Brief context of system integration of natural materials

The creation of new materials and products using natural resources has been occurring since ancient times. ^{Reference Callister and Rethwisch1} After decades of using synthetic materials for many industrial applications ( Figure 1 ), ^{Reference Möhring, Brecher, Abele, Fleischer and Bleicher2} in the late 1980s, interest in turning to natural materials started flourishing again, owing to the thrust toward sustainable design with the release of the Brundtland Commission Report. ^{Reference Durham3,4} Previous issues of MRS Bulletin have tangentially touched on the use of functional natural materials, including in the construction industry, ^{Reference Bonfield5,Reference Kurtis6} health applications, ^{Reference Kossovsky and Millett7–Reference Truss9} energy production and supply, ^{Reference Hurd, Kelley, Eggert and Lee10,Reference Ku and Shapiro11} chemical products, ^{Reference Glasser12,Reference Halley and Dorgan13} fibers, ^{Reference Vollrath, Porter and Holland14} and sustainable development in general. ^{Reference Green, Espinal, Traversa and Amis15,Reference Halada and Yamamoto16} However, there has not been a complete issue focused on the development of functional materials from natural resources with a systems integration approach. In this issue, materials such as hydrogels, ^{Reference Akintewe, DuPont, Elineni, Cross, Toomey and Gallant17,Reference Toomey, Vidyasagar, Ortiz, Knoll and Advincula18} cellulose, ^{Reference Hubbe, Rojas, Fingas and Gupta19–Reference Jocher, Gattermayer, Kleebe, Kleemann and Biesalski23} paper, ^{Reference Hubbe, Venditti and Rojas24–Reference Ruettiger, Mehlhase, Vowinkel, Cherkashinin, Liu, Dietz, Stark, Biesalski and Gallei29} cells, ^{Reference Zhernenkov, Ashkar, Feng, Akintewe, Gallant, Toomey, Ankner and Pynn30–Reference Falahat, Wiranowska, Toomey and Alcantar34} plants, ^{Reference Alvarez, Rojas, Rojano and Ganan35–Reference Bandyopadhyay, Peralta-Videa and Gardea-Torresdey40} nanocomposites, ^{Reference Hubbe, Rojas, Lucia and Sain20,Reference Habibi, Lucia and Rojas21,Reference Peralta-Videa, Zhao, Lopez-Moreno, de la Rosa, Hong and Gardea-Torresdey25,Reference Geissler, Loyal, Biesalski and Zhang41–Reference Auad, Mosiewicki, Richardson, Aranguren and Marcovich43} and biomass ^{Reference Trujillo-Reyes, Peralta-Videa and Gardea-Torresdey44–Reference Celikbag, Robinson, Via, Adhikari and Auad46} are featured as the core thematic thread that links how a systems integration approach is the essence of materials design.

Figure 1.

Timeline for the usage of materials from 10,000 Before the Common Era (BCE) to 2020 Common Era (CE). The green, blue, and red curves separate the four main types of materials. (Top to bottom): metals, polymers, composites, and ceramics. Adapted with permission from Reference Reference Möhring, Brecher, Abele, Fleischer and Bleicher2. © 2015 Elsevier. Note: PE, polyethylene; PC, polycarbonate; PMMA, poly(methyl methacrylate); PS, polystyrene; PP, polypropylene; GFRP, glass fiber-reinforced plastic; CFRP, carbon fiber-reinforced plastic; KFRP, knitted fabric-reinforced polymer.

The significance of working with natural materials

Natural materials have emerged as materials of choice for a wide range of applications, ranging from biomedical to energy to environmental. The many attributes of natural materials, such as intrinsic biocompatibility and surface active properties, make them prime sources for state-of-the-art applications. Regarding the use of natural products for medicinal applications, there are many examples such as the integration of aloe vera, genistein from soybean, green tea leaves, carrot root, mango pulp, and more recently, Carvalho’s cactus mucilage—the viscous liquid inside cactus pads—in wound healing. ^{Reference Carvalho, Soares, Blau, Menegon and Joaquim47,Reference Tabassum and Hamdani48} Natural plant-based products are classified as phytochemicals and have been found to exhibit anti-inflammatory, antioxidant, antimicrobial, regenerative, and biocompatible properties, and are viewed as safer and more affordable than conventional/standard therapies. ^{Reference Tabassum and Hamdani48–Reference Sivamani, Ma, Wehrli and Maverakis52}

