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# Additive-free 3D-printed nanostructured carboxymethyl cellulose aerogels

Sujie Yu, Romain Castellani, Anselmo Pereira, Rudy Valette, Tatiana Budtova*

Mines Paris, PSL University, Center for Materials Forming (CEMEF), UMR CNRS 7635, CS 10207, Rue Claude Daunesse, 06904 Sophia Antipolis, France

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## ARTICLE INFO

### Keywords:

- Direct ink writing

- Rheological properties

- Nanostructured materials

- Aerogels

- Density

- Surface area

## ABSTRACT

3D printing of polysaccharide solutions is widely recognized as a highly promising method in the biomedical field for achieving complex customized shapes. One of the main challenges is in selecting conditions, in particular, the rheological properties of the system, to retain the printed shape. For the first time, the direct ink writing (DIW) is successfully applied to neat carboxymethyl cellulose (CMC) solutions without any additives or crosslinking, only by adjusting solutions' rheological properties. The influence of CMC molecular weight, degree of substitution and polymer concentration on solutions' viscoelastic properties is investigated. Extrusion velocity at various pressures and pressure calibration curves are determined to optimize printing parameters. Lightweight and nanostructured materials, aerogels, are then made from the printed structures through drying with supercritical CO₂. 3D printed aerogels with high shape stability are of density (solid part) around 0.1 g/cm³ and specific surface area up to 140 m²/g, density being twice lower and surface area twice higher than those of the “bulk” (or moulded) counterparts. Customized aerogels with high specific surface area hold significant potential in biomedical applications, such as tissue engineering, wound dressings, drug delivery, etc.

## 1. Introduction

Transforming materials into desired specific shapes is a fundamental topic in materials science. 3D printing, also known as additive manufacturing, uses three-dimensional digital file prototypes, slicing them into thin layers that are then used as “blueprints” before transforming or depositing a given material, creating the desired object. This additive approach enables the fabrication of complex shapes and surfaces that are challenging to obtain using traditional methods [1]. The advantages of 3D printing in sustainability due to the minimisation of waste and emissions reduction, operational simplicity and low costs designate it as a technology for the future.

Direct Ink Writing (DIW), as an alternative to Fused Deposition Modelling (FDM), is considered to have broad potential across various fields, particularly in biomedical applications such as tissue engineering scaffolds, drug delivery systems, and wound dressing materials [2–4]. The use of DIW is restricted by the rheological properties of the ink as it should i) flow through the printing nozzle and ii) maintain sufficient viscosity and/or yield stress after printing to preserve structural integrity. In addition, the success of DIW depends on a comprehensive understanding and precise control of various printing parameters, including nozzle diameter, extrusion velocity, printing velocity and printing pressure [5–7]. These parameters play a crucial role in

determining the integrity, resolution, and mechanical properties of the printed structures. Therefore, while DIW holds considerable promise, it also presents several challenges. Selecting the printable ink formulation is often a complicated process, and balancing high-resolution printing with preserving the structure's stability can be difficult.

Polysaccharides are polymers extracted from nature; they are renewable, often soluble in water, biodegradable and usually biocompatible, making them highly promising for utilization in DIW for biomedical applications [8]. One of the main challenges in printing polysaccharide solutions is the adaptation of their rheological properties to keep stable the desired printed geometrical shapes. Printed gels (usually hydrogels) are often too soft, collapsing under their own weight. To improve shape stability and reinforce printed hydrogels, polysaccharide solutions are usually mixed with “fast-gelling” polymers that undergo sol-gel transition after being extruded and/or with particles or fibrous materials, such as clays or nanocelluloses, making the ink a yield-stress fluid [9–11]. However, mixing may alter biocompatibility. Very few articles have reported on printing neat polysaccharide solutions resulting in self-standing structures. For example, chitosan-solution was printed directly in the air, inducing “partial hardening via solvent evaporation” [12], followed by coagulation in aqueous NaOH and evaporative drying. Grids with the resolution of about 100 µm were obtained, but the filaments themselves were not porous.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: Tatiana.Budtova@minesparis.psl.eu (T. Budtova).</footnote>
In addition to natural polysaccharides, water-soluble cellulose ethers (carboxymethyl cellulose (CMC), methyl cellulose, etc) are widely used in biomedical applications due to their biocompatibility and cost effectiveness [13]. CMC is often utilized as an additive in inks, combined with nanofibrillated cellulose [14], hyaluronic acid [15], chitosan [16], alginate [17], gelatin [17], etc. Nanofibrillated cellulose/CMC inks were reported for potential use in bone tissue regeneration and 3D bioprinting applications [14]. Hyaluronic acid/CMC inks were suggested to be used in soft tissue engineering applications [15]. A potential lactoferrin-loaded CMC/glycol chitosan ink for tissue engineering applications showed high printability (continuous extrusion) and post-printing stability (without secondary crosslinking) [16]. A composite bioink composed of gelatin, CMC and alginate was optimized and tested for both direct and indirect printing for cartilage tissue engineering applications [17]. Few works explored norbornene functionalization, carbic functionalization [18], and chemically crosslinked CMC with Fe(III), poly(N-acryloyl glycinate) [19] and citric acid [20] for the preparation of inks. To date, there is no report showing self-standing 3D printed neat aqueous CMC solutions with no additives or crosslinking.

