On the Possible Use of Textile Fabrics for Vertical Farming

Vertical farming is one of several ideas that are being developed further by diverse research groups, companies and private citizens. Due to the growing problems of urbanisation and a growing population, vertical farming has presented itself as one possibility to feed people, particularly in large and densely crowded cities, in an effi cient and eco-friendly way. Interestingly, while agrotextiles are often used in agriculture and textile fabrics can be bought, for example, as frames for small vertical farming solutions for private balconies, only a few researchers have studied the possibilities of using textile fabrics as substrates for vertical faming to date. This study provides an overview of possible future applications of textile fabrics in vertical


Introduction
According to the latest World Population Prospects, the world's population will grow continuously over the next decades [1].Fighting hunger and malnutrition will become even more diffi cult than it is today, particularly in the poorest countries where population growth is expected to be highest [2].Even in wealthier countries, however, total agricultural land area is frequently insuffi cient to cover the demands of the population for food.For example, a recent statistical report indicates a demand of 18.3 • 10 6 ha for food production in Germany for 2016, while only 14.0 • 10 6 ha of the total available agricultural land area of 16.7 • 10 6 ha were used for food production.Th e missing food, corresponding to the diff erence of 4.3 • 10 6 ha, was bought abroad [3].Th is is one of the reasons why vertical farming is of major interest in research and development.Th e main idea of vertical farming is to grow vegetables Izvleček Vertikalno kmetovanje spada med ideje, ki jih razvijajo različne raziskovalne skupine, podjetja in zasebniki.Zaradi naraščajočih problemov urbanizacije in rasti svetovne populacije je vertikalno kmetovanje lahko ena od možnosti učinkovitega in okolju prijaznega načina zagotavljanja hrane, zlasti v velikih in gosto naseljenih mestih.Zanimivo pa je, da se kljub temu, da se agrotekstilije pogosto uporabljajo v kmetijstvu in da so na trgu na voljo tekstilni materiali za okvirne rešitve vertikalnega kmetovanja za zasebne balkone ipd., le manjše število raziskovalcev ukvarja z možnostjo uporabe tekstilij kot substrata za vertikalno kmetovanje.Članek predstavlja pregled možnosti za uporabo tekstilnih materialov za vertikalno kmetovanje v prihodnosti.Ključne besede: vertikalno kmetovanje, vertikalno vrtnarjenje, tekstilne tkanine, agrotekstilije, rastline, alge, hidroponika, aeroponika, akvaponika or other plants vertically, typically inside of high buildings.One possibility is to have several fl oors (or thinking on a smaller scale, several levels in a large shelf-like structure) that are used for farming.Water is oft en transported from the highest levels down to the next levels until it reaches the lowest level and is pumped back to the highest level, ideally achieving a closed water recycling system.On the largest scale, entire skyscrapers can be used for vertical farming, possibly including a restaurant or a supermarket where vegetables are sold.On a smaller scale, vegetables are harvested from the paths between the shelfs on which they grow.In this way, as Pinstrup-Andersen recently pointed out, indoor vertical farming may support improved nutrition, reduce water consumption and decrease the risks associated with outdoor farming due to increasing climate change and extreme weather conditions [4].Particularly in highly urbanised areas, vertical farming may off er a certain amount of independence [5].Interestingly, vertical farming can even compete economically with fresh food grown in greenhouses, despite the high cost of artifi cial lighting [6,7].From a technical point of view, relatively new technologies such as hydroponics, aeroponics and aquaponics facilitate effi cient farming in the city [8−10].Hydroponics is a hydroculture technology used to grow plants without soil, i.e. in water with additional nutrients, possibly using perlite or a similar structure, to support the roots of plants.Aeroponics goes one step further and grows plants in humid air or mist, but without inserting the roots in water.Finally, aquaponics uses another approach by creating a symbiosis between plants and aquatic animals, where water and animals' by-products are used as a source of nutrients for plants.It should be noted that the location and design of these technologies must be tailored carefully to achieve a sustainable food supply [11].Another technology based on the same idea of growing plants vertically to save space is demonstrated by the BIQ Algenhaus in Hamburg, Germany.Th e façade of this building is used as a bioreactor in which microalgae are grown.During growth, the microalgae absorb the radiation on the façade, converting it partly into biomass, while the residual radiation is used for heating the building.Nutrients are provided by the waste water from the building, again achieving a high recycling rate that is seen in common vertical farming technologies [12].
Despite these advantages and even the possible necessity of vertical farming that could be only be mentioned briefl y here, there is surprisingly little literature available about new technologies specifi cally for vertical farming.Even textile fabrics that are wellknown as agrotextiles used to protect diverse plants from wind, low temperatures or insects cannot be found in literature about vertical farming.An overview will be given here of the possible use of diff erent textile fabrics for vertical farming applications, subdivided into the farming of plants and algae.

