3D Printing of Cincau Perdu (Premna oblongifolia) Hydrogel for Dysphagia Patient’s Food Application

Abstract

This study investigates the feasibility of using cincau perdu (Premna oblongifolia) as a single-ingredient hydrogel ink for extrusion-based 3D food printing targeted for dysphagia-friendly applications. Cincau perdu (CP), also known as grass jelly, is a traditional jelly-like dessert popular in Southeast Asia. CP hydrogels could be rapidly prepared by microwave-assisted heating, followed by cooling to room temperature. The rheological properties, stiffness, and syneresis of the hydrogels could be adjusted by changing the hydrogels’ concentration. 3D constructs faithful to the blueprint could be fabricated using inks composed of 10 w/v% CP hydrogels. The textural properties were tunable by altering the geometry (grid type and height) of the printed construct. The CP hydrogels were categorized as level 5 (minced and moist) criteria based on the International Dysphagia Diet Standardization Initiative (IDDSI), making them suitable for diets of patients with dysphagia. Taken together, these findings demonstrate the potential of CP hydrogels as a sustainable, naturally gelling, and culturally relevant material for 3D-printed diets for patients with dysphagia.

1. Introduction

Dysphagia, or difficulty swallowing, is a complex medical condition with various underlying causes and serious health implications. It involves three phases: the oral, pharyngeal, and esophageal phases. The oral phase is the initial stage, where preparing food for safe swallowing becomes difficult due to muscle weakness, sensory deficits, or impaired motor control [1]. In the pharyngeal phase, challenges arise in safely moving food from the mouth to the esophagus, often caused by aspiration risk, weak pharyngeal contractions, delayed swallowing reflexes, or residue in the pharynx [2]. Individuals with dysphagia, therefore, rely on specially formulated diets that provide appropriate textures and can be easily prepared, manipulated, and safely swallowed.

In recent years, three-dimensional (3D) food printing has attracted increasing attention, particularly for developing customized diets for patients with dysphagia [3,4,5]. This technology offers multiple advantages, such as enabling the creation of visually appealing food to stimulate appetite, as well as providing precise control over texture and consistency to ensure safe swallowing [3,6]. Beyond these functional benefits, 3D food printing also allows on-demand production, making it highly adaptable to the number of patients, portion sizes, and the need for freshly prepared meals. This is especially valuable in clinical and elderly care settings, where dietary requirements must be tailored in real time. Various 3D printing modalities have been explored for food fabrication, including inkjet-, laser sintering-, adhesive spray-, microfluidic-, and extrusion-based printing systems [7,8,9,10]. Among them, extrusion-based printing is the most widely adopted for food printing, owing to its low cost and versatility in handling diverse materials such as pastes, purees, and gels [11,12,13].

An essential factor in 3D food printing is the selection of a suitable food ink. The ink must be edible, exhibit appropriate rheological properties to ensure smooth extrusion, and maintain structural integrity after printing [11,14]. For patients with dysphagia, the ink must also meet the textural requirements defined by the International Dysphagia Diet Standardization Initiative (IDDSI) [15]. Previous studies have explored a range of food inks tailored to dysphagia needs, including animal-based purees such as beef, pork, chicken, and fish [16], as well as plant-based purees like bok choy, apple, and shiitake mushroom [4]. However, these inks require the addition of hydrocolloid materials, such as xanthan gum [17], κ-carrageenan [18], and guar gum [19], to improve the printability.

In this study, we introduce Premna oblongifolia, commonly known as cincau perdu (CP) in Indonesia, as a hydrogel ink for 3D food printing. CP is abundantly available in several regions of Asia. CP is known for its ability to form a gel without the need for additional gelling agents. CP has a neutral taste, which allows it to be used in both sweet and savory applications. In addition, CP can be incorporated with proteins or lipids to enhance the nutritional value for patients with dysphagia without requiring synthetic additives. Beyond that, as a traditional food, CP has also been recognized for its medicinal properties. The leaves contain short-chain fatty acids (SCFAs) and pectin, which have shown anti-cancer activity against colon cancer cells through an apoptosis-independent chemopreventive pathway [20,21]. Additionally, CP leaf extracts possess antibacterial, antipyretic, anti-inflammatory, and antioxidant properties [22].

Traditionally, CP hydrogel is consumed as simple gel blocks mixed with beverages. The use of CP hydrogels as ink in 3D food printing provides a means of valorization by enabling the creation of complex and visually appealing structures. This approach not only enhances the esthetic appeal of CP-based products but also allows precise control over their texture, which can be beneficial for a patient with dysphagia. However, to our best knowledge, there are no reports on the utilization of CP hydrogel for 3D food printing for a patient with dysphagia.

This study aims to explore the feasibility of using CP hydrogels as an ink for 3D food printing for a patient with dysphagia. The rheological and mechanical properties, as well as the syneresis of CP hydrogels, were evaluated. The feasibility of using CP hydrogels as ink was evaluated using an extrusion-based printing system. The possibility of tuning the textural properties of the CP hydrogels by controlling the microarchitecture of the printed construct was evaluated. Finally, the IDDSI test, including spoon tilt, fork pressure, and spoon pressure test, was conducted on the CP hydrogels to evaluate their compliance for the food for a patient with dysphagia.