Many biomedical applications require porous materials, for example, for controlled drug delivery, absorbance of physiological liquids in wound dressings, support for cell growth, bioactivity, etc [21]. Aerogels, materials with open pores of tens to hundreds of nanometers, are ultra-lightweight (density <0.2 g/cm³) and with high specific surface area (above 100 m²/g). Bio-based aerogels have demonstrated significant potential in various biomedical applications [22]. Till now, the majority of bio-aerogels were made from natural polysaccharides (cellulose, starch, pectin, alginate and chitosan) via polymer dissolution, gelation or non-solvent induced phase separation, and drying with supercritical CO₂ (scCO₂) [23–27]. Here, we do not consider nanocellulose-based aerogels as it is not in the scope of our work. Very recently, aerogels based on cellulose ethers, methyl cellulose/nanohydroxyapatites and CMC, were reported [28,29]. CMC aerogels were of classical disk shape with bulk density below 0.25 g/cm³ and specific surface area up to 140 m²/g; their swelling and dissolution in water and in a simulated wound exudate were evaluated and materials were tested as a drug delivery device [29].

Bio-aerogels are new materials, and their properties and applications are under exploration. They are usually made in the shape of monoliths or beads [23–25,27,30]. Very few works report on aerogels based on 3D printed polysaccharide solutions, and printability is achieved, as mentioned above, either by “fast gelation” due to crosslinking, or by addition of a fibrous matter or particles leading to yield stress fluid [28,31–33]. Vancomycin-loaded 3D printed methyl cellulose aerogels were reported for potential use as scaffolds for bone tissue engineering, methyl cellulose was mixed with hydroxyapatites [28]. Methyl cellulose-bacterial cellulose aerogels with magnetic nanoparticles were reported for potential bone tissue engineering applications, utilizing a combination of 3D printing and scCO₂ drying techniques [32]. 3D printed alginate-hydroxyapatite aerogels were obtained by dual cross-linking with CaCl₂ and glutaraldehyde as post-processing reinforcement to achieve highly crosslinked aerogel scaffolds for bone tissue engineering [33]. 3D printed bio-aerogels present numerous advantages compared to either standard printed non-porous biomaterials or bio-based hydrogels: as porous materials, bio-aerogels can absorb large amounts of wound exudates, allow cell proliferation thanks to adjusted printed geometry and delivery of nutrients via material internal porosity. Because they are dry, bio-aerogels exhibit long-term shape stability and avoid using biocides to prevent contamination. Until now, there is no report on 3D printed CMC aerogels with no additives.

The novelty and the goal of this work is double: i) to demonstrate the feasibility of using DIW for printing self-standing neat CMC solutions without any additives or crosslinking, and ii) to make aerogels from them. As 3D printing of neat aqueous CMC solutions has never been reported, first we investigate the impact of CMC characteristics (molecular weight and degree of substitution (DS)) and of polymer

concentration on the ink rheological behavior, which is a key property for making self-standing structures. This screening allows the selection of potentially printable CMC solutions. Next, we determine solution yield stress and characterize the ink thixotropic behavior. To select printing conditions, we explore the relationship between extrusion velocity and the pressure applied by the printer. Finally, neat CMC solution self-standing 3D structures were printed using DIW, and aerogels were obtained from them through non-solvent induced phase separation and scCO₂ drying. 3D printed CMC aerogels were characterized by density, specific surface area and morphology, and compared with “bulk” (moulded) CMC aerogels of the same formulation.

## 2. Experimental section

### 2.1. Materials

Four CMC samples of different molecular weights and degrees of substitution were kindly provided by IFF (Walsrode, Germany). Their characteristics and nomenclature (M10D9, M16D9, M28D9 and M17D7 with “M” related to molecular weight and “D” to the degree of substitution) are outlined in Table 1. CMC molecular weight was determined using capillary viscometry [29], and the degree of substitution was provided by CMC producer, IFF. Absolute ethanol and glycerol were purchased from Fisher Chemical. All solutions were prepared in distilled water.