Textile fabrics for algae immobilisation and harvesting
Algae are eukaryotes that are, depending on the cited study, regarded as simple plants or as plantlike species [13,14] where they performed their experiment enabled cultivation using a simple technology and resulting in a reliable algae growth rate, and provided the local inhabitants a secure income.An integrated photobioelectrochemical system, combining a microbial fuel cell with algae growth for domestic wastewater treatment, was suggested by Luo et al. [17].In such a combination, electrogenic bacteria produce electricity through oxygen reduction, while at the same time degrading organic compounds.Th e algae produce oxygen by photosynthesis, thus eliminating the need for external aeration while also serving as biomass.Luo et al. studied six diff erent mesh membranes from polyester or nylon with varying pore sizes, ranging from 0.11 mm to 5.31 mm for algal attachment to support algae harvesting, and found polyester slightly superior to nylon, while the smaller pore sizes of 0.11 mm and 0.53 mm resulted in signifi cantly higher biomass productivity than the largest pore size.Nylon meshes were also used by Lee et al. to increase the biomass productivity of diverse microalgae [18].In addition, the harvesting and de-watering of the algal biomass were found to be easier and less expensive, as the meshes with adhered algae could simply be taken out of the culture.Focusing on microalgae, electrospun nanofi ber mats from polyamide and polyacrylonitrile were studied and compared with a non-woven polypropylene microfi ber fabric (Figure 1) [19].Cell adhesion, however, was poor, contrary to a previous study of Chlamydomonas reinhardtii on polysulfon nanofi ber mats [20].
Gross et al. studied the eff ect of materials and structures on algal cell attachment.Working with a non-sterile Chlorella vulgaris culture including other diverse green algae and cyanobacteria, they investigated various metals, polymers and rubber.In addition to smooth surfaces, meshes with diff erent pore sizes were studied.In general, polypropylene and nylon meshes with openings of 0.5-1.25 mm were found to be ideal for initial cell attachment and long-term adherent growth [21].Diff erent technologies were suggested for harvesting microalgae.Amongst them, fi ltration is a technology in which textile meshes or membranes are used.One possibility is the use of so-called microstrainers, rotating fi lters with fi ne meshes (pores measuring between 35 μm and 62 μm), combined with continuous backwash.Wilde et al. suggested a double-stage microstrainer to increase performance and cost-effi ciency [22].Th e exact mesh pore size for such fi ltration methods is determined by the size of the algae.Chlorella sp.
It should be mentioned that the opposite objective has been reported in several publications: textile fabrics can also be prepared to protect buildings from algae (particularly in consortia with bacteria) growing on them or to serve other biocidal functions [29].While the possible uses of nets, membranes and several other textile structures for algae growth and harvesting described above exist, no research can be found in literature regarding woven, warp or weft knitted structures as substrates for algae growth.Nevertheless, algae, particularly microalgae, belong to a species that can be of great interest for vertical farming, e.g. for vertical agriculture in space [30], while the outdoor cultivation of temperature-tolerant algae such as Chlorella sorokiniana in column photobioreactors may also be a research objective aimed at sustainable food production [31].Th is suggests that there is a broad area of research of possible new fi ndings and developments related to the growth of adherent algae on diverse textile fabrics with potential vertical farming applications.

