Merging Footwear Design and Functionality

2.1 Materials

Usable quality of leather depends on the type of raw skin or hide used, its strength, and degree of finishing as well as on the quality of the technological process used. Finished leather quality is expressed as a sum of all of its properties that ensure hide applicability in manufacture, stability in storage, and definite life of the materials under the conditions of use. Footwear manufacturers often check the quality of their eco-products according to the given criteria of eco-labs, even when they do not use the labels officially. This practice develops and raises quality standards and safety of their product brands.

The most often used raw materials in the manufacture of footwear nappa for uppers are calf, kip, or cow hide, as their processing offers full, soft, elastic, and pliable leathers of full and corrected grain for various types of intermediate and finishing treatments. Chrome and combined tannages have recently been often substituted by vegetable one, particularly for the footwear declared as chrome free. The type of tannage significantly impacts the properties of finished leather as well. Collagen fibers tanned with vegetable tannage are thicker and fuller, and the leather structure is denser and more closed than when tanned with mineral (chrome tannage). It is the result of tanning matter structure and the bonds they create between the collagen fibers and within them, as seen by the contents of tanning matter bonded. Fibrous composition is characteristic for all finished leathers, and their physical–mechanical properties depend upon this composition. When leather is exposed to tensile loads, fibers are often separated, due to elongation mostly, while leather generally offers significant resistance to elongation.

Key characteristics of leather microstructure are as follows: regularity of fiber interlacing, the angle of interlacing, interlacing density, fiber bending, separation degree, and fiber thickness. In addition to leather microstructure, monitoring some physical and chemical properties that could impact the mechanical properties of leather, such as humidity, fat, and leachable matter, is of key importance when evaluating the mechanical properties of leather. Sample should be mandatorily conditioned prior to mechanical testing, as the amount of moisture in leather is closely related to its end-use properties.

2.2 Methods

Mechanical properties of finished leather are also important in defining its end-use properties. Breaking strength and elongation are important mechanical properties that determine the resistance of leather to tensile loads. They are measured according to the technical standard HRN EN SO 336:2012. This standard specifies the method of determining tension strength, elongation at specified load, and the rate of stretching, as well as the elongation of leather at break, and can be used for all types of leather [4].

Breaking force was measured on samples of footwear nappa, tanned in different ways and with different types of leather grain finish, for the purpose of monitoring material quality and giving recommendations as to which type of tannage to be used. The measurements were performed on a Tensolab 3000 dynamometer, produced by Mesdan, with the stretching rate of 100 ± 20 mm/min and with the range of forces in accordance with the testing sample.

3.1 Mechanical properties

Breaking force and breaking elongation, together with leather thickness, necessary to calculate breaking strength, are summarized in Table 1. Testing samples included vegetable-tanned footwear nappa with semianiline retanning and chrome-tanned footwear nappa with aniline retannage. Each test included measurements of six testing samples, cut in the direction of the central axis and in the direction perpendicular to the central axis. Working conditions for testing were as follows: temperature of 23°C and relative air humidity of 50%.

The results obtained indicate that both vegetable- and chrome-tanned footwear leathers were of high end-use quality. The average values of the breaking force F and breaking strength Tn for chrome-tanned leather were, as expected, higher than the same values for vegetable-tanned samples.

Leather quality is additionally influenced by microstructure elements, differing according to the topological position on the skin, which results in variations of mechanical properties. Crupon (C) is a central part of the hide, situated at both sides of cow's spinal column. It is characterized by uniform thickness and the densest interlacing of almost vertically positioned collagen fibers, which makes it the part of the hide with the highest quality of microstructure. Bellies (side parts of the hide) are generally of nonuniform thickness, while collagen fibers are attenuated and positioned almost horizontally in space, which results in inferior mechanical properties.

The resistance of footwear nappa on tensile loads, in accordance with the standard method, was done on the samples of cow footwear nappa with combined tannage (chrome and vegetable) finished with polyurethane (PU) finish on the grain. Testing samples were taken from the belly and back parts of the hide, so as to establish the impact of the sampling area onto the mechanical properties of leather. Table 2 summarizes the average values obtained in testing six samples, for breaking force and breaking elongation, as well as for leather depth necessary to calculate breaking strength. Testing samples were cut in the direction of central axis (↑) and in the direction perpendicular to it (←). Working conditions for testing were as follows: temperature of 200°C and relative air humidity of 65%.