### 2.2. Formulation of CMC inks

Different concentrations of CMC solutions, from 1 to 7 wt/wt%, were prepared by dissolving CMC powder in distilled water using HEI-TORQUE Expert 400 mechanical stirrer at 50 rpm for at least of 30 mins at room temperature. High viscosity solutions were placed in a centrifuge (Hettich® Universal 320/320R) at 6000 rpm for 5 min to remove air bubbles. Centrifugation is a standard procedure to remove air bubbles leading to homogeneous CMC solutions with no phase separation [34,35], see also Fig. S1 showing an example of CMC M17D7 solution after centrifugation.

### 2.3. Rheological properties of CMC inks

Rheological measurements were performed using a Bohlin Gemini rheometer. The measurement system was 40 mm diameter parallel plate geometry (PU 40) at 1 mm gap. The measurement unit was covered with silicone oil and a lid to prevent water evaporation during measurements. All experiments were performed at 25 °C. Rheological data were collected in triplicate, and the mean values are presented.

Steady-shear flow tests of CMC solutions of different concentrations ranging from 1 to 7 wt/wt% were carried out in the range of shear rates ($\dot{\gamma}$) from 0.01 to 100 s⁻¹, and both shear stress ($\tau$) and apparent viscosity ($\eta$) were plotted.

Oscillatory shear measurements were performed at a strain amplitude of 1 % corresponding to the linear viscoelastic region. An example for 7 % M17D7 solution is shown in Fig. S2 in the Supporting Information; this was checked for all formulations. Storage modulus, G', and loss modulus, G'', were recorded as a function of frequency $\omega$ varied from 0.01 to 100 Hz.

Thixotropic properties of CMC solutions were characterized through

<table><caption>**Table 1**
Samples' nomenclature, degree of substitution and molecular weight.</caption><thead><tr><td>Nomenclature</td><td>Degree of substitution (DS)</td><td>Molecular weight (×10<sup>4</sup> g.mol<sup>-1</sup>)</td></tr></thead><tbody><tr><td>M10D9</td><td>0.9</td><td>10</td></tr><tr><td>M16D9</td><td>0.9</td><td>16</td></tr><tr><td>M28D9</td><td>0.9</td><td>28</td></tr><tr><td>M17D7</td><td>0.7</td><td>17</td></tr></tbody></table>
a three-step-deformation rate, with 300 s duration of each: first, at low shear rate (1 s⁻¹), then at high shear rate (100 s⁻¹) and, finally, at the original low value (1 s⁻¹).

## 2.4. 3D printing and pressure calibration

CELLINK BIO X6 printer was used for printing CMC solutions selected after the screening of solutions' rheological properties. Metal needles of a straight cylindrical shape and of various lengths were selected to simplify pressure and extrusion velocity calculations. As the printing of neat CMC solutions was not investigated previously, applied pressure and printing velocity shown by printer software will be correlated with the extrusion pressure and extrusion velocity, respectively, to avoid numerous trials and errors (Fig. S3 in the Supporting Information). All values are shown with standard deviations.

Extrusion velocity $v_{ext}$ was calculated from the displacement $\Delta d$ of the extruded filament during time $\Delta t$:

$$v_{ext} = \frac{\Delta d}{\Delta t} \qquad (1)$$

The displacement $\Delta d$ was measured using a Nikon 1 J5 camera with a Nikkor 105 mm Macro lens (at 400 frames per second), and the videos were analysed using Davinci Resolve 18 software (to extract still images) and ImageJ.

Furthermore, we developed a calibration method to determine the relationship between the extrusion pressure and applied pressure. The detailed information is provided in the Supporting Information, with the calibration curve shown in Fig. S4.

## 2.5. Preparation of CMC aerogels from printed solutions

CMC solutions were printed using a CELLINK BIO X6 printer equipped with an extrusion printhead, 3 mL syringe and a needle with 0.41 mm diameter at room temperature. The applied pressure was 50–90 KPa, and the printing velocity was 3–5 mm/s. The digital models were a grid (20 mm × 20 mm × 5 mm), a honeycomb and a gyroid (both 30 mm × 30 mm × 3.25 mm). The moulds for making “bulk” aerogel disks were polypropylene vials of diameter 27.5 mm, solution height was 12 mm.

