Mechanical metamaterial

3D printing, or additive manufacturing, has revolutionized the field in the past decade by enabling the fabrication of intricate mechanical metamaterial structures. Some of the unprecedented and unusual properties of classical mechanical metamaterials include:

Poisson's ratio defines how a material expands (or contracts) transversely when being compressed longitudinally. While most natural materials have a positive Poisson's ratio (coinciding with our intuitive idea that by compressing a material, it must expand in the orthogonal direction), a family of extreme materials known as auxetic materials can exhibit Poisson's ratios below zero. Examples of these can be found in nature, or fabricated,{{cite journal |last=Xu |first=B. |author2=Arias, F. |author3=Brittain, S. T. |author4=Zhao, X.-M. |author5=Grzybowski, B. |author6=Torquato, S. |author7=Whitesides, G. M. |year=1999 |title=Making Negative Poisson's Ratio Microstructures by Soft Lithography |journal=Advanced Materials |volume=11 |issue=14 |pages=1186–1189 |doi=10.1002/(SICI)1521-4095(199910)11:14<1186::AID-ADMA1186>3.0.CO;2-K|bibcode=1999AdM....11.1186X }}{{cite journal |last=Bückmann |first=Tiemo |author2=Stenger, Nicolas |author3=Kadic, Muamer |author4=Kaschke, Johannes |author5=Frölich, Andreas |author6=Kennerknecht, Tobias |author7=Eberl, Christoph |author8=Thiel, Michael |author9=Wegener, Martin |date=22 May 2012 |title=Tailored 3D Mechanical Metamaterials Made by Dip-in Direct-Laser-Writing Optical Lithography |journal=Advanced Materials |volume=24 |issue=20 |pages=2710–2714 |bibcode=2012AdM....24.2710B |doi=10.1002/adma.201200584 |pmid=22495906 |s2cid=205244958}} and often consist of a low-volume microstructure that grants the extreme properties. Simple designs of composites possessing negative Poisson's ratio (inverted hexagonal periodicity cell) were published in 1985.{{cite journal |last1=Kolpakovs |first1=A.G. |year=1985 |title=Determination of the average characteristics of elastic frameworks |journal=Journal of Applied Mathematics and Mechanics |volume=49 |issue=6 |pages=739–745 |bibcode=1985JApMM..49..739K |doi=10.1016/0021-8928(85)90011-5}}{{cite journal |last1=Almgren |first1=R.F. |year=1985 |title=An isotropic three-dimensional structure with Poisson's ratio=-1 |journal=Journal of Elasticity |volume=15 |issue=4 |pages=427–430 |doi=10.1007/bf00042531 |s2cid=123298026}} In addition, certain origami folds such as the Miura fold and, in general, zigzag-based folds are also known to exhibit negative Poisson's ratio.{{Cite book |last=Schenk |first=Mark |url=http://www.markschenk.com/research/files/PhD%20thesis%20-%20Mark%20Schenk.pdf |title=Folded Shell Structures, PhD Thesis |publisher=University of Cambridge, Clare College |year=2011}}{{Cite journal |last1=Wei |first1=Z. Y. |last2=Guo |first2=Z. V. |last3=Dudte |first3=L. |last4=Liang |first4=H. Y. |last5=Mahadevan |first5=L. |date=2013-05-21 |title=Geometric Mechanics of Periodic Pleated Origami |journal=Physical Review Letters |volume=110 |issue=21 |pages=215501 |arxiv=1211.6396 |bibcode=2013PhRvL.110u5501W |doi=10.1103/PhysRevLett.110.215501 |pmid=23745895 |s2cid=9145953}}{{Cite journal |last1=Eidini |first1=Maryam |last2=Paulino |first2=Glaucio H. |date=2015 |title=Unraveling metamaterial properties in zigzag-base folded sheets |journal=Science Advances |volume=1 |issue=8 |pages=e1500224 |arxiv=1502.05977 |bibcode=2015SciA....1E0224E |doi=10.1126/sciadv.1500224 |issn=2375-2548 |pmc=4643767 |pmid=26601253}}{{Cite journal |last=Eidini |first=Maryam |year=2016 |title=Zigzag-base folded sheet cellular mechanical metamaterials |journal=Extreme Mechanics Letters |volume=6 |pages=96–102 |arxiv=1509.08104 |doi=10.1016/j.eml.2015.12.006 |bibcode=2016ExML....6...96E |s2cid=118424595}}

