time for cultivation. In arid and desert regions of the world, where scarcity of water resources is an unbearable problem, the xerogel (i.e., dry hydrogel), in form of powder or granules, can be a helpful tool in agriculture. At the time of cultivation, water or nutrients mixed water are sprayed; the water is absorbed by the hydrogel, which then releases to the soil as needed, thus keeping the soil humid over long periods of time. Many research articles have been published mentioning application of hydrogels in agriculture; among them Sannino and coworkers recently developed cellulose-based superabsorbent hydrogels which are totally biodegradable and biocompatible and able to absorb up to 1 l of water per gram of dry material, without releasing it under compression .
5.3 Biomedical Application
Due to the property of biodegradability shown by the cellulosics, hydrogels synthesized from them also become biodegradable. In addition they have many similarity with body tissue, for this many researchers found potentiality to apply hydrogels from cellulose and its derivatives in biomedical application. As cellulose-based hydrogels swell in the presence of water and degrade, it can be applied as devices for the removal of excess water from the body and in the treatment of some pathological conditions, such as renal failure and diuretic-resistant edemas .
5.5 As Supporting Materials in Treatment
The World Health Organization gives information about modern health problem like obesity and overweight; one in every three adult population is overweight, and one in every ten is obese. Another alarming information is that more than 20 million children under age 5 are overweight. These two reasons are responsible for several chronic diseases, such as type 2 diabetes, cardiovascular disease, sleep apnea, hypertension, stroke, and certain forms of cancer. Doctors generally suggest supervised diet or dietary supplements combined with adequate physical exercise for the treatment of obese and overweight. Hydrogels can act as dietary supplements. It is reported that novel cellulose-based hydrogels, obtained by crosslinking aqueous mixtures of NaCMC and HEC, have been shown to be appealing for the production of dietary bulking agents .
5.6 Wound Dressing
One of the established applications of cellulosed-based hydrogels is wound dressing. Different types of dressing products are in the market, and in some cases, product with antimicrobial agents is also in the market to ensure sound cure. Few hydrogel dressings have been patented so far and are available in the market, based on synthetic or natural polymers or a combination of them. Due to high purity, good air permeability, and good water retention capacity, bacterial cellulose (BC) has been widely investigated for wound healing, and the product is also in the market.
6 Health Risk of Hygiene Product and Impact on Nature
Many health disorders are found to take place in relation to the use of napkins, and the common incidents are toxic shock syndrome, skin irritants, staph infections, and urinary tract infection and may be related to other health problems.
Most of the absorbent center in disposable diapers and napkin is made of sodium polyacrylate (SAP), which is a petrochemical-originated synthetic chemical. In addition many hygiene products use dyes, and most of the dyes are chemically derived from either petroleum or coal tars. Coal tar is a carcinogenic liquid or semiliquid that is obtained from bituminous coal and often contains a number of toxins, including benzene, xylene, naphthalene, phenol, and creosol. Again in some cases, the dyes are resistant to biodegradation. Therefore these chemicals can be harmful even after they leave the hands of the user, as they become water pollutants and are highly toxic to aquatic life.
SAP was responsible for the cases of toxic shock syndrome in both diapers and napkins and skin irritation, urinary tract infection, and other health problems.
Moreover handling of such huge number of solid garbage created from diapers and other hygiene product is very much difficult (Fig. 8b). It is found from a statistical data that there are about 50,000 diaper user among 1 million population in the USA and this requires necessity to remove 150 m3 of diapers from a city . The addition of millions of tons of untreated waste of diaper and napkins added to landfills each year is a national problem.
7 Suitability of Cellulosic to Overcome the Hindrance
Cellulose is the polymer of glucose, renewable, abundant in nature, and found more or less in all parts of the world. Though it is mainly originated from plants, but some bacteria are able to synthesize cellulose . Both the celluloses are insoluble in water and other solvents due to high crystallinity in the structure. To make the cellulose dissolve or transform into working condition, it requires modification. As cellulose contains numerous hydroxyl groups, these sites can be modified through chemical reaction. Etherification and esterification are two processes to bring change in hydroxyl groups of cellulose. Cellulose derivatives are called cellulosics which are found eco-friendly as they are decomposed by several bacteria and fungi present in air, water, and soil . It is also investigated that modification lowers crystallinity which results rapid degradation and enhances solubility.
Biocompatibility of cellulose and its derivatives prompted it to apply in various fields especially in personal hygiene product as well as biomedical application. As human or animal tissue cannot synthesize cellulase (enzyme), cellulose cannot be absorbed by them. The thing is that cellulose will degrade slowly and the matter was observed by many scientists, and Martson and his coresearchers are pioneer of them [54, 55, 56].
Most of the etherified derivatives of cellulose are water soluble, and during the reaction, the hydroxyl groups are most vulnerable sites where alkyl units can be attached. Many attempts have been made by scientists to synthesize hydrogels from cellulose derivatives like methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), and sodium carboxymethylcellulose (NaCMC), and many articles are also published, and finally they are used in food, pharmaceutical, and cosmetic industries, due to their high solubility, biodegradability, and biocompatibility, non-toxicity, and competitive low cost than the synthetic one.
In addition sodium carboxymethylcellulose (NaCMC) is polyelectrolyte which can show smart behavior (responsive to pH and ionic concentration); these effects impose positive effects to the hydrogels containing CMC during swelling and show suitability for smart application .
Modern age is the age of science, and the society is accompanied with many conscious, sincere people and well-wisher to mankind. Though concept of hygiene product is primitive, but research has brought newness of the items. The authors tried to give introduction of the products to general readers as well as some clues where scientists have opportunity to keep contribution. Actually science is always giving restless effort to make the world better for living. If personal hygiene product becomes flawless, the newborn, women, and elders will lead a sound life. In this respect, cellulose is the eligible candidate to solve the problem.
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