To obtain aerogels using drying with supercritical CO₂, water in the sample should be replaced by a fluid which is miscible with both;

ethanol was used according to our previous work on making CMC aerogel disks [29] (Fig. 1a). For “bulk” samples ethanol was carefully poured on the top of CMC solutions (ethanol volume 3–4 times that of CMC solution). For printed objects ethanol was first added drop by drop on sample surface, and then the sample was immersed in ethanol bath (Fig. 1b). In both cases water was then exchanged with ethanol 6 times; consequently, the final water concentration was <0.00095 %. The drying process using scCO₂ was conducted at PERSEE centre of Mines Paris-PSL as described elsewhere [29].

## 2.6. Characterisation of CMC aerogels

The shape fidelity of the 3D printed structures was determined by measuring their dimensions (length L, width W and height H) using a calliper with a precision of 0.01 mm and comparing with the dimensions defined by the model (length L₀, width W₀ and height H₀, respectively). The relative difference in dimensions between the model and printed structure are defined as $\Delta L/L₀$, $\Delta W/W₀$ and $\Delta H/H₀$, where $\Delta X = X - X₀$. Linear shrinkage during coagulation and total linear shrinkage from the printed structure to dry material (after scCO₂ drying) was calculated in the same way, with $X₀$ corresponding here to the dimension after printing.

The bulk density ($\rho_{bulk}$) of the solid part of the aerogel was determined as a ratio of sample mass to its volume. A bar from the 3D printed structure was cut, and the mass was measured using a digital analytical balance with a precision of 0.01 mg. The volume of this part was measured using the GeoPyc 1360 (Micromeritics), employing the chamber with a diameter of 19.1 mm and an applied force of 21 N. Each sample was measured five times, and the error was <0.01 g/cm³.

The porosity and pore volume $V_p$ of the solid part of aerogel were estimated from bulk and skeletal densities:

$$\text{Porosity (\%)} = \left( 1 - \frac{\rho_{\text{bulk}}}{\rho_{\text{skeletal}}} \right) \times 100\% \qquad (2)$$

$$V_p (\text{cm}^3/\text{g}) = \frac{1}{\rho_{\text{bulk}}} - \frac{1}{\rho_{\text{skeletal}}} \qquad (3)$$

where $\rho_{skeletal} = 1.5$ g/cm³ for all aerogel samples as no additive was used.

The internal pore's surface area was measured using nitrogen

<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic presentation of the preparation of (a) bulk aerogel and (b) 3D printed aerogel based on CMC solution. The red arrows represent the direction of ethanol diffusion. The figure was drawn partly using Figdraw software.</figcaption></figure>
adsorption with the ASAP 2020 (Micromeritics) using Brunauer-Emmett-Teller (BET) method. The aerogels were degassed under vacuum at 70 °C for 10 h. It should be noted that it is not possible to apply standard methods (Barrett, Joyner and Halenda method, BJH) using nitrogen adsorption or mercury porosimetry to determine pore size distribution in bio-aerogels [30,36–38]. BJH approach considers pores sizes in the mesopores and small macropores region, which takes in account only around 10–20 % of the total pore volume [30,36–38]. An example for M17D7 bulk aerogel is shown in Fig. S5: it shows that BJH method accounted for only 6.6 % of the total pore volume of the aerogel, calculated from Eq. (3). When mercury porosimetry is applied, bio-aerogels are compressed without mercury penetrating the pores, resulting in a measurement that is considered an artefact [37,39]. All characteristics of aerogels are shown with standard deviations.

The inner morphology of the aerogels was observed with the Scanning Electron Microscope (SEM) MAIA3 (Tescan), which is equipped with both secondary and backscattered electron detectors. To minimize electrostatic charge build-up and image default, a 14 nm layer of platinum was sputtered onto the aerogels surface using the Q150T Quarum metallizer. The observations were carried out under an accelerating voltage of 3 kV.

## 3. Results and discussion

### 3.1. Rheological properties of CMC solutions

The rheological properties of the CMC solutions were investigated in detail for selecting suitable samples and solution concentrations for printing 3D self-standing structures. The main requirement for a solution to exhibit elasto-viscoplastic properties is to be “gel-like” at rest (i.e. to possess storage modulus higher than loss modulus, and to display sufficient yield stress).