Negative stiffness (NS) mechanical metamaterials are engineered structures that exhibit a counterintuitive property: as an external force is applied, the material deforms in a way that reduces the applied force rather than increasing it. This is in contrast to conventional materials that resist deformation.{{Cite journal |last1=Lakes |first1=R. |last2=Rosakis |first2=P. |last3=Ruina |first3=A. |date=1993-02-15 |title=Microbuckling instability in elastomeric cellular solids |url=http://dx.doi.org/10.1007/bf00414256 |journal=Journal of Materials Science |volume=28 |issue=17 |pages=4667–4672 |doi=10.1007/bf00414256 |bibcode=1993JMatS..28.4667L |issn=0022-2461|url-access=subscription }}{{Cite journal |last1=Hewage |first1=Trishan A. M. |last2=Alderson |first2=Kim L. |last3=Alderson |first3=Andrew |last4=Scarpa |first4=Fabrizio |date=December 2016 |title=Double-Negative Mechanical Metamaterials Displaying Simultaneous Negative Stiffness and Negative Poisson's Ratio Properties |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.201603959 |journal=Advanced Materials |language=en |volume=28 |issue=46 |pages=10323–10332 |doi=10.1002/adma.201603959 |pmid=27781310 |bibcode=2016AdM....2810323H |issn=0935-9648}}{{Cite journal |last1=Tan |first1=Xiaojun |last2=Wang |first2=Bing |last3=Zhu |first3=Shaowei |last4=Chen |first4=Shuai |last5=Yao |first5=Kaili |last6=Xu |first6=Peifei |last7=Wu |first7=Linzhi |last8=Sun |first8=Yuguo |date=2019-12-10 |title=Novel multidirectional negative stiffness mechanical metamaterials |url=https://iopscience.iop.org/article/10.1088/1361-665X/ab47d9 |journal=Smart Materials and Structures |volume=29 |issue=1 |pages=015037 |doi=10.1088/1361-665x/ab47d9 |issn=0964-1726|url-access=subscription }}{{Cite journal |last1=Correa |first1=Dixon M |last2=Klatt |first2=Timothy |last3=Cortes |first3=Sergio |last4=Haberman |first4=Michael |last5=Kovar |first5=Desiderio |last6=Seepersad |first6=Carolyn |date=2015-01-01 |title=Negative stiffness honeycombs for recoverable shock isolation |url=https://doi.org/10.1108/RPJ-12-2014-0182 |journal=Rapid Prototyping Journal |volume=21 |issue=2 |pages=193–200 |doi=10.1108/RPJ-12-2014-0182 |issn=1355-2546|url-access=subscription }} NS metamaterials are typically constructed from periodically arranged elements that undergo elastic instability under load. This instability leads to a negative stiffness behavior within a specific deformation range. The overall effect is a material that can absorb energy more efficiently and exhibit unique mechanical properties compared to traditional materials.

These mechanical metamaterials can exhibit coefficients of thermal expansion larger than that of either constituent. {{Cite journal |last1=Lehman |first1=Jeremy |last2=Lakes |first2=Roderic |date=2013-09-01 |title=Stiff lattices with zero thermal expansion and enhanced stiffness via rib cross section optimization |url=https://doi.org/10.1007/s10999-012-9210-x |journal=International Journal of Mechanics and Materials in Design |language=en |volume=9 |issue=3 |pages=213–225 |doi=10.1007/s10999-012-9210-x |issn=1573-8841|url-access=subscription }}{{Cite journal |last1=Zhang |first1=Qiao |last2=Sun |first2=Yuxin |date=2024-01-01 |title=Novel metamaterial structures with negative thermal expansion and tunable mechanical properties |url=https://www.sciencedirect.com/science/article/pii/S0020740323005945 |journal=International Journal of Mechanical Sciences |volume=261 |pages=108692 |doi=10.1016/j.ijmecsci.2023.108692 |issn=0020-7403|url-access=subscription }}{{Cite journal |last1=Liu |first1=Siyao |last2=Li |first2=Yaning |date=September 2023 |title=Thermal Expansion of Hybrid Chiral Mechanical Metamaterial with Patterned Bi-Strips |url=https://onlinelibrary.wiley.com/doi/10.1002/adem.202300478 |journal=Advanced Engineering Materials |language=en |volume=25 |issue=17 |doi=10.1002/adem.202300478 |issn=1438-1656|doi-access=free }} The expansion can be arbitrarily large positive or arbitrarily large negative, or zero. These materials substantially exceed the bounds for thermal expansion of a two-phase composite. They contain considerable void space.