2. Materials and Methods

2.1. Preparation of CP Ink

Powder of cincau perdu (CP; Premna oblongifolia) leaf was purchased from Greennow (Rembang, Indonesia). The CP powder was used as received without further treatment. Hydrogels were prepared by heating a solution containing 6, 8, and 10 w/v% CP in MilliQ (MilliporeSigma, Burlington, MA, USA) using a microwave (ER-NC3, Toshiba, Tokyo, Japan) at 980 W for 20–60 s until the solutions turned into darker color and higher viscosity, followed by cooling down to room temperature (~25 °C) for 20 min.

2.2. Proximate and Nutritional Analyses of CP

2.2.1. Moisture Content

Moisture content was determined based on mass loss during oven drying. Approximately 1–2 g of sample was weighed into a pre-weighed covered weighing bottle. Liquid samples were prepared using quartz sand or folded filter paper to facilitate drying. Samples were dried at 105 °C for 3 h, cooled in a desiccator, and weighed. Drying and weighing were repeated until a constant mass was obtained. The moisture (%) was determined using the following formula:

$$\mathrm{Moisture}(\%)={\displaystyle \frac{W_1}{W}}\times 100$$

where $W$ is the initial sample mass (g), and $W_1$ is the mass loss after drying (g)

2.2.2. Ash Content

Ash content was measured based on the removal of organic matter by high-temperature incineration, leaving inorganic mineral residue. Approximately 2–3 g of sample was placed in a pre-weighed porcelain crucible, dried if necessary, charred over an open flame, and incinerated in a muffle furnace at 550 °C until complete ashing. The crucible was cooled in a desiccator and weighed to a constant mass. The ash content was determined as:

$$\mathrm{Ash}(\%)={\displaystyle \frac{W_1-W_2}{W}}\times 100$$

where $W$ is the sample mass (g), $W_1$ is the mass of crucible plus ash (g), and $W_2$ is the mass of the empty crucible (g).

2.2.3. Total Fat Content

Total fat was determined by solvent extraction using n-hexane. Samples were acidified to pH ≤ 2, mixed with n-hexane, shaken, and allowed to separate into phases. The organic phase was collected, and the solvent was evaporated. The remaining lipid residue was weighed gravimetrically. The total fat content was calculated as:

$$\mathrm{Fat}(\mathrm{mg}/\mathrm{L})={\displaystyle \frac{Y-X}{V}}\times 1000$$

where $Y$ is the mass of beaker plus lipid residue (g), $X$ is the mass of the empty beaker (g), and $V$ is the sample volume (L).

2.2.4. Protein Content

Protein content was determined using the Kjeldahl method, which quantifies total nitrogen after acid digestion and converts it to protein using a nitrogen conversion factor. Samples were digested with concentrated H₂SO₄ in the presence of a catalyst until a clear solution was obtained. After neutralization with NaOH, the released ammonia was distilled into boric acid and titrated with standardized acid. The protein content was calculated based on the nitrogen content (%N) as calculated from titration volumes, as follow:

$$\%\mathrm{N}={\displaystyle \frac{\left(Vs-Vb\right)\times N_{acid}\times 14.007}{m}}$$

%Protein = %N × 6.25

where $Vs$ is the sample titration volume (mL), $Vb$ is the blank volume (mL), $N_{acid}$ is acid normality, and $m$ is sample mass (g).

2.2.5. Carbohydrate Content

Total carbohydrate content was calculated by difference based on proximate composition.

%Carbohydrate = 100 − (%Moisture + %Ash + %Protein + %Fat)

2.2.6. Energy Value and Calories from Fat

Total energy content was calculated using Atwater factors: 4 kcal/g for protein, 4 kcal/g for carbohydrates, and 9 kcal/g for fat, using the following formula:

Energy (kcal) = (4 × Protein) + (9 × fat) + (4 × Carbohydrate)

(1)

Calories derived from fat were calculated as:

Calories from fat = 9 × %Total fat

2.2.7. Sodium Content

Sodium content was analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES; Agilent 700, Agilent Technologies, Santa Clara, CA, USA) following nitric acid digestion. The emission intensity of sodium was measured and quantified against calibration standards. The results were expressed as mg per 100 g of sample.

2.2.8. Total Sugar Content

Total sugar was determined using the Luff–Schoorl method, based on the reduction in Cu²⁺ by reducing sugars under alkaline conditions, followed by iodometric titration. Sugar concentration was obtained using standard Luff–Schoorl conversion tables.

2.2.9. Saturated Fat Content

Saturated fat content was determined by gas chromatography with flame ionization detection (GC-FID; Clarus 580, PerkinElmer, Waltham, MA, USA) following lipid extraction, methylation, and fatty acid profiling. Lipids were released by acid hydrolysis, extracted using ether solvents, and converted into fatty acid methyl esters (FAMEs) using BF₃–methanol. FAMEs were analyzed by GC-FID, and saturated fat content was calculated as the sum of individual saturated fatty acids (${\Sigma Wi}_{\left(saturated\right)}$) compared to total weight of the initial sample ($W_{sample}$), using the following equation:

$$\mathrm{Saturated} \mathrm{fat}(\%)={\displaystyle \frac{{\Sigma Wi}_{\left(saturated\right)}}{W_{sample}}}\times 100$$

(2)

2.3. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR was conducted on CP powder using the FT/IR-4100 instrument (JASCO, Tokyo, Japan). Measurement was conducted at 60 scans with a resolution of 2.0 cm⁻¹ for each sample.