The apparent viscosity η as a function of the shear rate ˙γ of four CMC solutions at different polymer concentrations is shown in Fig. 2. At low polymer concentration (1 %), the solutions of DS 0.9 behave as a Newtonian fluid (Fig. 2a), while CMC of DS 0.7 (M17D7) shows some weak shear thinning (Fig. 2b). With the increase of polymer concentration and molecular weight, solution viscosity increases, as expected for polymer solutions, and shear thinning becomes more pronounced. The slopes of viscosity vs shear rate in the shear thinning region are around - 0.3 to - 0.4 for 3 % M17D7, 3 % M28D9 and 5 % M16D9 and around - 0.55 to - 0.6 for 5 % M17D7 and 5 % M28D9. The solution of lower DS, M17D7, exhibits a higher viscosity compared to the viscosity of CMC solution of higher DS, M16D9 (Fig. S6), despite both having the same concentration and similar molecular weight. The influence of DS on CMC viscosity was reported to be convex, with viscosity decrease for DS above 0.9–1, studied for CMC of high molecular weight, (1.4–1.7) × 10⁶ [40]. At lower DS, but for CMC still soluble in water, more hydroxyl groups are available to form intra- and intermolecular bonds, and above the overlap concentration the solution becomes gel-like. This is confirmed in Fig. 3 which shows that both G' and G" of M17D7 are higher than those of M16D9, and for M17D7 G' > G" which reflects gel-like behavior opposite to M16D9 with G' < G".

The results for storage and loss moduli of 5 % solutions are presented in Fig. 3. For low molecular weight CMC solutions of DS 0.9, M10D9 and M16D9, the storage modulus G' is lower than the loss modulus G" practically at all frequencies. M10D9 behaves as a viscoelastic fluid with G' ~ ω² and G" ~ ω. For M28D9, G' = G" indicating that a critical concentration, resembling a gel point at which polymer chains create a percolated structure, is reached. For M17D7, the storage modulus is higher than the loss modulus at all frequencies studied; this CMC solution acts like a gel.

Considering the results presented in Figs. 2 and 3, and that M17D7-based “bulk” aerogels possess the highest specific surface area among aerogels made from 1 %, 3 % and 5 % M10D9, M16D9 and M28D9 solutions [29], M17D7 was selected to be used for printing and making

<figure><img src="image_4.png" /><figcaption>Fig. 2. Viscosity as a function of shear rate of (a) M10D9, M16D9 and M28D9, and (b) M17D7 at three polymer concentrations: 1 %, 3 % and 5 %.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Storage (G', open symbols) and loss (G", filled symbols) moduli as a function of the angular frequency, for 5 % CMC solutions.</figcaption></figure>

aerogels. Even though 5 % M17D7 solution is gel-like in the linear viscoelastic region, preliminary printing tests showed that shape stabilization was poor (Fig. S7), and thus M17D7 concentration in the solution was increased. Fig. 4a shows viscoelastic properties of 6 % and 7 % M17D7 solutions, together with the ink provided by the CELLINK BIO X6 printer producer (named “Cellink”, composition not provided by the company). Cellink was used only as a reference which allows printing self-standing structures. In all cases G' > G", with about one-decade difference for Cellink and half-decade for the 7 % M17D7 solution. 7
<figure><img src="image_6.png" /><figcaption>Fig. 4. Viscoelastic (a) and thixotropic (b) behavior of M17D7 solutions. Open and filled points correspond to storage and loss moduli, respectively.</figcaption></figure>

% M17D7 shows the best match with Cellink viscoelastic properties, with storage modulus being higher than the loss modulus within three decades of frequencies. Similar rheological properties were reported for CMC solutions with DS = 0.77 in the concentration range of 4 % to 7 % [41]. 7 % M17D7 has the potential for printing self-standing samples. 6 % M17D7 solution will also be used for comparison, and 5 % M17D7 will not be considered in the following.

In order to have a better understanding of the recovery of CMC solutions' viscosity after high shear, as it happens during printing, the thixotropic behavior of the solutions was investigated (Fig. 4b). The flow of Cellink and of 6 % and 7 % M17D7 is rather similar in terms of the order of magnitude of viscosity values, but CMC solutions are thixotropic while Cellink does not appear to be in this range of shear rate variations (Fig. 4b). The time needed for the recovery of viscosity of M17D7 solutions after a rest period is attributed to the need for reforming chains' entanglements, which is of the order of magnitude of about 10 s for 80 % variation in apparent viscosity. A similar thixotropic behavior was also reported for other CMC solutions [42,43].

Finally, both 6 % and 7 % M17D7 solutions were examined in terms of yield stress, which is an important characteristic ensuring shape stability after printing and sustaining the deposition of continuous layers. Herschel-Bulkley equation [44] was used to determine the yield stress $\tau_0$:

$$|\tau| = K|\dot{\gamma}|^m + \tau_0 \quad (4)$$

where $\tau$ is the shear stress, $K$ is the consistency and $m$ is the flow index.