A high strength-to-density ratio mechanical metamaterial is a synthetic material engineered to possess exceptional mechanical properties relative to its weight. This is achieved through carefully designed internal microstructures, often periodic or hierarchical, which contribute to the material's overall performance.{{Cite journal |last=Lakes |first=Roderic |date=February 1993 |title=Materials with structural hierarchy |url=https://www.nature.com/articles/361511a0 |journal=Nature |language=en |volume=361 |issue=6412 |pages=511–515 |doi=10.1038/361511a0 |bibcode=1993Natur.361..511L |issn=1476-4687|url-access=subscription }}

In a closed thermodynamic system in equilibrium, both the longitudinal and volumetric compressibility are necessarily non-negative because of stability constraints. For this reason, when tensioned, ordinary materials expand along the direction of the applied force. It has been shown, however, that metamaterials can be designed to exhibit negative compressibility transitions, during which the material undergoes contraction when tensioned (or expansion when pressured).{{cite journal |last1=Nicolaou |first1=Zachary G. |last2=Motter |first2=Adilson E. |year=2012 |title=Mechanical metamaterials with negative compressibility transitions |journal=Nature Materials |volume=11 |issue=7 |pages=608–13 |arxiv=1207.2185 |bibcode=2012NatMa..11..608N |doi=10.1038/nmat3331 |pmid=22609557 |s2cid=13390648}} When subjected to isotropic stresses, these metamaterials also exhibit negative volumetric compressibility transitions.{{cite journal |last1=Nicolaou |first1=Zachary G. |last2=Motter |first2=Adilson E. |year=2013 |title=Longitudinal Inverted Compressibility in Super-strained Metamaterials |journal=Journal of Statistical Physics |volume=151 |issue=6 |pages=1162–1174 |arxiv=1304.0787 |bibcode=2013JSP...151.1162N |doi=10.1007/s10955-013-0742-8 |s2cid=32700289}}

In this class of metamaterials, the negative response is along the direction of the applied force, which distinguishes these materials from those that exhibit negative transversal response (such as in the study of negative Poisson's ratio).