The average values of elongation percentage in the direction of testing sample man axis for the crupon part are lower than for the belly part, which is explained by the hide microstructure, or spatial position and density of the collagen fiber interlacing. As opposed to elongation, breaking force and breaking strength of the crupon part in the direction of the man axis are higher than those of the belly part. The results obtained confirm the dependence of mechanical properties of leather resistance to breaking force, stretching and breaking strength on the type of tannage used, and microstructure characteristics of the crupon and belly part of the hide. Naturally, the impact of the type of raw hide, its quality, and the selection of the processing technology should not be neglected as well. Footwear sector has been trying to improve at the moment all the factors that impact comfort and durability of the products, which requires using newly developed materials of new functionalities.

There is a prevailing public opinion that the usage of artificial materials in footwear manufacture contributes to the feeling of discomfort, as these materials are not able to sustain optimal microclimate within the footwear. Therefore, footwear designers and technologists have to find structural and technological solutions to eliminate the negative impact of the materials on the microclimatic conditions within the footwear, meaning also ensuring healthy conditions for the feet.

Thermophysiological properties of both the footwear manufactured and the materials used for the footwear are important functional properties of the footwear. However, the present state of the art in this area of research leaves enough space and opportunity for further investigation. One of the recent investigations revealed that even though particular footwear upper was box calf, the shoe lining and insole were of synthetic textile fabric and synthetic polymer, which resulted in shoe discomfort and initiated further investigation to solve the problem [5]. It was also established that poor water vapor permeability and poor water absorption of the synthetic soles used in the manufacture of protective footwear directly impacted the quality of the products.

To improve the thermal comfort of the footwear, nonwoven textile materials, foams, and leather that are most used for footwear have been treated with microcapsules and nano-finishes, so as to be able to keep the feet temperature constant for a longer period of time. The investigations related to the mechanism of structural and dimensional changes of leather surface under the impact of various weather conditions are also important. They can offer a considerable contribution to the development of new materials for the treatment of footwear leather [6, 7].

Footwear, as opposed to clothing, is generally not washed, meaning that its care and maintenance are minimal. However, there are some top-quality products that ask for the leather or footwear to be washable. New generation of materials has been manufactured for finished leather with pronounced washability, the most important among them being acrylic polymer-based materials. New generation of soft polymeric agents is based on hydrophobic and hydrophilic elements. Hydrophobic ones offer softness, while hydrophilic ones initiate emulsification in water and efficiently bond the agent to the surface of the leather. Properly designed technological procedure results in a leather that is light, soft, of full handle, resistant to washing, dry-cleaning, intense light, heat, humidity, of good dyeing properties throughout the cross-section, with intensive hue, offering at the same time good resistance to stretching, tearing, and increased waterproofness effect. Leather washability is tested following the standards EN SO 1502:2004 – resistance of coloration to machine washing and EN SO 1503:2004 – resistance of coloration to mild washing [8, 9]. Washing agent is also an important factor in the process of leather washing. Various recipes of environmentally friendly liquid washing agents in low and high concentration, applied to washing nubuck and nappa washable leathers, yielded a range of different results. The best results for both leather samples were those obtained with a recipe of anionic and nonionic surfactants, improved with a lanoline component. The impact of washing agents with various contents of surfactants and additives was also tested when drying in the open and in a drying machine, comparing spectral values of washed and unwashed samples. Footwear are in general subjected to extreme end-use conditions, while closed space inside them and the lack of ventilation create a good basis for the growth of microorganisms. These microorganisms are in principle responsible for unpleasant smell and can also be a health hazard, especially when chronic diseases are present, such as diabetes [10]. All of these clearly indicate a need for antimicrobial finish of footwear.

3.2 Antimicrobial functionalization

The purpose of antimicrobial treatment is to improve material protective and sanitary properties, protecting the wearer from detrimental impact of the environment, actions of microorganisms included. There are three categories of microorganisms: bacteria, fungi, and algae. Cellulosic fibers are more sensitive to the activity of fungi than protein ones, which are, on their part, more susceptible to the impact of bacteria. Microorganisms play an important part in treatment and processing textile materials and leather, since they, through their life activities, impact favorable or detrimentally onto the production process and the properties of finished materials alike. Natural materials are sensitive to the activities of microorganisms, especially under the conditions of adequate (higher) humidity and heat. As opposed to natural fibers, synthetic ones are, due to their high hydrophobicity, more resistant to the impact of microorganisms [11]. Bacteria are the most important microorganisms in leather making. Bacteria are common on skin and leather alike; they can develop bad odors, which is especially the common case in the area of the feet.