2.4. Rheological Properties

The rheological properties of the CP solution and hydrogels was measured using a HAAKE MARS III rheometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a parallel plate geometry with a top plate diameter of 40 mm. The viscosity-shear rate profiles of the ink (6–10 w/v% CP) before and after heating were measured by placing 0.65 mL of the ink between the plates with the gap set to 0.5 mm. To evaluate the influence of temperature, the viscosity-shear rate profiles of 8 w/v% CP hydrogel ink were measured at 25, 37, and 50 °C. The viscosity was measured by applying a shear rate between 0.1 and 1000 s⁻¹. The yield stress of the CP hydrogel ink (6–10 w/v%) was evaluated by measuring the storage (G′) and loss (G″) modulus under oscillatory shear stress sweep (1–1000 Pa) at a frequency of 1 Hz at 25 °C. The yield stress was determined based on the crossing point where G″ become higher than G′ following linear viscoelastic region (LVER) (Figure S1). Thixotropy test of the CP hydrogels was conducted by measuring the viscosity of the hydrogels under the shear rate of 5.0 s⁻¹ for 2 min, 500 s⁻¹ for 1 min, and 5.0 s⁻¹ for 2 min at 25 °C.

2.5. Scanning Electron Microscopy (SEM)

CP hydrogel was frozen by immersion in liquid nitrogen and subsequently freeze-dried for 2 days. The microstructure of the hydrogel was observed using a scanning electron microscope (JCM-6000 plus, JEOL, Tokyo, Japan).

2.6. Mechanical Properties

The mechanical property of CP hydrogels was measured using a material tester (EZ-SX, Shimadzu, Kyoto, Japan). CP hydrogels prepared as described in Section 2.1 were added (0.5 mL) to a polydimethylsiloxane (PDMS) mold (diameter: 11 mm, height: 5 mm). Young’s modulus of the hydrogels was measured using an 8 mm-diameter probe at a compression rate of 6 mm/min, using a sensor pressure of 5 N. Young’s modulus was calculated based on the stress–strain curve with a strain range of 1–3%.

2.7. Syneresis

Measurement of syneresis or water spreading was conducted according to previous reports [4]. Briefly, 1 g of CP hydrogel (6–10 w/v%) was placed within a 1 cm radius on a grade 4 Whatman filter paper. After left undisturbed for 30 min at room temperature, photographs of the water spreading was taken. The syneresis area was measured using ImageJ (version 1.54g, NIH, Bethesda, MO, USA).

2.8. Extrusion-Based 3D Printing

3D printing was conducted using an extrusion-based 3D printer (BIO X, Cellink Bico, Gothenburg, Sweden). The printing system was composed of a syringe pump, a 2.5 mL syringe (BD Bioscience, Franklin Lakes, NJ, USA), and an 18-gauge tapered nozzle (inner diameter: 0.84 mm; Musashi Engineering, Tokyo, Japan). The layer height was set to 0.86 mm; the first layer height was 80%, while the movement speed was 7–10 mm/s.

Printability of the ink was evaluated by fabricating a rectangular construct (20 × 20 × 1 mm). Line width was quantified using ImageJ from photographs of constructs produced under different printing temperatures (25, 37, and 50 °C), flow rates (2.5, 5, 10, and 15 µL/s), and CP concentrations (6, 8, and 10 w/v%).

Assessment of the ink’s ability to support tall and geometrically complex structures was performed by printing a 5 mm-high rectangular construct using 6–10 w/v% CP hydrogels. Additionally, more intricate geometries—including a tube (15 mm diameter, 15 mm height), a pyramid (25 × 25 × 20 mm), a helical hexagon (40 × 40 × 10 mm), and a Voronoi slab (30 × 30 × 5 mm)—were produced with the 10 w/v% formulation to demonstrate structural fidelity and design versatility.

Further investigation into texture modulation through structural design involved printing lattice constructs (12–14 mm in length and width, 9 mm in height) with grid densities of 2 × 2, 3 × 3, and 4 × 4. The influence of construct height on mechanical texture was examined using square constructs (14 × 14 mm, 4 × 4 grid) fabricated at heights of 4, 9, and 15 mm, representing variations in vertical layering.

2.9. Texture Profile Analysis

The hardness, springiness, cohesiveness, adhesiveness, chewiness, and gumminess of the hydrogels were assessed using a Probe Texture Analyzer CTA-15 (Nanbei, Zhengzhou, China). Hydrogels produced either by 3D printing or by molding (20 mm diameter, 15 mm height) were compressed under a 5 g trigger force, with a deformation of 4 mm, pre-test and post-test speeds of 1 mm/s, a test speed of 0.5 mm/s, and a 0.5 s interval between compression cycles.