Mechanical metamaterials with negative effective bulk modulus exhibit intriguing and counterintuitive properties. Unlike conventional materials that compress under pressure, these materials expand. This anomalous behavior stems from their carefully engineered microstructure, which allows for internal deformation mechanisms that counteract the applied stress. Potential applications for these materials are vast. They could be employed to design acoustic or phononic metamaterials,advanced shock absorbers, and energy dissipation systems.{{cite journal |last=Lee |first=Sam Hyeon |author2=Park, Choon Mahn |author3=Seo, Yong Mun |author4=Wang, Zhi Guo |author5=Kim, Chul Koo |date=29 April 2009 |title=Acoustic metamaterial with negative modulus |journal=Journal of Physics: Condensed Matter |volume=21 |issue=17 |pages=175704 |arxiv=0812.2952 |bibcode=2009JPCM...21q5704L |doi=10.1088/0953-8984/21/17/175704 |pmid=21825432 |s2cid=26358086}}{{cite journal |last=Lee |first=Sam Hyeon |author2=Park, Choon Mahn |author3=Seo, Yong Mun |author4=Wang, Zhi Guo |author5=Kim, Chul Koo |date=1 December 2009 |title=Acoustic metamaterial with negative density |journal=Physics Letters A |volume=373 |issue=48 |pages=4464–4469 |bibcode=2009PhLA..373.4464L |doi=10.1016/j.physleta.2009.10.013}}{{cite journal |last=Yang |first=Z. |author2=Mei, Jun |author3=Yang, Min |author4=Chan, N. |author5=Sheng, Ping |date=1 November 2008 |title=Membrane-Type Acoustic Metamaterial with Negative Dynamic Mass |url=http://repository.ust.hk/ir/bitstream/1783.1-6034/1/PhysRevLett.101.204301.pdf |journal=Physical Review Letters |volume=101 |issue=20 |pages=204301 |bibcode=2008PhRvL.101t4301Y |doi=10.1103/PhysRevLett.101.204301 |pmid=19113343 |s2cid=714391}}{{cite journal |last=Ding |first=Yiqun |author2=Liu, Zhengyou |author3=Qiu, Chunyin |author4=Shi, Jing |date=August 2007 |title=Metamaterial with Simultaneously Negative Bulk Modulus and Mass Density |journal=Physical Review Letters |volume=99 |issue=9 |pages=093904 |bibcode=2007PhRvL..99i3904D |doi=10.1103/PhysRevLett.99.093904 |pmid=17931008}}{{cite journal |last=Lee |first=Sam Hyeon |author2=Park, Choon Mahn |author3=Seo, Yong Mun |author4=Wang, Zhi Guo |author5=Kim, Chul Koo |date=1 February 2010 |title=Composite Acoustic Medium with Simultaneously Negative Density and Modulus |journal=Physical Review Letters |volume=104 |issue=5 |page=054301 |arxiv=0901.2772 |bibcode=2010PhRvL.104e4301L |doi=10.1103/PhysRevLett.104.054301 |pmid=20366767 |s2cid=119249065}}{{cite journal |last=Li |first=Jensen |author2=Fok, Lee |author3=Yin, Xiaobo |author4=Bartal, Guy |author5=Zhang, Xiang |year=2009 |title=Experimental demonstration of an acoustic magnifying hyperlens |journal=Nature Materials |volume=8 |issue=12 |pages=931–934 |bibcode=2009NatMa...8..931L |doi=10.1038/nmat2561 |pmid=19855382}}{{cite journal |last=Christensen |first=Johan |author2=de Abajo, F. |year=2012 |title=Anisotropic Metamaterials for Full Control of Acoustic Waves |journal=Physical Review Letters |volume=108 |issue=12 |pages=124301 |bibcode=2012PhRvL.108l4301C |doi=10.1103/PhysRevLett.108.124301 |pmid=22540586 |s2cid=36710766 |hdl-access=free |hdl=10261/92293}}{{cite journal |last=Farhat |first=M. |author2=Enoch, S. |author3=Guenneau, S. |author4=Movchan, A. |year=2008 |title=Broadband Cylindrical Acoustic Cloak for Linear Surface Waves in a Fluid |journal=Physical Review Letters |volume=101 |issue=13 |page=134501 |bibcode=2008PhRvL.101m4501F |doi=10.1103/PhysRevLett.101.134501 |pmid=18851453}}{{cite journal |last=Cummer |first=Steven A |author2=Schurig, David |year=2007 |title=One path to acoustic cloaking |journal=New Journal of Physics |volume=9 |issue=3 |pages=45 |bibcode=2007NJPh....9...45C |doi=10.1088/1367-2630/9/3/045 |doi-access=free}} Furthermore, their unique elastic properties may find utility in creating novel structural components with enhanced resilience and adaptability to dynamic loads.

File:Pentamode.png

A pentamode metamaterial is an artificial three-dimensional structure which, despite being a solid, ideally behaves like a fluid. Thus, it has a finite bulk but vanishing shear modulus, or in other words it is hard to compress yet easy to deform. Speaking in a more mathematical way, pentamode metamaterials have an elasticity tensor with only one non-zero eigenvalue and five (penta) vanishing eigenvalues. Pentamode structures have been proposed theoretically by Graeme Milton and Andrej Cherkaev in 1995 {{cite journal|last=Milton|first=Graeme W.|author2=Cherkaev, Andrej V.|title=Which Elasticity Tensors are Realizable?|journal=Journal of Engineering Materials and Technology|date=1 January 1995|volume=117|issue=4|pages=483|doi=10.1115/1.2804743}} but have not been fabricated until early 2012.{{cite journal|last=Kadic|first=Muamer|author2=Bückmann, Tiemo |author3=Stenger, Nicolas |author4=Thiel, Michael |author5= Wegener, Martin |title=On the practicability of pentamode mechanical metamaterials|journal=Applied Physics Letters|date=1 January 2012|volume=100|issue=19|pages=191901|doi=10.1063/1.4709436|arxiv = 1203.1481 |bibcode = 2012ApPhL.100s1901K |s2cid=54982039}} According to theory, pentamode metamaterials can be used as the building blocks for materials with completely arbitrary elastic properties. Anisotropic versions of pentamode structures are a candidate for transformation elastodynamics and elastodynamic cloaking.