Recent investigations continue to diversify the benefits of using extracts from plants such as cactus mucilage in the functional form of a filtration membrane. ^{Reference Thomas, Devisetty, Katakam, Perez, Guo, Stebbins, Alcantar, Muppaneni, Abelson, Granqvist and Traversa38,Reference Thomas, Pais and Alcantar53,Reference Thomas, Alcantar, Pais, Bermudez, Majewski, Alcantar and Hurd54} Opuntia ficus-indica cactus mucilage extracts have been functionalized in the form of a nanofiber membrane ( Figure 2 ) for integration into rural or urban point-of-use filtration systems. ^{Reference Thomas, Devisetty, Katakam, Perez, Guo, Stebbins, Alcantar, Muppaneni, Abelson, Granqvist and Traversa38,Reference Young, Anzalone, Pichler, Picquart, Alcantar, Shannon, Ginley and Weiss39,Reference Thomas, Pais and Alcantar53–Reference Vecino, Devesa-Rey, de Lima Stebbins, Moldes, Cruz and Alcantar57} Mucilage is a natural, nontoxic, biocompatible, biodegradable, inexpensive, and abundant material and is composed of proteins, monosaccharides, and polysaccharides ( Figure 3 ). Two fractions of the mucilage can be extracted. The solids portion after maceration leads to the gelling extract (GE), which is a pectin-rich polysaccharide; while the nongelling extract (NE), which is considered a galactomannan polysaccharide, can be obtained from the liquid supernatant. ^{Reference Young, Anzalone, Pichler, Picquart, Alcantar, Shannon, Ginley and Weiss39,Reference Buttice, Stroot, Lim, Stroot and Alcantar56,Reference Alcantar, Joseph and Young58–Reference Fox, Stebbins and Alcantar61}

Figure 2.

Example of the use of natural materials for the system integration in a point-of-use (POU) filtration system. The cactus mucilage is extracted from the (a) Opuntia ficus-indica plant. (b) It is then dried and ground into a fine powder. (c) Atomic force microscope (AFM) scans of the nanostructure for the nongelling (NE) cactus mucilage extract show a fishing net-like topography. The AFM scans reveal the nanostructure of the NE extract at (right) high (500 nm) and (left) low magnification (2 µm). The color bar scale on the left-hand side shows the color scheme depending on the depth of the scanned surface. Next, cactus mucilage is processed into nanofibers by electrospinning. (d) The subsequent AFM images show greater details of the nanofiber structure; respective scales noted. All AFM scans were taken in height mode. (e) This image shows a typical electrospun membrane. The typical area of the membranes ranged between 2.5 and 3.0 cm². (f) The membranes are then assembled into water purification filters. ^{Reference Thomas, Devisetty, Katakam, Perez, Guo, Stebbins, Alcantar, Muppaneni, Abelson, Granqvist and Traversa38,Reference Young, Anzalone, Pichler, Picquart, Alcantar, Shannon, Ginley and Weiss39,Reference Thomas, Pais and Alcantar53–Reference Vecino, Devesa-Rey, de Lima Stebbins, Moldes, Cruz and Alcantar57}

Figure 3.

Proteins, monosaccharides, and polysaccharides of mucilage gelling extract (GE) and nongelling extract (NE) are composed of the following main sugars: (a) arabinose, (b) galactose, (c) xylose, (d) glucose, and (e) uronic acids (e.g., hexoses and pentoses sugars). ^{Reference Young, Anzalone, Pichler, Picquart, Alcantar, Shannon, Ginley and Weiss39,Reference Buttice, Stroot, Lim, Stroot and Alcantar56,Reference Alcantar, Joseph and Young58–Reference Fox, Stebbins and Alcantar61} GE is a pectin-rich polysaccharide; hence, it contains more uronic acid-type sugars than NE. Conversely, the NE has a higher arabinose and galactose content than GE.