2.10. International Dysphagia Diet Standardization Initiative (IDDSI) Test

The IDDSI test was performed using the spoon tilt, fork pressure, and spoon pressure methods according to the IDDSI Framework detailed definitions 2.0 [15]. In the spoon tilt test, CP hydrogels (6–10 w/v%) were placed on a spoon and allowed to slide off as the spoon was tilted to assess cohesiveness and ease of release. For the fork and spoon pressure tests, hydrogel samples (15 × 15 mm) were compressed using a fork and a spoon, respectively, with pressure applied until the thumbnail blanched. This procedure evaluated the ease with which the hydrogels could be deformed or broken down under minimal force.

2.11. Statistical Analysis

All data were presented as the mean ± standard error (S.E.). Data compilation and analysis were performed using Microsoft Excel (version 2501, Microsoft, Seattle, WA, USA). Statistical differences were evaluated using one-way analysis of variance (ANOVA), followed by Tukey’s HSD post hoc test, with a p-value < 0.05 considered statistically significant. Asterisk (*) and double-asterisk (**) indicate a p-value < 0.05 and <0.005, respectively, while n.s. indicates no significant difference (p > 0.05).

3. Results and Discussion

3.1. Proximate and Nutritional Composition of Cincau Perdu Hydrogel

First, we conducted characterization of CP. The proximate and nutritional composition of CP, including ash content, calorie from fat, total fat, moisture content, total calories, carbohydrate, protein content, sodium (Na), and saturated fat, were determined to establish its baseline physicochemical characteristics relevant to dysphagia-oriented food applications. Additionally, these parameters are essential for contextualizing the rheological and printability behavior observed in subsequent sections, as well as for benchmarking CP against conventional hydrocolloids commonly used in texture-modified foods.

Table 1 shows the nutritional content of CP. Comparison between CP with carrageenan and xanthan gum [23], well-known hydrocolloids used in the inks for 3D printing of the food for patients with dysphagia, showed that CP exhibited the highest moisture content, approximately 9–11% higher than the other hydrocolloids, indicating superior water retention and a more hydrated gel matrix (Table S1). In contrast, CP showed markedly lower fat and protein contents compared with carrageenan and xanthan gum samples. Ash content in CP was also slightly lower than that in carrageenan and xanthan gum. Collectively, these results indicate that CP functions primarily as a high-moisture, low-fat, and low-protein polysaccharide matrix, differing substantially from carrageenan- and xanthan gum-based systems that exhibit higher macronutrient density. This result indicates the suitability of CP for applications requiring hydration-rich and softer textural profiles, such as dysphagia-oriented food formulations.

Table 1. Nutritional content of CP.

No	Nutritional Component	Content
1	Ash Content	2.13%
2	Calorie from fat	5.94 kcal/100 g
3	Total fat	0.66%
4	Moisture content	78.74%
5	Total calories	79.84 kcal/100 g
6	Carbohydrate	15.42%
7	Protein content	3.06%
8	Sodium (Na)	9.68 mg/100 g
9	Saturated fat	0.22%

Next, FT-IR spectroscopy was employed to identify the principal functional groups of the CP polysaccharide and to confirm its pectin-like chemical structure, which supports its ability to form hydrogels. The FT-IR spectrum of CP powder exhibited characteristics of polysaccharide bands (Figure 1). This includes a broad O–H stretching vibration at 3550–3200 cm⁻¹, C–H stretching at 2984 and 2932 cm⁻¹, an ester C=O band near 1740 cm⁻¹, as well as asymmetric and symmetric COO⁻ stretching at 1630 cm⁻¹ and 1415 cm⁻¹, respectively [24,25,26]. Intense C–O–C/C–O vibrations were also observed in the fingerprint region (around 1145, 1070, and 1030 cm⁻¹), indicating a pectin-rich structure composed mainly of pyranose-type sugar units with ordered glycosidic linkages [24].

Figure 1. FT-IR spectra of CP powder.

3.2. Hydrogelation of CP

Next, the hydrogelation behavior of CP was examined using a thermal treatment involving heating, followed by cooling of the CP solution (Figure 2). In this study, microwave heating was employed due to its convenience, wide acceptance, and utilization in the food industry and household [27,28]. In addition, microwave heating also offers shorter heating time, timely control, and higher energy efficiency compared to conventional heating [29,30]. Upon microwave heating of the aqueous CP solution (6–10 w/v%) for 20–60 s, an increase in the viscosity and turbidity, as well as flow resistance upon inverting the bottle, was observed, indicating the onset of gelation. Subsequent cooling to room temperature (25 °C) for 20 min yielded a free-standing hydrogel (Figure 2). Similar behavior has been reported for pectin and other galacturonic-acid-based hydrogels, where thermal treatment leads to the formation of an amorphous hydrogen bond network that stabilizes the gel structure [31,32].

Figure 2. Photographs of CP hydrogel preparation by thermal treatment of CP solution by heating using a microwave, followed by cooling to room temperature. The concentration of CP was 10 w/v%.