Microorganisms on natural leather can cause rot and putrefaction, fermentation, or generate pigments, glitter, and various stains. Higher concentration of microorganisms on textiles look like dirt causes bad odor, discoloration, reduced strength, all of which, generally and in higher or lower extent reduces product use value. The other aspect of possible impact of microorganisms includes causing irritations, allergies, and contamination of the wearer or the people who manipulate contaminated textiles or leather [12]. This is why materials are treated antimicrobially, with the aim to control and prevent the development and reproduction of microorganisms. In this way, contagious diseases are prevented, and the incidence of allergic reactions and respiratory problems is reduced. At the same time, material degradation in the form of discoloration, dirty smears, and unpleasant smells is also prevented [13].

3.3 Merging the requirements for footwear design, functionalization, and environmental protection

In designing haute couture footwear, the accent is usually on new and different shapes of footwear, new shapes of soles and heels, new cuts, material combinations and variations, trimmings, and decorations (Figure 1), while functional treatments of the material involved are in most cases taken for granted, as high fashion prefers the usage of high-quality materials that follow most recent technological developments. Such models are made in small and limited quantities, involve numerous hand operations, which in effect result in extremely high price of the product.

Figure 1

Examples of haute couture footwear: a) Holmes of Norwich 1958, b) John Fluevog 1992, c) Fnsk 2009, and d) Nicholas Kirkwood 2010 [24]

Footwear design in higher- and middle-price class follows the trend and tendencies of haute couture. However, most of the above segments are more moderate and often simpler, due to industrial way of producing them. These products often include top design and high-quality footwear, made of natural leather exclusively, certified materials, and high-quality manufacture as well as numerous special treatments, finishes, and innovations in industrial footwear production. Sometimes, some of these functional treatments are recognizable in the market and constitute even a logo or trade mark of the brand. This group includes many of more or less well-known footwear manufacturers and renowned global brands.

The success of the Italian footwear producer Geox on global market is based on excellent product design and a number of functional innovations, the most revolutionary and well known among them being the patent-protected system for ventilation with waterproof rubber sole (Geoxbreathes^®), often advertised as “breathing shoes” (Figure 2). The long-term development concept and systematic investigations of innovative ideas and notions, many of which concern new materials, production processes, equipment, and machines, have resulted in the protected Geox technology, with over 60 different patents register in Italy and globally [20, 25].

Figure 2

a) Illustration of Geoxbreathes^® efekta; b) and c) advertising samples of Geox footwear with their functionalities specially presented [26]

Vapor-permeable footwear with waterproof (Figure 3c) is the new generation of innovative and patent-protected waterproof technology, protecting from rain, mud, and snow and simultaneously increasing permeability by natural thermoregulation. The notion of vapor-permeable footwear with waterproof and vapor-permeable sole includes the bonded assembly of uppers and sole, containing a segment made of waterproof material and a layer permeable to water vapor, or waterproof and vapor-permeable membranes, which cover the area toward the inner part of the footwear. The footwear contains at least one waterproof and vapor-permeable protective barrier, situated at the inner part of the shoe on the membrane, covering it at least partially. Protective barrier and the membrane are sealed in a watertight manner onto the body of the sole at least at one sealing zone [27]. The technology has been applied to a wide range of products and covers a broad spectrum of styles and solutions for formal, casual, and active purposes.

Figure 3

a, b) Shamees Aden Amoeba prototype of tennis shoes [30]

Meeting increasingly stringent requirements of environmental protection in leather-making industry includes advocating products made of leather with lower environmental impact. It is thus necessary to monitor the impact of products and manufacturing on the depletion of natural resources as well as on the emissions into water, air, and ground. For example, the manufacture of washable leather, despite chrome tanning applied, has been made more environmentally acceptable by considerable reduction in the amount of chrome in finished leather, waste water, and residual waste. This can be done by measures taken within the technological process and using technologies that introduce new and environment-friendly materials, together with recycling tanning solutions. Maximum level of bath exhaustion in retanning, dyeing, and fatliquoring also contributes to lower pollution by interventions within the technological process.

