Keywords

1 Introduction

In the last three decades, 3D printing, as one of the additive manufacturing processes, has been defined, developed, and improved [1]. Unlike traditional subtractive manufacturing methods, 3D printing works by building pieces layer by layer based on a computer-aided design (CAD) file. This method allows for more frequent geometry iterations than conventional manufacturing approaches such as casting or forging. Because of this, 3D printing-assisted product prototypes can have faster iterations and more adjustable dimensions, also known for improved customizing capability. A wide range of materials can be 3D printed, such as polymers [2], ceramics [3], metal alloys [4], and their composites [5]. Even more, these materials can blend with different feedstocks to improve the functionalities of the 3D printed products. These feedstocks include: solid particles [6], viscus semi-solids [7], liquids [8], and aqueous solutions [9]. Hydrogels have been explored and refined to fit the 3D printing approach as one of the polymer-liquid mixtures [2].

Because of the shear-thinning characteristics, hydrogels have a lower inclination to flow post extrusion from the nozzle under low-shear circumstances [2]. 3D printed hydrogels can assist wound healing [10], nutrient delivery [11], medication delivery [2, 12], and organ repair [13] applications. Aside from these characteristics, hydrogels and 3D printing are renowned for their porous structure and inhomogeneous material distributions. These characteristics lead to anisotropic mechanical properties in 3D printed objects, which promote the usage of 3D printed hydrogel in seed sprouting applications. The porous structure allows embedded seeds to breathe, while the anisotropic mechanical performance replicates solid planting conditions.

Soilless cultivation is one of the most common topics for studying plants using hydrogel sprouting. Soilless farming is the practice of cultivating plants without the use of soil. The system’s complexity is divided into aeroponics, aquaponics, and hydroponics [14]. Only the plant and the growth substrate are present in aeroponics. The materials should have the following attributes to ensure that the plant grows in a healthy state:

  • Maintain mechanical integrity of the connection and any suitable form changes during the growing cycle [15].

  • Give the gemination nourishment or water (e.g., sprouting, rooting) [16].

  • Prevent evaporation of water [17].

  • Keep diseases like fungus, bacteria, and other pathogens [18].

  • Create an atmosphere that is bioactive and non-toxic [19].

Since cellulose is the polymerizing repeating unit, carboxymethyl cellulose (CMC) chain carries a considerable amount of hydroxyl groups. They can preserve water and provide a bioactive environment while ensuring attachment from viscous polymer chains and hydrogen bonding. CMC is non-toxic to most plants when there are no halogen elements present. Pathogen isolation can be achieved using 3D printing by printing gels separately for different seeds. Moreover, CMC hydrogels possess reliable ability to retain 3D high-aspect ratio struc-tures, distinguishing itself from some alternative materials like methylcellulose and hydroxypropyl methylcellulose.

Growing edible plants and producing food in space is advantageous. They can absorb carbon dioxide and create oxygen gas on the one hand. It also provides food for astronauts and completes the carbon cycle. On the other hand, planting in space was more difficult than on Earth. Firstly, microgravity can cause soil particles to fly out of control. Second, the soil has a limited water retention capacity compared to hydrogel [20]. Hydrogel as a growing medium in space is both efficient and cost-effective. Finally, because of the reduced space requirements, hydrogel-based growing plants can be multilayer-packed, resulting in less weight and volume.

Bean orientation has been shown to affect light absorption and seedling emergence rate in previous studies [21]. It is still unknown if the direction has an effect on sprouting when no visible light is present. It is also uncertain how much water is required for soybeans to sprout. To tackle these two problems, more study is required. Different size 3D-printed hydrogels were created as growing media for sprouting soybeans in this study. Soybeans were planted in hydrogels with varying orientations for a 5-day sprouting observation using an infrared camera. The weights of printed hydrogel, dry beans, and sprouted bean weights were recorded, as well as the sprouting length, which was photographed and recorded for analysis.

2 Materials and Method

2.1 Materials

The soybeans were selected with a weight range from 0.26 to 0.29 g per bean, supplied by Well luck Co., Inc. (Jersey City, NJ). CMC-gel was formulated by swelling CMC 6000 Fine Powder curtesy of Ticalose (White Marsh, MD) with deionized water at 8%, 10%, and 12% w/w.

Fig. 1.
figure 1

Left: Printed models schematics with changing dimensions marked with X (unit: mm). Right: Bean orientations a) hilum faces up (HU), b) hilum faces down (HD), c) hilum faces up (HS), d) hilum faces side and corner (HSC), and d) hilum faces side and side (HSS).

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2.2 Hydrogel Printing

Twenty-four hours post swelling, CMC-gel were packed into a 50 mL syringe and centrifuged at 2000 rpm for 4 min to remove trapped air bubbles. Models with different diameters (5 mm, 6 mm, and 7 mm, X marked dimension, the outwards drafting (X + 2) were expanded accordingly) were designed and exported as STL files, as shown in Fig. 1 left. STL files were sliced with Procusini.club web interface [22]. The CMC gel was printed using Procusini Dual 4.0 (Freising, Germany) with an optimized setting. After printing, printed gels were temporarily stored in the fridge at 4 ℃.