3.3. Rheological Properties

Rheological properties of the ink are crucial in determining the printability and structural integrity of the construct in 3D printing, including 3D food printing [33]. Thus, we conducted a characterization of the rheological properties of the CP ink. In an initial experiment, the influence of thermal treatment on the viscosity of the CP was evaluated. As shown in Figure S2, thermal treatment resulted in an apparent increase in the viscosity of the CP hydrogel compared to the CP solution. In addition, the impact of temperature on the rheological properties of the CP hydrogel was assessed. As shown in Figure S3, no apparent difference was observed in the viscosity of the CP hydrogel at varying temperatures (25, 37, and 50 °C). This result can be attributed to the stable crosslinking, which resulted in a resistance to thermal changes [34].

The influence of CP concentration on the rheological properties of the ink was evaluated to understand how formulation affects printability. As shown in Figure 3A, CP concentration influenced the viscosity of the hydrogel ink. The highest viscosity was observed in 10 w/v% CP hydrogel, which was 2.3 and 4.0 times higher than 8 w/v% and 6 w/v% CP, respectively, at a 1000 s⁻¹ shear rate. All CP hydrogels demonstrated a pronounced decrease in viscosity with increasing shear rate, reflecting a characteristic shear-thinning behavior [35,36]. Shear-thinning property is crucial in 3D printing to allow smooth extrusion of the ink, which affects the printability of the construct [37,38]. The shear-thinning property could be attributed to the alignment of the polymer chain and temporal disruption of the hydrogen bond under a high shear rate [39,40]. The viscoelastic properties of the CP inks are comparable to those of peas, carrots, apples, and edible rose-based inks [4,41].

Figure 3. (A) Viscosity-shear rate profiles, (B) yield stress, and (C) thixotropy of CP hydrogels prepared at varying concentrations (6–10 w/v%), measured at 25 °C. Error bars in figure (B) indicate S.E. (n = 3). * p < 0.05, ** p < 0.005, Tukey HSD.

Additionally, the yield stress of the CP hydrogels was determined using an oscillatory shear stress sweep. Under a low shear stress, a linear viscoelastic region (LVER) with a storage modulus (G′) value higher than the loss modulus (G″) was observed on all CP hydrogels (Figure S1), indicating that the ink preserved its gel state. Upon increasing the shear stress, the G′ value decreased to below G″ (Figure S1), indicating the point where the ink transition to liquid-like flow. This point was used to determine the yield stress of the ink. As shown in Figure 3B, the yield stress of the ink was affected by the concentration of CP. Increasing the CP concentration to 10 w/v% resulted in a significantly higher yield stress (p < 0.05, Tukey HSD), a phenomenon that could be mediated by the higher crosslinking density, which resulted in higher resistance to flow initiation [42,43].

Furthermore, the thixotropic behavior of the ink, defined as its ability to recover viscosity after the removal of shear forces, was evaluated [4,44]. Here, the viscosity of the ink was measured to model the extrusion process: before extrusion in the nozzle, during extrusion through the nozzle, and post-extrusion, whereby the ink was removed from shear stress. The viscosity of the CP ink following removal of high shear stress was restored to a relatively similar level as the initial condition, regardless of the CP concentrations (Figure 3C). The recovery of the viscosity following extrusion is important to ensure that the ink can maintain its shape [45,46]. Taken together, CP hydrogel demonstrated a shear-thinning property and thixotropic behavior, which makes it suitable as an ink in 3D food printing.

3.4. Scanning Electron Microscopy

Next, the microstructure of the hydrogel was observed using scanning electron microscopy (SEM). SEM evaluation provides an insight into the network connectivity and porosity of the hydrogels, which can affect the mechanical property, texture, and water retention of the hydrogels [47,48,49]. CP hydrogels, in general, showed an interconnected porous network (Figure 4). The microstructure of the CP hydrogel that displayed a porous honeycomb-like network is similar to that observed in agar, wheat starch, and Mesona chinensis polysaccharide-based gels that were prepared by thermal treatment [50,51,52]. Furthermore, in 6 w/v% CP hydrogels, the hydrogels displayed a large pore (Figure 4A), while increasing the concentration of CP to 8 w/v% (Figure 4B) resulted in a denser network and smaller pore, with 10 w/v% CP exhibiting the smallest pore (Figure 4C). This result demonstrates the concentration-dependent crosslinking of the CP hydrogel, in which higher concentration provides a larger volume fraction of polymer chains that promotes greater intermolecular interaction and crosslinking points, which lead to a denser network and smaller pore [49,53,54].

Figure 4. Scanning electron microscopy of CP hydrogels prepared at a concentration of (A) 6, (B) 8, and (C) 10 w/v% observed at a magnification of 500× (top) and 1000× (bottom).

3.5. Mechanical Properties

The mechanical properties of the CP hydrogels were evaluated. Mechanical properties of the ink are important to ensure structural integrity, where the printed ink can support its own weight following layer-by-layer deposition [55]. Here, the mechanical properties were calculated based on the stress–strain curve (Figure 5A) following the application of compressive strain on CP hydrogels prepared at varying concentrations. Increasing the concentration of CP from 6 to 10 w/v% significantly increased the stiffness of the hydrogels (p = 0.03, one-way ANOVA) (Figure 5B). The pronounced increase in the mechanical property observed in higher CP concentration can be mediated by a denser polymer network and crosslinking density [47,48,54], as shown in the SEM observation (Figure 4).

