Photosynthesis-assisted remodeling of three-dimensional printed structures

Abstract

The mechanical properties of engineering structures continuously weaken during service life because of material fatigue or degradation. By contrast, living organisms are able to strengthen their mechanical properties by regenerating parts of their structures. For example, plants strengthen their cell structures by transforming photosynthesis-produced glucose into stiff polysaccharides. In this work, we realize hybrid materials that use photosynthesis of embedded chloroplasts to remodel their microstructures. These materials can be used to three-dimensionally (3D)-print functional structures, which are endowed with matrix-strengthening and crack healing when exposed to white light. The mechanism relies on a 3D-printable polymer that allows for an additional cross-linking reaction with photosynthesis-produced glucose in the material bulk or on the interface. The remodeling behavior can be suspended by freezing chloroplasts, regulated by mechanical preloads, and reversed by environmental cues. This work opens the door for the design of hybrid synthetic-living materials, for applications such as smart composites, lightweight structures, and soft robotics.

Keywords: 3D printing; photosynthesis; self-healing; self-remodeling; self-strengthening.

Fig. 1.

Concept of the photosynthesis-assisted remodeling of 3D-printed structures. (A) Schematics to illustrate photosynthesis-assisted remodeling of plants. The photosynthesis-produced glucose undergoes a condensation reaction to form stiff polysaccharide (e.g., cellulose). (B) Schematics to illustrate photosynthesis-assisted remodeling of a synthetic polymer. The photosynthesis-produced glucose undergoes a reaction with isocyanate (NCO) side groups to form additional cross-links. (C) Image sequence of a 3D-printed treelike structure with various light illumination periods (white light intensity 69.3 ${\text{W/m}}^2$) of the photosynthesis process. (D) Unstrengthened and strengthened 3D-printed treelike structures loaded by the same weight (1 g). (E) Image sequence of a 3D-printed Popeye-like structure with various light illumination periods of the photosynthesis process. (F) Unstrengthened and strengthened 3D-printed Popeye-like structures loaded by the same weight (200 g). The red dashed boxes denote glass slides. The unstrengthened Popeye’s height reduces by 34.7%, but the strengthened Popeye only by 7% (SI Appendix, Fig. S7).

Fig. 2.

Mechanism of photosynthesis-assisted strengthening. (A) Schematic of an experimental sample with free NCO groups and embedded chloroplasts undergoing 4-h light illumination and 4-h darkness. (B) Schematic of a control 1 sample with free NCO groups and embedded chloroplasts undergoing 8-h darkness. (C) Schematic of a control 2 sample with free NCO groups but without chloroplasts undergoing 4-h light illumination and 4-h darkness. (D–F) Samples and FTIR spectra before and after respective processes for (D) experiment, (E) control 1, and (F) control 2 cases, respectively. (G) Uniaxial tensile stress–strain curves of three groups of samples. (H) Young’s moduli, tensile strengths, and fracture toughnesses of three groups of samples. (I) Young’s moduli and tensile strengths of experimental samples with embedded chloroplasts of various weight concentrations (processed with 4-h illumination and 4-h darkness). (J) Young’s moduli and tensile strengths of experimental samples with 5 wt % chloroplasts after the photosynthesis processes with various light illumination periods. (K) Young’s moduli and tensile strengths of the processed experimental samples at three states: after 4-h darkness, after 2-h light and 2-h darkness at 0 °C, and after 2-h light and 2-h darkness at 0 °C followed by 2-h light and 2-h darkness at 25 °C. (L) Young’s moduli and tensile strengths of processed experimental samples at three states: after 8-h darkness, strengthened with 4-h light illumination and 4-h darkness, and strengthened and treated with 2 M HIO₄ solution to cleave the glucose cross-linkers. Error bars in H–L represent SDs of 3–5 samples.

Fig. 3.

Photosynthesis-assisted strengthening with patterned light. (A) Schematic of an experimental setup for the localized strengthening through a patterned light with an S shape. (B) Samples at the as-printed state and after 4-h illumination with an S-shaped light and 4-h darkness. (C) Young’s modulus distribution of the patterned sample measured with indentation tests. (D) Average stiffness of the unstrengthened and strengthened regions. (E) Crack detouring in a plate sample with a strengthened circle. (F) Straight crack in a plate sample without a strengthened circle. (G) Load-displacement curves of samples with and without the strengthened circle. (Inset) The loading setup. (H) Crack paths of samples with and without wavy strengthened regions. (I) Schematic to illustrate a 3D-printed lattice structure processed by a graded light (Left to Right: 69.3–0 ${\text{W/m}}^2$). (J) Samples of functionally graded, fully soft, and fully stiffened lattices. (K) Effective Young’s modulus distribution of three samples measured with indentation tests. (L) Compressive force-displacement curves of three samples with a loading rate of 10 mm/s. The loading is along the longitudinal gradient direction (x direction). (M) The absorbed energy of the three samples. The error bars in D and M represent SDs of 3–5 samples. Note that the inhomogeneous green color in E, F, and H is possibly due to some clusters of broken chloroplasts produced during the extraction experiments, which do not influence the result quality.

Fig. 4.

Photosynthesis-assisted strengthening regulated by preloads. (A) Schematics to illustrate the photosynthesis-assisted strengthening in experimental samples without and with a prestretch. (B) Stress–strain curves of three samples: with a prestretch of 1.3 after 4-h light illumination and 4-h darkness, without a prestretch after 4-h light illumination and 4-h darkness, and without a prestretch after 8-h darkness. (C) FTIR spectra corresponding to the above three processed samples. (D) Young’s moduli and tensile strengths of the processed samples with various prestretches. Error bars represent SDs of 3–5 samples. (E) Schematics to illustrate the photosynthesis process on a sample plate under nonuniform prestresses applied by a 3D-printed foot. (F) Young’s modulus distribution of the processed sample plate. (G) The master curve between the applied compressive prestrain and the resultant Young’s modulus of the sample after the photosynthesis process (4-h illumination and 4-h darkness). Error bars represent SDs of 3–5 samples. (H) The compressive prestrain distribution translated from the Young’s modulus distribution in (F).

Fig. 5.

Photosynthesis-assisted healing. (A) Schematic of photosynthesis-assisted healing of a fractured polymer through forming additional cross-links between free NCO groups and photosynthesis-produced glucose around the fracture surface. (B) Samples and interfacial microscope images at the virgin, damaged, and healed states. The healing process consists of 4-h light illumination and 4-h darkness. (C) Uniaxial tensile stress–strain curves of samples with various periods of light illumination time compared with that of the virgin sample. The virgin sample went through the photosynthesis process with 4-h light illumination and 4-h darkness. (D) Healing strength ratios of healed samples for various illumination periods. The healing strength ratio is defined as the tensile strength of the healed polymer normalized by that of the virgin sample. The error bars represent SDs of 3–5 samples. (E) Three-dimensional-printed experimental propeller structure at the virgin, damaged, and healed state. (Insets) Crack regions on a sector wing. (F) The healed experimental propellers assembled on a remotely controlled boat can facilitate the forward movement. (G) The unhealed propellers made of control 2 polymer ink (with free NCO groups but without chloroplasts) assembled on a remotely controlled boat cannot facilitate the forward movement.