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Highly-stretchable 3D-architected Mechanical Metamaterials

View Article: PubMed Central - PubMed

ABSTRACT

Soft materials featuring both 3D free-form architectures and high stretchability are highly desirable for a number of engineering applications ranging from cushion modulators, soft robots to stretchable electronics; however, both the manufacturing and fundamental mechanics are largely elusive. Here, we overcome the manufacturing difficulties and report a class of mechanical metamaterials that not only features 3D free-form lattice architectures but also poses ultrahigh reversible stretchability (strain > 414%), 4 times higher than that of the existing counterparts with the similar complexity of 3D architectures. The microarchitected metamaterials, made of highly stretchable elastomers, are realized through an additive manufacturing technique, projection microstereolithography, and its postprocessing. With the fabricated metamaterials, we reveal their exotic mechanical behaviors: Under large-strain tension, their moduli follow a linear scaling relationship with their densities regardless of architecture types, in sharp contrast to the architecture-dependent modulus power-law of the existing engineering materials; under large-strain compression, they present tunable negative-stiffness that enables ultrahigh energy absorption efficiencies. To harness their extraordinary stretchability and microstructures, we demonstrate that the metamaterials open a number of application avenues in lightweight and flexible structure connectors, ultraefficient dampers, 3D meshed rehabilitation structures and stretchable electronics with designed 3D anisotropic conductivity.

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Mechanical behaviors of elastomer lattices under large-strain compressions.(A) The mechanical response to compressive loading on a Kelvin unit cell. (B) Nominal stresses of a Kelvin elastomer lattice in a function of compressive strains with loading rate 0.0167 mm/s. (C) Sequential images (i–vi) of the Kelvin lattice under large-strain compression denoted in (B). (D) The mechanical response to compressive loading on an Octet unit cell. (E) Nominal stresses of an Octet elastomer lattice (2 × 2 × 2) in a function of compressive strains with loading rate 0.0167 mm/s. (F) Sequential images (i–vi) of the Octet lattice under large-strain compression denoted in (E). The red arrows in (F) denote the buckling layers in the Octet lattice. (G) Nominal stresses of 2 × 2 × 1 and 2 × 2 × 3 Octet elastomer lattices in functions of compressive strains with loading rate 0.0167 mm/s. Shadow regions in (E) and (G) indicate the negative stiff regions. (H) The critical strains of the first buckling of the Octet lattices in a function of the beam aspect ratios (beam length/diameter). (I) Energy absorption efficiencies of Octet, Octahedron, Kelvin and Dodecahedron elastomer lattices, and an elastomer foam, whose relative densities are all ~26%. The error bars denote the standard deviation among at least 3 data points. Scale bars in (C,F) denote 2 mm.
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f4: Mechanical behaviors of elastomer lattices under large-strain compressions.(A) The mechanical response to compressive loading on a Kelvin unit cell. (B) Nominal stresses of a Kelvin elastomer lattice in a function of compressive strains with loading rate 0.0167 mm/s. (C) Sequential images (i–vi) of the Kelvin lattice under large-strain compression denoted in (B). (D) The mechanical response to compressive loading on an Octet unit cell. (E) Nominal stresses of an Octet elastomer lattice (2 × 2 × 2) in a function of compressive strains with loading rate 0.0167 mm/s. (F) Sequential images (i–vi) of the Octet lattice under large-strain compression denoted in (E). The red arrows in (F) denote the buckling layers in the Octet lattice. (G) Nominal stresses of 2 × 2 × 1 and 2 × 2 × 3 Octet elastomer lattices in functions of compressive strains with loading rate 0.0167 mm/s. Shadow regions in (E) and (G) indicate the negative stiff regions. (H) The critical strains of the first buckling of the Octet lattices in a function of the beam aspect ratios (beam length/diameter). (I) Energy absorption efficiencies of Octet, Octahedron, Kelvin and Dodecahedron elastomer lattices, and an elastomer foam, whose relative densities are all ~26%. The error bars denote the standard deviation among at least 3 data points. Scale bars in (C,F) denote 2 mm.

Mentions: Unlike the large-strain tensile behaviors, we find that the compressive behaviors of bending and stretching dominant lattices are remarkably different. For a bending-dominant Kelvin elastomer lattice, the stress response primarily originates from the large-deflection bending of the elastomer beams (Fig. 4A), and thus increases monotonically with the applied strain (Fig. 4B,C, Supplementary Movie 2)42. This observation is similar to the compressive behaviors of the elastomer foams with stochastic bending-dominant architectures142. Differently, the compressive loading on a stretching-dominant Octet elastomer lattice is primarily applied along the beam axes (Fig. 4D), and can trigger beam buckling after a critical load. Therefore, the stress response of the Octet lattice is non-monotonic, but feature domains with negative stiffness after the occurrence of beam buckling (Fig. 4E,F, Supplementary Movie 3). It is noted that these different compressive behaviors are not limited to Kelvin and Octet lattices, but also occur in other bending-dominant (e.g., Dodecahedron) and stretching-dominant (e.g., Octahedron) lattices (Supplementary Fig. 13). To understand the distinct behaviors, we perform finite element analyses for Octet and Kelvin lattices under large-strain compressions. The computational results show that the stress response of the compressed Kelvin lattice indeed monotonically increases with the applied strain (Supplementary Fig. 14), while the stress response of the compressed Octet lattice goes up and down due to the beam buckling (Supplementary Fig. 15). The computational predictions quantitatively agree with the experimentally measured stress-strain curves (Fig. 4B,E).