Figure 5. (A) Stress–strain curve and (B) Young’s modulus of CP hydrogels fabricated using 6–10 w/v% CP. Error bars in figure (B) indicate S.E. (n = 3). Asterisk (*) indicates p < 0.05, Tukey’s HSD.

3.6. Syneresis

Syneresis is a phenomenon of water release from foods, including hydrogel [56,57]. Syneresis is important in 3D printing since excessive syneresis can compromise the integrity of the deposited layers, which leads to the deformation of the printed construct. From a consumer perspective, foods exhibiting high syneresis are also undesirable due to unappealing appearance and texture [57,58]. Here, the syneresis of the CP hydrogels was evaluated by quantitatively measuring the area of the water spreading after placing the hydrogels on a filter paper (Figure 6A). As shown in Figure 6B, increasing the CP concentration in the hydrogels resulted in lower syneresis, from 4.87 cm² in 6 w/v% CP to 2.85 cm² in 10 w/v% CP. This result can be attributed to the denser intermolecular network in the hydrogel with higher polymer concentration, which restricts water mobility and thereby limits water release [47,59].

Figure 6. (A) Photographs of the water spreading (syneresis) from CP hydrogels on the filter paper. Scale bars indicate 1 cm. (B) Area of syneresis. Error bars indicate S.E. (n = 3). * p < 0.05, Tukey HSD.

3.7. 3D Printing

The feasibility of using CP hydrogels as ink in 3D food printing was assessed utilizing an extrusion-based printing system, as shown in Figure 7A. Initially, the printability was evaluated by measuring the line width of the construct printed at varying printing parameters, including printing temperature, flow rate, and concentration of CP (Figure 7B–G). As presented in Figure 7B,C, varying the printing temperature from 25 to 50 °C resulted in no significant differences (p > 0.05, one-way ANOVA) in the line width. This result can be attributed to heat-induced covalent crosslinking, which can form a strong network resistant to thermal changes [34]. Based on this result, the temperature of 25 °C was selected for printing in this study.

Figure 7. (A) Photographs of the extrusion-based 3D printing system used in this study. (B–G) Line width of construct printed by varying (B,C) printing temperature, (D,E) flow rate, and (F,G) concentration of CP. Scale bars in Figure (B,D,F) indicate 1 mm. Error bars in Figure (C,E,G) indicate S.E. (n ≥ 4). n.s.: no significant difference (p > 0.05), * p < 0.05, ** p < 0.005, Tukey HSD.

Next, the influence of flow rate on the printability was assessed. In general, a larger line width was observed by increasing the flow rate from 2.5 to 15 µL/s (Figure 7D,E). Higher flow rate can lead to larger line width, which is caused by the force that is given from the speed of extrusion; faster speed gives more force to extrude, which makes hydrogels get pressed and have a wider line shape without getting collapsed [60]. Due to the considerably large filament width at a flow rate of 10 and 15 µL/s, a flow rate of 5.0 µL/s was selected for this study.

In addition, the influence of CP concentration in the ink on the printability was investigated. Ink containing hydrogels prepared using 6 w/v% CP showed a larger filament width (2.30 ± 0.06 mm) compared to 8 w/v% CP (1.77 ± 0.05 mm) (Figure 7F,G). The denser polymer in 8 w/v% CP creates a stiffer matrix, which allows the ink to maintain its shape post-extrusion. In contrast, the lower crosslinking in 6 w/v% CP led to spreading post-printing [61]. Interestingly, further increasing the CP concentration to 10 w/v% resulted in a larger line width (3.20 ± 0.13 mm) compared to 8 w/v% CP. This result can be attributed to the high viscosity and yield stress of the 10 w/v% CP hydrogels (Figure 3A,B), which leads to higher resistance to deformation during extrusion, causing the spreading of the ink [62].

The optimum CP concentration for use as printing ink was assessed by fabricating a 5 mm high rectangular construct (Figure 8A). Constructs printed using 6 and 8 w/v% CP hydrogels were unable to retain their shape and collapsed, likely due to insufficient stiffness at lower polymer concentrations, which limits the ability of the material to support its own weight [63]. Meanwhile, the high viscoelastic (Figure 3) and mechanical properties (Figure 5) of 10 w/v% CP hydrogels allowed the construction to maintain its shape (Figure 8A). These findings indicate that 10 w/v% represents the optimal formulation for stable extrusion-based 3D printing. Utilizing this optimum ink, a variety of tall (cm-scale) and complex 3D constructs could be fabricated, including a tube (Figure 8B), a pyramid (Figure 8C), a helical hexagon (Figure 8D), and a Voronoi slab (Figure 8E).

Figure 8. Blueprint and photographs of (A) rectangle construct fabricated using 6–10 w/v% CP hydrogel ink, (B) tube, (C) pyramid, (D) helical hexagon, and (E) Voronoi slab printed using 10 w/v% CP hydrogel ink. Scale bars indicate 5 mm.

3.8. Texture Profile Analysis

Food texture is a vital consideration in managing dysphagia, as it has a direct impact on the safety, efficiency, and palatability. Controlling food texture, for example, altering its viscosity, hardness, and cohesiveness, can help prevent choking and aspiration by guaranteeing the bolus passes safely through the oral and pharyngeal stages of swallowing. A review study highlights the significance of compatible food textures in reducing airway invasion and residue, contrasting with poorly matched textures, which compromise swallowing control and safety [64]. Thus, an evaluation was conducted on the texture profile of the printed construct and the possibility of controlling the texture by altering the printing dimension.