Mucilage as an organic material is capable of interacting with metals, cations, and biological substances promoting flocculation—clumping pollutants into flocs—for removing arsenic, bacteria such as Escherichia coli and Bacillus cereus, and other particulates from drinking water ( Figure 4 ). ^{Reference Buttice, Stroot, Lim, Stroot and Alcantar56,Reference Buttice, Alcantar and Ahuja59,Reference Fox, Stebbins and Alcantar61,Reference Alcantar, Fox, Thomas and Toomey65} This natural material has the potential to be used as a sustainable method for water filtration and contaminant sensing. A mucilage nanofiber membrane has been integrated into a filtration system to treat a 50-parts-per-billion (ppb) arsenic solution. Results demonstrate the natural functionality of the mucilage to absorb arsenic (As) atoms in the filtration system, and this is measured from filtration data in terms of the percentage of arsenic removed. ^{Reference Thomas, Devisetty, Katakam, Perez, Guo, Stebbins, Alcantar, Muppaneni, Abelson, Granqvist and Traversa38} For instance, when mucilage was added in combination with iron, >90% removal of As was attained in just 10 min. ^{Reference Fox, Stebbins and Alcantar61} When mucilage was added as a pure powder or in membrane form, up to 50% As or 18–20% As was removed, respectively. ^{Reference Eppili55,Reference Vecino, Devesa-Rey, de Lima Stebbins, Moldes, Cruz and Alcantar57} Sibaja et al., ^{Reference Sibaja, Culbertson, Marshall, Boy, Broughton, Solano, Esquivel, Parker, De la Fuente and Auad62} Vollrath et al., ^{Reference Vollrath, Porter and Holland14} and Truss ^{Reference Truss9} have also used natural fibers in functional applications such as textiles and tissue scaffolds. These studies elucidate that mucilage nanofiber membranes, silks, and biocomposite fibers have the potential to serve as the basis for the next generation of economically sustainable filtration devices that make use of a natural nontoxic material for sustainable systems integration designs.

Figure 4.

Functional applications of cactus mucilage. In all cases, cactus mucilage has been added to separate/flocculate contaminants at minute concentrations (i.e., between 2 and 100 ppm). ^{Reference Buttice, Stroot, Lim, Stroot and Alcantar56,Reference Buttice, Alcantar and Ahuja59,Reference Fox, Stebbins and Alcantar61,Reference Alcantar, Fox, Thomas and Toomey65} (a) Nongelling extract (NE) can effectively flocculate bacteria from solution. Gelling extract (GE), can effectively remove (b) sediments and (c) heavy metals when added to water. The inset in (b) is a lower magnification image. (d) Effectiveness of cactus mucilage as a dispersant. The example shows crude oil droplets for 3% volume/volume oil/water dispersions (3% v/v). The dispersant/oil ratio (DOR) is 1:20.

Nanocomposites, polymer hybrids, hydrogels, and cactus mucilage have also been demonstrated as natural dispersants; ^{Reference Green, Espinal, Traversa and Amis15,Reference Hubbe, Rojas, Fingas and Gupta19,Reference Dubois, Herzog, Ruttiger, Geissler, Grange, Kunz, Kleebe, Biesalski, Meckel, Gutmann, Gallei and Andrieu-Brunsen26,Reference Trujillo-Reyes, Peralta-Videa and Gardea-Torresdey44,Reference Jabbari, Veleta, Zarei-Chaleshtori, Gardea-Torresdey and Villagran63–Reference Alcantar, Fox, Thomas and Toomey65} hence, they have the potential of being used for oil spill cleanup operations. ^{Reference Alcantar, Fox, Thomas and Toomey65} It is well known that extensive damage to marine and wildlife habitats can occur due to oil spills. Chemical dispersants can potentially bioconcentrate (when dispersant concentration exceeds that in water), causing damage to the marine life and environment. On the other hand, the use of surface active natural chemicals leads to nontoxic dispersants. John et al. ^{Reference Jadhav, Vemula, Kumar, Raghavan and John66,Reference John, Shankar, Jadhav and Vemula67} have developed bio-based dispersants that can gelate crude oil and enhance its recovery. ^{Reference Jadhav, Vemula, Kumar, Raghavan and John66,Reference Divya, Miroshnikov, Dutta, Vemula, Ajayan and John68} Similarly, NE and GE extracts from the cactus plant can be made less toxic by formulating oil-in-water (O/W) emulsions (Figure 3). Mucilage disperses oil in O/W emulsions by lowering the surface and interfacial tension. Higher emulsion stabilities were shown to have smaller droplet size in the systems with cactus mucilage. ^{Reference Fox, Stebbins and Alcantar61,Reference Alcantar, Fox, Thomas and Toomey65}