Plants
Agrotextiles are typically used as covers, not as substrates for growing plants.Although they can be degraded by UV irradiation and the eff ects of weather, and then destroyed by mechanical impacts [32], their advantages seem to outweigh these problems.
In an unheated greenhouse during the winter, lettuce was found to grow best if covered by a combination of mulch and an agrotextile [33].Outdoor studies showed that an agrotextile or polyethylene foil resulted in an increased lettuce yield in the early growth phase [34].In another study, a cloth cover resulted in the strongest lettuce plants, while straw mulch resulted in the weakest plants [35].Agrotextiles were found more suitable than black plastic and sawdust for strawberries with a high ascorbic acid content [36].Traditional straw mulch, however, resulted in the best fruit size and yield [37].For an early potato harvest, a covering made from agrotextiles resulted in larger potatoes than a covering made from a perforated plastic fi lm or no covering at all [38].On the other hand, litchi fruit harvesting periods could be delayed successfully by shading the trees with an agronet of 30% or 50% light transmission, thus preventing the ripening of all litchis within a few days [39].
Several research groups are working on diff erent approaches to further develop these simple agrotextiles.Dan et al. developed a new textile composite material that was UV resistant and clearly increased the growth of spinach and two lettuce varieties [40].
A knitted alternative to typical non-woven fabrics was suggested by Scarlat et al., who prepared warp knitted nets from polyester and polyamide, and found good mechanical properties compared with typical agrotextiles [41].
It should be mentioned that agrotextiles such as row covers are used to protect plants, not only from the wind and sun, but also from virus vectors and other undesirable pests.Honeydew melons could be protected from the sweet potato whitefl y and vegetable leaf miner using diff erent row covers with varying success [42].For tomatoes, whitefl ies are also a major pest that sometimes results in complete crop loss.In combination with planting aromatic basil between tomato rows, an agronet cover could reduce whitefl y infestation by more than two thirds [43].Th e same eff ect was found when French bean plants were protected from the silverleaf whitefl y and black bean aphids.In addition, the covered plants developed faster and yielded a higher quality [44].While the latter study showed no clear correlation between the protective eff ect and the impregnation of the net with an alphacypermethrin-based insecticide, another study found that the mortality rate of whitefl ies exposed to alphacypermethrintreated agronets doubled [45].To prevent crops from bacterial contaminations, antibacterial nanosilver coatings were deposited on high-density polyethylene nets and studied with respect to their antibacterial properties on gram-positive and gramnegative bacteria [46].
To summarise these fi ndings, textile fabrics can be used for a broad variety of agricultural applications, in particular protecting plants from excessively cold or warm weather, insects, bacterial contaminations, etc.It should be mentioned, however, that there are also some negative aspects that are usually not reported in literature, but must not be overlooked, such as birds and other animals becoming entangled in nets meant to protect fruit against them, or the exposure of the environment to non-biodegradable polymer parts if agrotextiles are destroyed by heavy wind, etc. Th ese aspects, however, are not relevant for indoor vertical farming, and can be addressed through intelligent textile constructions for outdoor vertical farming ideas.Because the step from covering plants with agrotextiles to letting plants grow on agrotextiles in hydroponic applications is not actually large, it is surprising that no reports regarding other textile applications in agriculture can be found in scientifi c literature.Textile fabrics are commercially available as base materials for outdoor vertical farming systems for individuals, and contain bags in which plant pots can be fi xed.However, the direct combination of textile and plant, i.e. using a fabric as a substrate to which the plant roots adhere, is only scarcely found.Th is is particularly hard to understand, as the shape and chemistry of textile fabrics can be tailored according to the requirements of the roots of each plant (Figure 2).

Conclusion and outlook
Textile fabrics are used for several applications in agriculture and algae farming.Surprisingly, no reports can be found in scientifi c literature about the obvious implications of studying the ability of diverse textile fabrics for their possible use in hydroponics and other vertical farming applications.Due to the increasing need for vertical farming solutions in the coming decades, vertical farming can be developed further based on the well-known advantages of textile fabrics: the wide variety of structures and chemical compositions of fi bres, the possibilities of creating one-, two-and, to a certain extent, even three-dimensional fabrics, and relatively inexpensive and long-established textile production technologies.
In the author's personal opinion, vertical farming is a technologically available method to address the severe problem of supplying suffi cient food for a growing population in cities with increasingly less space, without destroying even more forests and other natural wildlife habitats.Numerous examples of the use of textile fabrics for diverse agricultural applications suggest that recent progress in innovative agrotextiles and algae harvesting nets should be transferred into textile-based solutions for vertical farming.Th is transfer could pave the way to new technologies for ecologically and economically reasonable food production.

Figure 1 :
Figure 1: Growth of Chlamydomonas reinhardtii on a polyamide nanofi ber mat (a) and on a polypropylene non-woven fabric (b).For a detailed description of the experiments, cf.Ref. 19.