2.3 Seed Planting

Each seed with no visible defects was selected and planted in the central cavity of the printed seed planter made of CMC-gel. To investigate the effect of seed orientation on sprouting performance, three seed orientations were chosen: hilum faces up (HU), hilum faces down (HD), and hilum face sides (HS), as shown in Fig. 1 right a. Furthermore, two seed orientations were chosen: hilum face sides and corner (HSC) and hilum face side and side (HSS), as shown in Fig. 1 right b. To eliminate the hole size effects in the orientation-controlled group, the HSC and HSS groups were seeded in 6 mm hydrogel. All seeds were planted facing one of the corners of the hydrogels to eliminate the orientation effects in the diameter-controlled groups.

3 Results and Discussion

3.1 Hydrogel Printing Quality Control and Mass Evaluation

Printed CMC-hydrogel weight was measured after the printing, as shown in Fig. 2. On one hand, with the same composition of the hydrogel, increasing diameter resulted in mass decreases due to the theoretical volume decrease. On the other hand, with the same diameter, reducing CMC weight percentage will result in a non-significant mass decrease due to the density of the formulated CMC hydrogel decrease. With a maximum of 3% error rate, the printing performance was stable and reliable under the sample size and manufacturing method [23].

Fig. 2.
figure 2

Soybean weight yield on Day 5 normalized by Day 0.

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Under the consideration of size difference from each soybean, the yielding masses at Day 5 were normalized by the dry mass of the soybeans at Day 0 shown in Fig. 3. And 4. In general, the average mass yields decrease when the CMC weight percentage increases. According to previous studies [2], increasing CMC weight percentage leads to the hydrogels’ mechanical strength and viscosity of the increase. It prevents the beans from swelling, therefore reducing the water absorption. This also affected the volume changes.

3.2 Volume Evaluation

The soybean swelled significantly during the sprouting process due to the high-water content in the CMC-gel contact. In general, water absorption is the major reason for volume increases, as shown in Fig. 4. Changes in the orientation of downward planted soybeans (HD) were observed by examining infrared images. The soybeans changed to hilum facing sides (HS). In addition, HSC and HSS conditions were evaluated to seek difference when hilum faces to the corner or the side of the printed hydrogel from a top-down view.

Fig. 3.
figure 3

Infrared images show volume changes for the embedded soybean of first 24 h of swelling the size of the CMC-gel at 0 h is 15 mm square.

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To understand the relationship between sprouting and experimental controlled variables, soybean volumes at Day 0 and Day 5 was estimated using the elliptical sphere volume equation. The swelling volume change ratio is calculated by dividing soybean volume at Day 5 by Day 0. Results are shown in Fig. 4. The volume expansion rate decreases in average at the condition of HSC and CMC percentage increase. This is due to the improved mechanical strength provided with an increasing percentage of the CMC. This orientation requires the beam to swell against the share force instead of linear modules. On the other hand, when HSS, the improved mechanical strength reduces the shape deformation caused by swelling from the perpendicular direction of the hilum location, preventing the hydrogel from distortion, and providing less compression against the swelling process.

3.3 Soybean Density Evaluation

It is uncommon to evaluate the density changes of the sprouted seed. However, with limited space and hydrogel substrate, density changes are critical for seeds to grow in healthy conditions. This is important information providing guidance in choosing appropriate soil or sprouting substrate. Previously, evidence indicated soybean’s density decreased overall during the sprouting stage [24]. This study evaluates the density change by the ratio between mass changes over volume changes shown in Fig. 5. When the density change ratio is larger than one, it indicates that the condition of density increases.

Soybeans’ density, on average, decreased in the orientation control group except for 12% CMC for HSC condition and 8% CMC for HSS conditions. This density increase is evidence of water absorption shortage. Therefore, these conditions are not suitable for soybean sprouting. From another perspective, the density of the soybean sprouted in the 8% CMC hydrogel showed density decrease on average. Moreover, the 95% confidence interval upper limits of 5 mm and 7 mm gel are less than one, indicating they are the optimal condition for soybean sprouting across this study.

Fig. 4.
figure 4

Soybean volume yield on Day 5 normalized by Day 0.

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Fig. 5.
figure 5

Soybean density yield on Day 5 normalized by Day 0.

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4 Conclusions

With the assistance of accessible and customizable 3D printing, high water retention, biodegradable, non-toxicity, and capacity of extrusion at ambient conditions, CMC was chosen to formulate hydrogel providing attachment, water, and isolation in the application of soybean sprouting. Compared with different orientations, HD soybeans rotate to HS providing sprouts with better growing freedom. HU orientation limited the water absorption by not allowing hilum to contact the hydrogel. Within the HSC and HSS conditions, only 12% gel with HSC and 8% gel with HSS condition yielded density increased soybean sprouting, demonstrating these conditions limited water availability to the soybean. Among different weight percentages of the CMC content in the hydrogel formula, 8% CMC hydrogel sprouted soybean yields the highest mass gain in average. Compared with different diameters of the central cavity and different planting orientations, both 5 mm and 7 mm gel printed with 8% CMC gel yields density decreased soybean in the 5-day sprouting process, which suggested the optimal sprouting conditions in this study.

These findings provide guidelines and further research suggestions regarding seed sprouting with limited resources and complex environments. Further in-space experiments will assist in analyzing the ultimate performance of sprouting seeds, using hydrogel as growing media.