Numerous footwear manufacturers follow contemporary development trends and meet requirements in all the segments of footwear industry, which makes them competitive on the global and European markets. The footwear is made of certified materials only, produced in the environment-friendly manner, with no harmful matter or compounds, while the manufacturing processes use only water-based glues. Such companies also possess the certificate ISO 14001, for the system of managing environmental protection.

There is a trend among footwear manufacturers to address eco-labels on their products directly to end users, who is then in the position to actively participate in environmental protection and advocating sustainable manufacture and consumption by selecting such a product. Eco-labels are voluntary instruments of protecting the environment showing a contribution of a product to one or more aspects of environmental protection. Branding eco-designed footwear is increasingly the present manner of improving competitiveness, as it offers a broader contribution to the society in general by introducing the concept of sustainable development into the traditional sector of textiles/leather/footwear, and its popularization in the market and the development of innovative solutions.

According to the EC decision 2016/1349 on establishing environmental measures for granting the EU environmental label for footwear, products that contribute to the environmental dimension of sustainable development throughout their whole life cycle are promoted, products that are durable and in which hazardous materials are present in very limited amounts. Footwear durability is an important aspect in reducing waste from the industry, while the Measure-7 Parameters that increase the durability in the abovementioned decision, is an important measure to establish whether a product is environmentally friendly or not [28].

3.4 Novelties in the field of functional eco-materials and technologies

Scientists and multidisciplinary designers have created a conceptual prototype Amoeba tennis shoes (Figure 3) in the laboratory of Shamees Aden in London. The aim is to investigate the future of new materials obtained from protocells. Protocells are chemical cocktails mixed from basic inanimate molecules under laboratory conditions and combined to create matter that exhibits some of the characteristics of living cells, e.g., the ability to metabolize food, to move, and to propagate. These investigations have initiated a series of discussions on social and ethical implications of these new technologies for the future of design [29].

A new method of designing leather products employs basic postulates of the infrared design theory. The design of leather face/grain for the manufacture of shoes, purses, and belts is approached with predetermined double design. One design is visible through the eyes of the viewer in the daylight, and the other in near-infrared spectrum. The application of this duality on leather involves familiarity and control of specific properties of various types of leather materials. In this way, leather products can be branded, designed, individualized, and protected in a completely new way [31, 31].

Apart from defining complete individuality, or uniqueness, of leather products, the duality method developed raises the level of brand protection for these products to new heights. The combination of para-visible and invisible offers completely new marketing possibilities for each leather product [32].

The application of the research described and IRD technology on leather and leather products is shown in Figure 4.

Figure 4

Application of infrared design method on footwear design: a) low boots made of goat nappa leather designed by a mosaic, visible by the viewer at daylight; b) the same boots in near-infrared spectrum [32]

Additive technologies, with the three-dimensional (3D) printing included, have become increasingly popular in various branches not only of industry, but also in fine arts, culture in general and elsewhere. There is almost no area of everyday life where additive technologies are not present. Positive examples of 3D printing and design of garment and footwear parts include the manufacture of wearable fashion prototypes using low-budget 3D printers [33], production of anatomic inserts for footwear which involve functionalization of anatomic materials, and building-in inner structure into the base of the insert, which all influences absorption properties of the footwear in case of an impact [34], manufacture of footwear using 3D CAD models and 3D printing [35], or extravagant examples of footwear, hybrid fusion of traditional footwear, and 3D print technology [36].

Industrial designers at the Massachusetts Institute of Technology, Christophe Guberan, and Carlo Clopath, developed by the end of 2015, in collaboration with computer scientist Skylar Tibbits, also at the MIT, a project of 3D printed recreational footwear, which changed the shape reacting to the impulses from the wearer (Figure 5) [36]. After printing a precise two-dimensional (2D) sample from the material of various layer thicknesses and properties onto a stretched piece of textile, the shape of the footwear can be independently reshaped into preprogrammed forms. This program is exclusively innovative and scientifically valid also by the fact that no robotics or sensors are used in it, which proves that the so-called advanced technologies are not necessary components for interactive footwear.

Figure 5

MIT designers: 3D print wearer-reactive footwear [36] 3D, three-dimensional