Very often Cauchy elasticity is sufficient to describe the effective behavior of mechanical metamaterials. When the unit cells of typical metamaterials are not centrosymmetric it has been shown that an effective description using chiral micropolar elasticity (or Cosserat {{cite journal |last1=Rueger |first1=Z. |last2=Lakes |first2=R. S. |title=Strong Cosserat Elasticity in a Transversely Isotropic Polymer Lattice |journal=Physical Review Letters |date=8 February 2018 |volume=120 |issue=6 |pages=065501 |doi=10.1103/PhysRevLett.120.065501|pmid=29481282 |bibcode=2018PhRvL.120f5501R |doi-access=free }}) was required.{{cite journal |last1=Frenzel |first1=Tobias |last2=Kadic |first2=Muamer |last3=Wegener |first3=Martin |title=Three-dimensional mechanical metamaterials with a twist |journal=Science |date=23 November 2017 |volume=358 |issue=6366 |pages=1072–1074 |doi=10.1126/science.aao4640|pmid=29170236 |bibcode=2017Sci...358.1072F |doi-access=free }} Micropolar elasticity combines the coupling of translational and rotational degrees of freedom in the static case and shows an equivalent behavior to the optical activity.

In addition to the well-known unprecedented mechanical properties of mechanical metamaterials, "infinite mechanical tunability" is another crucial aspect of mechanical metamaterials. This is particularly important for structural materials as their microstructure and stiffness can be tuned to effectively achieve theoretical upper bounds for specific stiffness and strength.{{Cite journal |last1=Ritchie |first1=Robert O. |last2=Zheng |first2=Xiaoyu Rayne |date=September 2022 |title=Growing designability in structural materials |url=https://www.nature.com/articles/s41563-022-01336-9 |journal=Nature Materials |language=en |volume=21 |issue=9 |pages=968–970 |doi=10.1038/s41563-022-01336-9 |bibcode=2022NatMa..21..968R |issn=1476-4660|url-access=subscription }}{{Cite journal |last1=Berger |first1=J. B. |last2=Wadley |first2=H. N. G. |last3=McMeeking |first3=R. M. |date=2017 |title=Mechanical metamaterials at the theoretical limit of isotropic elastic stiffness |url=https://www.nature.com/articles/nature21075 |journal=Nature |language=en |volume=543 |issue=7646 |pages=533–537 |bibcode=2017Natur.543..533B |doi=10.1038/nature21075 |issn=0028-0836 |pmid=28219078 |s2cid=205253514 |hdl-access=free |hdl=2164/9176}}{{Cite journal |last1=Crook |first1=Cameron |last2=Bauer |first2=Jens |last3=Guell Izard |first3=Anna |last4=Santos de Oliveira |first4=Cristine |last5=Martins de Souza e Silva |first5=Juliana |last6=Berger |first6=Jonathan B. |last7=Valdevit |first7=Lorenzo |date=2020-03-27 |title=Plate-nanolattices at the theoretical limit of stiffness and strength |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=1579 |bibcode=2020NatCo..11.1579C |doi=10.1038/s41467-020-15434-2 |issn=2041-1723 |pmc=7101344 |pmid=32221283}} While theoretical composites that achieve the same upper bound have existed for some time,{{Cite journal |last=Milton |first=G. W. |date=2018 |title=Stiff competition |url=https://www.nature.com/articles/s41586-018-0724-8 |journal=Nature |language=en |volume=564 |issue=7734 |pages=E1 |bibcode=2018Natur.564E...1M |doi=10.1038/s41586-018-0724-8 |issn=1476-4687 |pmid=30518886}} they have been impractical to fabricate as they require features on multiple length scales.{{Cite journal |last1=Berger |first1=J. B. |last2=Wadley |first2=H. N. G. |last3=McMeeking |first3=R. M. |date=2018 |title=Berger et al. reply |url=https://www.nature.com/articles/s41586-018-0725-7 |journal=Nature |language=en |volume=564 |issue=7734 |pages=E2–E4 |bibcode=2018Natur.564E...2B |doi=10.1038/s41586-018-0725-7 |issn=1476-4687 |pmid=30518891|url-access=subscription }} Single length scale designs are amenable to additive manufacturing, where they can enable engineered systems that maximize lightweight stiffness, strength and energy absorption.