The results are presented in Fig. 5 for stresses measured upon increasing and then immediately decreasing shear rate for 6 % and 7 % M17D7 solutions, and also for Cellink. Consistency, flow index and yield stress are in Table 2. Both 6 % and 7 % M17D7 solutions are thixotropic, with stress showing a hysteresis upon shear rate increase/decrease cycle (Fig. 5a, b). The yield stress of 6 % M17D7 solution is very low, with the difference for increase/decrease cycle being within the experimental error. For 7 % M17D7 solution, the yield stress calculated from the

<figure><img src="image_7.png" /><figcaption>Fig. 5. Shear stress vs shear rate for increasing and decreasing shear rate for (a) 6 % and (b) 7 % M17D7 solutions and (c) Cellink, together with Herschel-Bulkley equation fits (dashed lines).</figcaption></figure>

increasing shear rate dependence is about 350 Pa, while it is approximately 10 Pa for the decreasing shear rate part. The reason for such difference is that 7 % M17D7 solution is highly thixotropic, as mentioned above and also shown in Fig. 5b. Cellink is not thixotropic with rather low yield stress (Fig. 5c and Table 2).

Using yield stress values, the maximum number of layers of 3D printed structures was estimated and compared with the experiment. To achieve solid structures, it is necessary to ensure that $\tau_0 \ge \rho \times g \times h_{max}$, where $\rho$ represents the density of the printing matter (for simplicity we use the density of water, $10^3 \text{ kg/m}^3$, as polymer concentration is low), $g$ is the acceleration of gravity, and $h_{max}$ refers to the height of the 3D printed structure. Considering that the height of one layer is 0.41 mm, the number of layers that can be printed with 7 % M17D7 should be around $10^0-10^2$ (e.g., 1 mm < $h_{max}$ < 30 mm, using the yield stress
<caption>**Table 2**</caption>

The values of the Herschel-Bulkley equation parameters for 6 % and 7 % M17D7 solutions as well as for Cellink, upon increasing and decreasing shear rate.

<table><thead><tr><th rowspan="2">Shear rate</th><th colspan="2">6 % M17D7</th><th colspan="2">7 % M17D7</th><th colspan="2">Cellink</th></tr><tr><th>Increasing</th><th>Decreasing</th><th>Increasing</th><th>Decreasing</th><th>Increasing</th><th>Decreasing</th></tr></thead><tbody><tr><td>Consistency</td><td>190 ± 10</td><td>100 ± 5</td><td>60 ± 3</td><td>60 ± 2</td><td>80 ± 3</td><td>80 ± 3</td></tr><tr><td>Flow index</td><td>0.35 ± 0.01</td><td>0.445 ± 0.01</td><td>0.6 ± 0.01</td><td>0.6 ± 0.01</td><td>0.3 ± 0.01</td><td>0.3 ± 0.01</td></tr><tr><td>Yield stress (Pa)</td><td>2 ± 1</td><td>2 ± 1</td><td>350 ± 50</td><td>10 ± 3</td><td>15 ± 5</td><td>15 ± 4</td></tr></tbody></table>

values reported in Table 2 and according to the maximum height equation shown above). Fig. 6 shows printed grids: 10 × 10 × 11.25 mm with 56 layers; 10 × 10 × 12.5 mm with 62 layers; and 10 × 10 × 20 mm with 97.6 layers. The object collapsed during printing of the 98th layer (the slender printed object folds once the gravitational stress overpasses the τ₀). This result showed an excellent fit between a simple theoretical prediction of how many layers can be printed and the printing experiment. Neat CMC aqueous ink can be used to print 3D structures with up to 97 layers, at least for this specific CMC formulation.

## 3.2. Correlation between extrusion velocity and applied pressure

It is known that if the printing velocity is much higher than the extrusion velocity, the filament may break. To set the printing velocity for making a continuous filament based on the selected CMC solutions, extrusion velocity was calculated using Eq. (1) (see Experimental section) and correlated with applied pressure. Fig. 7 shows extrusion velocity for 6 % and 7 % M17D7 solutions extruded through type 22G and 25G needles of diameter 0.41 mm and 0.25 mm, respectively, with length 13.45 mm. As expected, the higher the applied pressure, the faster the extrusion velocity, while the extrusion velocity decreases with increased polymer concentration at the same pressure. The obtained calibration dependences are used in the following for the selection of printing parameters.