Highly-stretchable 3D-architected Mechanical Metamaterials
Mechanical behaviors of elastomer lattices under large-strain compressions.(A) The mechanical response to compressive loading on a Kelvin unit cell. (B) Nominal stresses of a Kelvin elastomer lattice in a function of compressive strains with loading rate 0.0167 mm/s. (C) Sequential images (i–vi) of the Kelvin lattice under large-strain compression denoted in (B). (D) The mechanical response to compressive loading on an Octet unit cell. (E) Nominal stresses of an Octet elastomer lattice (2 × 2 × 2) in a function of compressive strains with loading rate 0.0167 mm/s. (F) Sequential images (i–vi) of the Octet lattice under large-strain compression denoted in (E). The red arrows in (F) denote the buckling layers in the Octet lattice. (G) Nominal stresses of 2 × 2 × 1 and 2 × 2 × 3 Octet elastomer lattices in functions of compressive strains with loading rate 0.0167 mm/s. Shadow regions in (E) and (G) indicate the negative stiff regions. (H) The critical strains of the first buckling of the Octet lattices in a function of the beam aspect ratios (beam length/diameter). (I) Energy absorption efficiencies of Octet, Octahedron, Kelvin and Dodecahedron elastomer lattices, and an elastomer foam, whose relative densities are all ~26%. The error bars denote the standard deviation among at least 3 data points. Scale bars in (C,F) denote 2 mm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC5035992&req=5

f4: Mechanical behaviors of elastomer lattices under large-strain compressions.(A) The mechanical response to compressive loading on a Kelvin unit cell. (B) Nominal stresses of a Kelvin elastomer lattice in a function of compressive strains with loading rate 0.0167 mm/s. (C) Sequential images (i–vi) of the Kelvin lattice under large-strain compression denoted in (B). (D) The mechanical response to compressive loading on an Octet unit cell. (E) Nominal stresses of an Octet elastomer lattice (2 × 2 × 2) in a function of compressive strains with loading rate 0.0167 mm/s. (F) Sequential images (i–vi) of the Octet lattice under large-strain compression denoted in (E). The red arrows in (F) denote the buckling layers in the Octet lattice. (G) Nominal stresses of 2 × 2 × 1 and 2 × 2 × 3 Octet elastomer lattices in functions of compressive strains with loading rate 0.0167 mm/s. Shadow regions in (E) and (G) indicate the negative stiff regions. (H) The critical strains of the first buckling of the Octet lattices in a function of the beam aspect ratios (beam length/diameter). (I) Energy absorption efficiencies of Octet, Octahedron, Kelvin and Dodecahedron elastomer lattices, and an elastomer foam, whose relative densities are all ~26%. The error bars denote the standard deviation among at least 3 data points. Scale bars in (C,F) denote 2 mm.
Mentions: Unlike the large-strain tensile behaviors, we find that the compressive behaviors of bending and stretching dominant lattices are remarkably different. For a bending-dominant Kelvin elastomer lattice, the stress response primarily originates from the large-deflection bending of the elastomer beams (Fig. 4A), and thus increases monotonically with the applied strain (Fig. 4B,C, Supplementary Movie 2)42. This observation is similar to the compressive behaviors of the elastomer foams with stochastic bending-dominant architectures142. Differently, the compressive loading on a stretching-dominant Octet elastomer lattice is primarily applied along the beam axes (Fig. 4D), and can trigger beam buckling after a critical load. Therefore, the stress response of the Octet lattice is non-monotonic, but feature domains with negative stiffness after the occurrence of beam buckling (Fig. 4E,F, Supplementary Movie 3). It is noted that these different compressive behaviors are not limited to Kelvin and Octet lattices, but also occur in other bending-dominant (e.g., Dodecahedron) and stretching-dominant (e.g., Octahedron) lattices (Supplementary Fig. 13). To understand the distinct behaviors, we perform finite element analyses for Octet and Kelvin lattices under large-strain compressions. The computational results show that the stress response of the compressed Kelvin lattice indeed monotonically increases with the applied strain (Supplementary Fig. 14), while the stress response of the compressed Octet lattice goes up and down due to the beam buckling (Supplementary Fig. 15). The computational predictions quantitatively agree with the experimentally measured stress-strain curves (Fig. 4B,E).

View Article: PubMed Central - PubMed

ABSTRACT

Soft materials featuring both 3D free-form architectures and high stretchability are highly desirable for a number of engineering applications ranging from cushion modulators, soft robots to stretchable electronics; however, both the manufacturing and fundamental mechanics are largely elusive. Here, we overcome the manufacturing difficulties and report a class of mechanical metamaterials that not only features 3D free-form lattice architectures but also poses ultrahigh reversible stretchability (strain > 414%), 4 times higher than that of the existing counterparts with the similar complexity of 3D architectures. The microarchitected metamaterials, made of highly stretchable elastomers, are realized through an additive manufacturing technique, projection microstereolithography, and its postprocessing. With the fabricated metamaterials, we reveal their exotic mechanical behaviors: Under large-strain tension, their moduli follow a linear scaling relationship with their densities regardless of architecture types, in sharp contrast to the architecture-dependent modulus power-law of the existing engineering materials; under large-strain compression, they present tunable negative-stiffness that enables ultrahigh energy absorption efficiencies. To harness their extraordinary stretchability and microstructures, we demonstrate that the metamaterials open a number of application avenues in lightweight and flexible structure connectors, ultraefficient dampers, 3D meshed rehabilitation structures and stretchable electronics with designed 3D anisotropic conductivity.

No MeSH data available.


Related in: MedlinePlus