Initially, the textural profile of CP hydrogels fabricated using mold and 3D printing was compared (Figure 9A). As shown in Figure 9B–G, the fabrication method significantly impacts the textural profile of the CP hydrogels, which has direct implications for their potential use in dysphagia diets. CP hydrogels fabricated using 3D printing exhibited significantly lower hardness (Figure 9B), springiness (Figure 9C), gumminess (Figure 9E), and chewiness (Figure 9F), compared to those fabricated using a mold. These properties make the 3D printed foods easier and safer to swallow, reducing the risk of choking and aspiration. In addition, the textural properties were also affected by the pore size of the printed hydrogels [65]. In particular, increasing the pore size from 4 × 4 to 2 × 2 grid design resulted in lower hardness (Figure 9B), gumminess (Figure 9E), chewiness (Figure 9F), and adhesiveness (Figure 9G). The lower grid density, such as that in the 2 × 2 grid, reduces the number of load-bearing hydrogel filaments in the construct. This can lead to a more porous, open structure, weakening adhesion and resistance to compression and deformation, which results in lower hardness, gumminess, chewiness, and adhesiveness [66,67,68]. In contrast to other textural properties, no significant difference in cohesiveness can be seen between molded and printed constructs (Figure 9D). The degree of cohesiveness in a substance is influenced by its intermolecular bonding strength, typically measured by how much deformation it can withstand before breaking [5].

Figure 9. (A) Photographs and (B–G) texture profile analysis, including (B) hardness, (C) springiness, (D) cohesiveness, (E) gumminess, (F) chewiness, and (G) adhesiveness of the 9 mm-height constructs printed using 10 w/v% CP hydrogels at varying grid design (2 × 2 to 4 × 4). Scale bars in figure (A) indicate 5 mm. Inset in figure (A) shows the blueprint. Error bars in figure (B–G) indicate S.E. (n = 3). n.s.: p > 0.05, * p < 0.05, ** p < 0.005, Tukey HSD.

Next, the influence of construct height on the textural properties of the printed hydrogels was examined using 4 × 4 lattice constructs fabricated at heights ranging from 4 to 15 mm (Figure 10A). Increasing the construct height resulted in significantly higher hardness (Figure 10B), springiness (Figure 10C), gumminess (Figure 10E), and chewiness (Figure 10F), with 15 mm height showing the highest value. Taller constructs contain a greater volume of material, which provides more elastic recovery (springiness) and requires increased force and energy to compress during biting or instrumental testing (hardness, gumminess, and chewiness) [69]. As for the cohesiveness (Figure 10D) and adhesiveness (Figure 10G), no significant difference (p > 0.05, one-way ANOVA) was observed by altering the construct height, as these properties are intrinsic material properties independent of sample geometry or size [70]. Overall, these findings demonstrate that the textural characteristics of CP hydrogels can be modulated through geometric design of the printed structures.

Figure 10. (A) Photographs and (B–G) texture profile analysis, including (B) hardness, (C) springiness, (D) cohesiveness, (E) gumminess, (F) chewiness, and (G) adhesiveness of the constructs printed using 10 w/v% CP hydrogels at varying heights (4–15 mm). Scale bars in figure (A) indicate 5 mm. Error bars in figure (B–G) indicate S.E. (n = 3). n.s.: p > 0.05, * p < 0.05, ** p < 0.005, Tukey HSD.

Comparative data are presented in Table 2, which summarizes the composition of CP hydrogel alongside reported values for xanthan gum- and carrageenan-based systems from previous studies. This table provides a benchmark for situating CP relative to established hydrocolloids and highlights similarities and differences in nutritional profile that may influence formulation strategies, fortification potential, and application scope in dysphagia-friendly 3D-printed foods.

Table 2. TPA comparation of CP hydrogel with carrageenan- and xanthan gum-supplemented formulations.

Material	Hardness (N)	Springiness (mm)	Chewiness (mJ)	Cohesiveness	Adhesiveness (mJ)	Gumminess (N)	References
6–10% CP	19.79–231.79	0.72–0.88	43.85–57	0.25–0.75	0.36–0.91	15.11–64.4	This study
0.4–0.8% Xanthan Gum	225–350	N/A	N/A	0.85–0.88	38–98	198–310	[71]
Carrageenan Gummy Candy	8.22	0.95	405.8	0.87	−39.3	413.7	[72]
k-carrageenan gum in sturgeon surimi	40.21	6.33	135.20	0.50	0.05	21.34	[73]
Xanthan gum in sturgeon surimi	9.17	4.30	9.62	0.24	0.11	2.43	[73]
2–8 gr of Xanthan gum in 1 KG pork pastes	65.87–79.47	0.49–0.58	13.23–24.89	0.41–0.54	−3.51–−4.61	N/A	[74]
2–8 gr k-carrageenan gum in 1 KG pork pastes	73.18–86.15	0.55–0.69	17.89–32.70	0.44–0.56	−4.72–−5.46	N/A	[74]