## 3.3. 3D printed CMC aerogels and comparison with “bulk” counterparts

Self-standing structures were printed by setting the printing velocity equal to the extrusion velocity. Stable structures in the shapes of “grids” (20 mm × 20 mm × 5 mm), “honeycombs” and “gyroids” (30 mm × 30 mm × 3.25 mm) based on 6 % and 7 % M17D7 solutions were obtained (see examples illustrating the printing process and the self-standing structures in Videos S1–S4 in the Supporting Information). Similar structures were obtained for solutions based on crosslinked CMC-glycol chitosan through Schiff base reaction and ionic interactions (6 layers), as well as for CMC crosslinked with citric acid via thermal treatment after printing (Fig. S8) [16,20]. Aerogels were then obtained by solvent/non-solvent exchange and drying with supercritical CO₂ (see details in Fig. 1). “Bulk” (or moulded) aerogels in the shape of disks were made from the same CMC solutions. All printed aerogels kept their shape and integrity for about two years.

The examples of printed and coagulated samples (after solvent/non-

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Extrusion velocity as a function of applied pressure for 6 % and 7 % M17D7 solutions; needles were 22G and 25G (length 13.45 mm and diameter 0.41 and 0.25 mm).</figcaption></figure>

solvent exchange) and of the corresponding aerogels based on 7 % M17D7 are shown in Fig. 8a together with “bulk” aerogels in the shape of a monolithic disk; the same for 6 % M17D7 is shown in Fig. S9. The morphology of the corresponding aerogels at different magnifications is also presented in Figs. 8b and S9. “Bulk” aerogel possesses a homogeneous fine network of entangled polymer fibrils. Printed aerogel possesses the same morphology with excellent adhesion between the layers (see red arrows showing the position of the borders between layers).

The shape fidelity (see Experimental section) of the 7 % M17D7 3D printed structures was estimated. The dimensions of the printed structures were the same as those of digital models (ΔL = ΔW = ΔH = 0). This is an excellent result compared to some other bio-based printed structures, such as 3D printed nanocellulose structures with shrinkage deviation in all directions within 10–50 % [45], alginate structures with printability from 0.81 to 1.36 [46], and 3D printed CMC-alginate-montmorillonite clay structure and CMC-laponite nanocomposite structures, with printability ranging from 0.1 to 1.5 [47] and 0.9 to 1 [48], respectively.

Linear shrinkage of samples during coagulation and total shrinkage

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Examples of (a) sliced model, and printed structures using 7 % M17D7: (b) 10 × 10 × 11.25 mm with 56 layers, (c) 10 × 10 × 12.5 mm with 62 layers and (d) 10 × 10 × 20 mm with 97.6 layers (collapsed).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** (a) Representative examples of sliced models, of printed 7 % M17D7 solutions and their coagulated and dried (aerogel) counterparts (the scale is 5 mm), together with (b) a comparison of their internal morphology with that of bulk aerogel at different scales. Red arrows show the borders between printed layers.</figcaption></figure>

from printed solution to aerogel are listed in Table S1. The linear shrinkage during coagulation was within 10 %, and the total linear shrinkage was within 15 % (which is approximately 40 % of volume shrinkage). The total volume shrinkage of monolithic aerogels, around 75 %, was significantly higher than that of 7 % M17D7 printed aerogels. 3D printed methyl cellulose aerogel structures possessed high volume shrinkage, around 80 % [28]. During coagulation, printed CMC structures showed higher shrinkage in the X-direction (length) and Y-direction (width) compared to Z-direction (height), but total linear shrinkage was practically the same in all directions, within 10–15 %. A similar trend was reported for nanocellulose-based 3D printed structures [45].

Bulk density of the solid parts of printed and bulk aerogels is presented in Fig. 9a for the samples made from 6 % and 7 % M17D7 solutions. Interestingly, the density of the solid part of printed samples, 0.11–0.14 g/cm³, is twice lower than that of “bulk” (or moulded) discs, 0.19–0.28 g/cm³. In both cases, higher polymer concentration in solution induces higher density, as expected, and reported for various bio-aerogels [24,27,29,49]. Consequently, the porosity of the solid part of printed aerogel structures (91 % - 93 %) is higher than that of “bulk” counterparts (81 % - 87 %). We hypothesise that the difference in the density and porosity of printed and “bulk” aerogels is different duration of coagulation upon the contact with non-solvent. For bulk aerogels, ethanol was poured on the top of CMC solution; ethanol was then slowly diffusing inside CMC solution coagulating the polymer. The complete penetration of ethanol through CMC solution took approximately one to

two weeks leaving time for polymer chains to contract, relax and “pack”, leading to high shrinkage. In contrast, during solvent/non-solvent exchange process of 3D printed aerogels, ethanol was diffusing into the sample from all sides, also in between the printed layers, and the process was completed within few minutes. While ethanol diffusion coefficient in coagulating CMC solution of the same polymer concentration should be the same, fast solvent exchange is supposed to be due to the additional porosity in-between the printed layers (see homogeneous morphology of “bulk” aerogels vs layered one for the printed ones in Fig. 8b). Fast completion of non-solvent penetration did not allow chains to relax and shrink; this fast structure “freezing” resulted in lower density and higher porosity of the printed aerogel.

