Lorenzo
Bonetti
*a,
Aron
Cobianchi
a,
Daniele
Natali
b,
Stefano
Pandini
c,
Massimo
Messori
d,
Maurizio
Toselli
b and
Giulia
Scalet
a
aDepartment of Civil Engineering and Architecture, University of Pavia, Via Ferrata 3, Pavia 27100, Italy. E-mail: lorenzo.bonetti@unipv.it
bDepartment of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale Risorgimento 4, Bologna 40136, Italy
cDepartment of Mechanical and Industrial Engineering, University of Brescia, Via Branze 38, Brescia 25133, Italy
dDepartment of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino 10129, Italy
First published on 2nd May 2024
We propose an approach to 4D print solvent-triggered, gradient-based bilayers made of semi-crystalline crosslinked polymer networks. Out-of-plane bending is obtained after immersion in the solvent, exploiting the different swelling degrees of the layers resulting from crosslinking gradients. Lastly, a beam model of the shape transformation is applied and experimentally validated.
In this panorama, 4D printing represents a powerful and flexible technology to manufacture actuators with complex architectures and enhanced shape-morphing over time (the 4th dimension) after fabrication.1,4 In particular, 4D printed bilayers are among the simplest and most investigated actuation systems,5 where their shape transformation is generally an out-of-plane bending. Narrowing the focus to extrusion-based 4D printing, that offers several advantages over other printing techniques (e.g., low costs, ease of use, and versatility),6–8 bending of bilayer structures has been reported by means of different approaches, among which multi-material layering,9 layer pre-stretching,10 and crosslinking gradients.11,12 In particular, the latter approach simplifies the design and fabrication steps by achieving bending using a single material whose properties vary along the thickness of the structure; this avoids the need for multi-material printers, adhesion issues, and additional setups for pre-stretch application. However, despite its advantages, this approach is still poorly investigated within the panorama of extrusion-based 4D printing and only limited to direct ink writing (DIW) of hydrogels.11
In this work, a versatile and simple extrusion-based 4D printing method is reported for the first time to fabricate solvent-triggered gradient-based bilayers made of semi-crystalline crosslinked polymer networks. This latter class of materials was selected as a case study, given the general interest it holds in the field of shape-memory materials.6,13 Notably, the approach based on the crosslinking gradient here explored can be conveniently extended to other polymeric materials. Particularly, fused particle fabrication is the extrusion-based technique exploited. Alongside, a theoretical model to predict the bending behavior is applied and experimentally validated.
Printing parameter | Value |
---|---|
Nozzle diameter | 22 G (0.41 mm) |
Printhead temperature | 70 °C |
Pressure | 15 kPa |
Speed | 10 mm s−1 |
Infill | 98% (concentric) |
A bilayer (30 × 5 × 0.82 mm, Fig. 2A) was fabricated controlling both the print bed temperature and UV exposure time of each layer during UV irradiation (Table 2).
Layer | Thickness (mm) | UV exposure time (s) | Print bed temperature (°C) |
---|---|---|---|
Note: the bottom layer was obtained through double UV irradiation, varying the print bed temperature. | |||
Bottom | 0.41 | 120 | 20 |
120 | 60 | ||
Top | 0.41 | 120 | 20 |
Specifically, after its deposition, the bottom layer was exposed to a double UV irradiation (first: 120 s, 20 °C; second: 120 s, 60 °C) by controlling the print bed temperature. After cooling the print bed to 20 °C, the top layer was then deposited on the bottom layer and UV irradiated (120 s, 20 °C).
Single layer samples (30 × 5 × 0.41 mm) with the same UV irradiation conditions of the bottom and top layers of the bilayer were fabricated and used as control.
![]() | (1) |
Similarly, swelling tests were carried out on 4D printed single layer samples through immersion in THF to get information on the dimensional variation due to swelling. The longitudinal swelling-induced strain (εswelling) was calculated from eqn (2):
![]() | (2) |
![]() | (3) |
The relative error was calculated as follows (eqn (4)):
![]() | (4) |
The obtained bilayer structures were thus studied for their capacity to undergo solvent-triggered shape change. For this purpose, single layer structures were first investigated from a physical-mechanical point of view. Table 3 reports the main parameters obtained from this investigation. In particular, the control of UV light exposure time and print bed temperature resulted in a control over the gel content of the two layers and, accordingly, of the swelling-induced longitudinal strain (εswelling) after immersion in THF.
Layer | G (%) | ε swelling | E (MPa) |
---|---|---|---|
Bottom | 92.0 ± 1.0 | 0.52 ± 0.01 | 140.0 ± 1.3 |
Top | 84.5 ± 0.1 | 0.57 ± 0.02 | 123.4 ± 8.8 |
Moreover, increased crosslinking extent resulted in increased mechanical properties (i.e., E) among the two layers (Table 3 and Fig. 1).
![]() | ||
Fig. 1 Representative stress–strain curves for single layer structures. Top = dotted line, bottom = solid line. |
Following this investigation, the solvent-triggered behavior of the bilayer structures was explored. The 3D printed bilayer underwent out-of-plane bending (or folding) when exposed to the solvent (Fig. 2B), as a result of the crosslinking gradient generated during printing among the bottom and the top layers.
![]() | ||
Fig. 2 Representative images of the 4D printed bilayer (A) after printing and (B) at the swelling equilibrium (24 h, RT in THF). The dotted yellow line allows to compute the radius of curvature ρ, used for the experimental calculation of K values. Note: the swollen specimen (Fig. 2B) was rotated by 90° (along its length axis) to acquire the image. |
Consequently, different crosslinking extents led to different swelling degrees in the two layers, with the bottom layer swelling less than the top one. Overall, such preferential swelling led to a bending deformation towards the bottom layer (Fig. 2B). Conversely, the as-printed single layer structures immersed in the solvent did not bend at their swelling equilibrium, but simply underwent isotropic swelling-induced expansion (Table 3). This confirmed that the governing mechanism of folding of the bilayer structures was the crosslinking gradient obtained during printing.
The curvature of the bilayers structures (K) was then assessed as the inverse of the radius of curvature (ρ, Fig. 2B) via image analysis and compared with the theoretical one.
We computed the radius of curvature of the folded bilayer in the swollen state using the modified Timoshenko equation (eqn (3)). The original Timoshenko model describes the curvature of a bilayer structure composed of two materials with different coefficients of thermal expansion.14 Here, we extended and applied such a model to a bilayer structure undergoing preferential swelling (top vs. bottom layer) due to the difference in the crosslinking degree of the two layers.16 The quantities needed in eqn (3) were taken from Tables 2 and 3. It is worth noting that the model assumes that the two elastic moduli and layer thicknesses remain constant upon swelling, and their values are equal to those measured experimentally in the dry state. A discussion on this model assumption is available elsewhere.17
Interestingly, an optimal fit was obtained, with K values of 0.0927 ± 0.001 vs. 0.0914 mm−1 for the measured (i.e., experimental) and theoretical cases, respectively. The relative error (eqn (4)) was equal to 0.0140. Despite the simplified assumptions, such result indicates the suitability of the adopted model in the description of the solvent-triggered shape change in the proposed bilayer for the investigated range of strains.
Several different industrial applications can be envisioned for such systems, ranging from actuators responsive to organic solvents to sensors capable to respond/detect toxic or explosive chemicals (e.g., liquids, vapors).18
Notably, this work can be easily extended to other bilayers undergoing solvent-triggered shape change, thus laying the foundations for model-assisted optimal design of different bilayers (e.g., different material(s), geometrical, and physical-mechanical features) and/or of even more complex layered designs.
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