3.9. International Dysphagia Diet Standardization Initiative (IDDSI) Test

The suitability of CP hydrogels for dysphagia-friendly diets was assessed using the International Dysphagia Diet Standardization Initiative (IDDSI) tests, including the spoon tilt test, fork pressure test, and spoon pressure test [15]. The spoon tilt test was conducted to evaluate the stickiness and cohesiveness of the hydrogels [75]. As shown in Figure 11A, CP hydrogels, regardless of their concentration, showed to be cohesive on the spoon and easily slid off the spoon upon tilting, with minimal remaining residue on the spoon. The material also formed a soft, stable mound on the plate without exhibiting flow or runniness. This result indicates that 6–10 w/v% CP hydrogels form a moist and cohesive bolus that is neither sticky nor adhesive, a condition that is suitable for the diet of a patient with dysphagia [76].

Figure 11. (A) Spoon tilt test, (B) fork pressure test, and (C) spoon pressure test of CP hydrogels prepared at varying concentrations (6–10 w/v%).

Fork and spoon pressure tests were performed to assess whether the hydrogels could be easily compressed or broken down under minimal force, simulating tongue pressure during swallowing [77,78]. In the fork pressure test, the hydrogels showed deformation after pressure was applied, and the hydrogels did not return to their original shape (Figure 11B). A similar response was observed in the spoon pressure test, where the CP hydrogels were readily compressed and remained permanently deformed after the pressure was released (Figure 11C). The results from the fork and spoon pressure tests showed that CP hydrogels prepared at a concentration of 6–10 w/v% have a tender and soft texture for safe chewing with weak tooth or tongue force for a patient with dysphagia. Overall, the results from the IDDSI tests using the spoon tilt test as well as the fork and spoon pressure tests indicate that CP hydrogels can be categorized as level 5 (minced and moist).

According to clinical studies on dysphagia diets, foods categorized as Level 5 (minced and moist) must possess low enough hardness to be easily mashed by the tongue against the palate [76]. The Young’s Modulus of 10 w/v% formulation (approximately 1.5 kPa) falls within the reported threshold for soft solid foods suitable for dysphagia management, which typically remain below the high-stiffness range (>30–40 kPa) associated with Level 6 or 7 foods. This mechanical profile ensures that while the 3D-printed structure maintains its geometric shape (as seen in Figure 8), it remains compliant enough to meet the safety requirements for patients with oral-phase swallowing difficulties.

Taken together, this study demonstrates the feasibility of utilizing CP hydrogels as food ink for fabricating constructs with a texture suitable for patients with dysphagia. In particular, the CP hydrogel developed in this study, which could be categorized as level 5 (minced and moist), could hold significant translational potential for older adults with mild–moderate oropharyngeal dysphagia, cancer, or frailty-related dysphagia (e.g., head and neck cancer treatment and sarcopenia), as well as patients with neurological disorders (e.g., stroke, Parkinson’s disease, and dementia) who have impaired oral phase control and fatigue with chewing, yet are able to manipulate small soft boluses [79]. Furthermore, the natural antioxidant and bioactive compounds found in CP may offer supplementary health benefits for geriatric patients who often suffer from chronic oxidative stress.

Future studies should be conducted to address the limitations of the current study. As commonly found in plant-based materials, there is a potential for batch-to-batch variability in the properties of the CP, depending on the sources and method of extraction from the raw leaves [80]. A standardization of the method could reduce the batch-to-batch variability, which results in a more uniform rheological behavior and printability of the CP hydrogels, which could be beneficial to ensure reproducibility and scale-up of the system. In addition, sensory evaluations are critical for patients with dysphagia. A previous study has reported that foods with unfavorable sensory perception, e.g., taste, flavor, and mouthfeel, despite having a safe texture, could affect the appetite and compliace of the patient [81]. Thus, designing CP hydrogels that not only possess a safe texture based on controlling the rheological properties or construct design but also by reinforcing the positive sensory perception in CP hydrogels, such as by combining with additional flavorings, could further promote CP hydrogels as a potential food for patients with dysphagia.

4. Conclusions

This study reported the application of CP hydrogels as an ink in 3D food printing for patients with dysphagia. CP hydrogel could be prepared by heating–cooling thermal treatment of the CP solution. Rheological analysis showed that CP hydrogels exhibited a shear-thinning and thixotropic property, which makes them suitable as ink in an extrusion-based 3D printing system. SEM observation of the CP hydrogels showed an interconnected network with pore size and density affected by the concentration of CP. The concentration of CP also influenced the mechanical properties and syneresis of the CP hydrogels. Using an extrusion-based printing system, a variety of complex and tall constructions that were faithful to the blueprint could be fabricated. The textural properties of the constructs could be controlled by adjusting the design of the printed constructs, including grid type and construct height. Based on the IDDSI test, including the spoon tilt and fork and spoon pressure tests, the hydrogels could be categorized as level 5 (minced and moist) with suitable adhesiveness, cohesiveness, and ease of compression, or broken down for patients with dysphagia. Taken together, the results of this study demonstrate the potential of 3D printing using CP hydrogels as ink to fabricate 3D food constructs for patients with dysphagia.