The specific surface area of bulk and 3D printed aerogels are compared in Fig. 9b. The specific surface area of 3D printed CMC aerogels is twice higher (128–141 m²/g) than that of their bulk counterparts (around 70 m²/g). We suggest two reasons explaining this intriguing result: i) fastness of coagulation and ii) formation of additional porosity between the printed layers. Fast completion of coagulation does not allow chains to tightly pack (vide supra), resulting in additional porosity. To the best of our knowledge, there is no study investigating the influence of the speed of coagulation on bio-aerogel properties. Few articles report on the influence of sample dimensions: surface area of cellulose aerogel particles increased with the decrease of particle diameter [50], and surface area of cellulose aerogel beads was higher than that of monolith (or bulk aerogels) [51]. The input of the
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** (a) Bulk density and (b) specific surface area of bulk aerogels (in red) and 3D printed aerogels (in blue) from 6 % and 7 % M17D7 solutions.</figcaption></figure>

"added" surface due to the decrease of macroscopic sample size should be low, and thus we suggest that it may be fast completion of coagulation that does not allow chains to tightly pack, thus creating additional fine porosity. Additional porosity may also appear because of some gaps between printed layers (Fig. 8b). Both reasons correlate with the density results discussed above (Fig. 9a).

## 4. Conclusions

This work presents the very first report on printing self-standing neat CMC solutions without any additives or crosslinking and making from them nanostructured and lightweight materials, aerogels. To select printable solutions, CMC solutions' rheological properties were investigated varying CMC molecular weight, DS, and polymer concentration. Solutions of CMC with lower DS (0.7) and higher concentration (6 % and 7 %), M17D7, were chosen for printing because of their viscoelastic properties (yield stress, G' > G'' in three decades of frequencies). Correlation between extrusion velocity and applied pressure was built. It was demonstrated that extrusion pressure is higher than the applied pressure displayed on the 3D printer CELLINK BIO X6.

Based on yield stress values, the maximum number of 7 % M17D7 solutions printed layers was predicted to be up to 100 layers. The actual prints achieved 3D structures with up to 97 layers. Self-standing 3D structures (grid 20 mm × 20 mm × 5 mm, honeycomb and gyroid, both of the same dimensions 30 mm × 30 mm × 3.25 mm) based on neat CMC solutions were printed. Printed structures showed excellent shape fidelity, with no deviation between printed structures and digital models. After replacing water with ethanol, aerogels were obtained by drying with scCO₂. The adhesion between the layers was excellent, as revealed by SEM. Compared to bulk aerogels, 3D-printed counterparts exhibit

twice lower density and twice higher specific surface area, which was suggested to be due to fast CMC coagulation and additional porosity in between the printed layers, confirmed by SEM. On the nanoscale level, the inner morphology of printed and "bulk" aerogels is the same.

This work opens a new avenue for making novel materials, 3D printed CMC aerogels, with no additives. Next, their mechanical and biomedical properties are to be explored. Bio-aerogels with customizable shape, lightweight and with high specific surface area have significant potential applications in life science applications such as tissue engineering, wound dressings, drug delivery, etc.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijbiomac.2025.140277.

## CRediT authorship contribution statement

**Sujie Yu:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Romain Castellani:** Writing – review & editing, Validation, Methodology, Investigation. **Anselmo Pereira:** Writing – review & editing, Validation, Methodology, Investigation. **Rudy Valette:** Writing – review & editing, Validation, Supervision, Methodology, Investigation. **Tatiana Budtova:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Sujie Yu reports financial support was provided by China Scholarship Council. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We acknowledge the financial support from China Scholarship Council, IFF for providing CMC samples, J. Jaxel (PERSEE, Mines Paris, France) for supercritical drying and I. Lahouij and F. Georgi (CEMEF, Mines Paris, France) for guiding in SEM analysis and G.L. Tian (LMD, Ecole Polytechnique, France) for coding pressure calibration.

## Data availability

Data will be made available on request.

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