Toxicity is also an important issue to consider when working with natural materials. According to the US Environmental Protection Agency (US EPA) toxicity categories are used to classify chemicals based on their acute toxicity (US EPA, 2010b). ^{Reference Denton, Miller and Stuber69} One of the most common parameters to measure toxicity is the LC₅₀, which stands for the lethal concentration of a substance that causes 50% of a population of selected sensitive organisms to perish after 24 h of exposure. If the LC₅₀ is less than 100 mg/L, by convention, the chemical in question can be considered toxic. The lesser the value of LC₅₀, the more toxic a chemical. In our experience, mucilage from cactus, such as the NE and GE extracts, would be classified as practically nontoxic. When aquatic life is exposed to the mucilage, specifically Daphnia magna, the survival rate is almost 100% for concentrations equal to or lower than 100 mg/L. For concentrations ranging from 200–2000 mg/L of GE and NE, the survival rate of Daphnia was higher than 70%. In general, nonexistent toxicity is a beneficial property of working with natural materials. The contribution by Medina-Velo et al. shows in detail how toxicity could be a significant problem in plant biology when plants are exposed to different kinds of materials.

The previous examples mentioned that cactus mucilage properties demonstrate the ability of a natural material to be immersed in fresh and aquatic water systems, and how such systems can be integrated into water-treatment technologies. There are several other natural materials that offer critical benefits to systems integration of functional natural materials, and these will be discussed in the different contributions in this issue. Figure 5 shows a schematic representation of the contributions for this issue in regards to the integration of natural materials into functionalized sustainable systems. For instance, it illustrates how natural materials such as cellulose, lignin, cactus plants, biomass, and cells can be processed to produce paper for light, nonpower sensor applications, lignin nano- and macroparticulates, membrane filtration systems for water purification, bio-oils for energy and high-strength polymers, and functionalized tissue substrates. It is also worth mentioning that the analysis of how natural materials react to potential contaminants produced during nanomanufacturing will be discussed in this issue.

Figure 5.

Schematic representation of the contributions for this issue related to the theme, “System integration of functionalized natural materials.” From top right, counterclockwise: Baksh et al. illustrate the preparation of functionalized tissue substrates. Böhm et al. demonstrate the transformation of cellulose fiber into sensors made of functionalized paper. Sibaja Hernández et al. discuss the integration of the production of bio-oils for functionalized polymeric systems in the realm of biomass. Medina-Velo et al. depict plant uptake and transport mechanisms of different metal contaminants that affect plant growth and integrity. Ago et al. show the preparation of functionalized cellulosic fibers with lignin nanoparticles for antibacterial, antioxidant, and colloid active substrates.

In this issue

System integration in sustainable design

Federal agencies started recognizing the importance of supporting projects that touched on the integration of natural materials in sustainable design. In the early 1990s, the EPA and the National Science Foundation (NSF) started supporting projects that included sustainable materials and processes. The NSF implemented “Biocomplexity in the Environment Programs” in the early 2000s. Consequently, the “Materials Use: Science, Engineering and Society” Program was promoted in 2003 with approximately USD$65 million investment. Subsequently, the creation of various institutes for sustainability along with different undergraduate and graduate curricula that relate to this issue increased exponentially. The current program that supports research and development across disciplines in this area is the “Sustainable Chemistry, Engineering, and Materials Program” at NSF. The great majority of technologies, new materials, and novel designs that resulted from such initiatives have shown that system integration of natural materials is key to enhancing our understanding of how to best learn from nature.

Summary

The successful application of natural materials in the construction of membranes, filtration devices, sensors, organic electronics, flexible smart materials, support structures, hydrated systems, chemical synthesis, and hydrophobic surfaces are displayed in the articles in this issue of MRS Bulletin. The contributions present various types of functionalized natural materials/composites engineered for high chemical and mechanical performance, which have a significant role in manufacturing and materials flow analysis. The benefits of fundamental science related to natural systems include prime relationships to sustainability such as life-cycle assessments, and life-cycle costs in health and safety that will continue to pave pathways to transform materials science and create better materials systems.