To date, most mainstream studies on mechanical metamaterials have focused on passive structures with fixed properties, lacking active sensing or feedback capabilities.{{Cite journal |last1=Pishvar |first1=Maya |last2=Harne |first2=Ryan L. |date=2020-08-18 |title=Foundations for Soft, Smart Matter by Active Mechanical Metamaterials |url=http://dx.doi.org/10.1002/advs.202001384 |journal=Advanced Science |volume=7 |issue=18 |doi=10.1002/advs.202001384 |pmid=32999844 |issn=2198-3844|pmc=7509744 }} Deep integration of advanced functionalities is a critical challenge in exploring the next generation of metamaterials.{{Cite book |url=https://www.nap.edu/catalog/25244 |title=Frontiers of Materials Research: A Decadal Survey |date=2019-08-12 |publisher=National Academies Press |others=Committee on Frontiers of Materials Research: A Decadal Survey, National Materials and Manufacturing Board, Board on Physics and Astronomy, Division on Engineering and Physical Sciences, National Academies of Sciences, Engineering, and Medicine |isbn=978-0-309-48387-2 |location=Washington, D.C.|doi=10.17226/25244 }} Composite mechanical metamaterials could be the key to achieving this goal. However, the entire concept of composite mechanical metamaterials is still in its infancy. Obtaining programmable behavior through the interplay between material and structure in composite mechanical metamaterials enables integrating advanced functionalities into their texture beyond their mechanical properties. The “mechanical metamaterial tree of knowledge” implies that chiral, lattice and negative metamaterials (e.g., negative bulk modulus or negative elastic modulus) are ripe followed by origami and cellular metamaterials.

Recent research trends have been entering a space beyond merely exploring unprecedented mechanical properties. Emerging directions envisioned are sensing, energy harvesting, and actuating mechanical metamaterials.The tree of knowledge reveals that digital computing, digital data storage, and micro/nano-electromechanical systems (MEMS/NEMS) applications are one of the pillars of the mechanical metamaterials future research. Along this direction of evolution, the final target can be active mechanical metamaterials with a level of cognition. Cognitive abilities are crucial elements in a truly "intelligent mechanical metamaterials". Similar to complex living organisms, intelligent mechanical metamaterials can potentially deploy their cognitive abilities for sensing, self-powering, and information processing to interact with the surrounding environments, optimizing their response, and creating a sense–decide–respond loop.

File:Mechanical metamaterial tree of knowledge.jpg

Programmable response is an emerging direction for mechanical metamaterials beyond mechanical properties. {{Cite journal |last1=Florijn |first1=Bastiaan |last2=Coulais |first2=Corentin |last3=van Hecke |first3=Martin |date=2014-10-24 |title=Programmable Mechanical Metamaterials |url=https://link.aps.org/doi/10.1103/PhysRevLett.113.175503 |journal=Physical Review Letters |volume=113 |issue=17 |pages=175503 |doi=10.1103/PhysRevLett.113.175503|pmid=25379923 |arxiv=1407.4273 |bibcode=2014PhRvL.113q5503F |hdl=1887/51767 }}{{Cite journal |last1=Tang |first1=Yichao |last2=Lin |first2=Gaojian |last3=Yang |first3=Shu |last4=Yi |first4=Yun Kyu |last5=Kamien |first5=Randall D. |last6=Yin |first6=Jie |date=March 2017 |title=Programmable Kiri-Kirigami Metamaterials |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.201604262 |journal=Advanced Materials |language=en |volume=29 |issue=10 |doi=10.1002/adma.201604262 |pmid=28026066 |bibcode=2017AdM....2904262T |issn=0935-9648|url-access=subscription 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|first2=Damiana |last3=Formoso |first3=Olivia Irene B. |last4=Kostitsyna |first4=Irina |last5=Ochalek |first5=Megan E. |last6=Olatunde |first6=Taiwo J. |last7=Park |first7=In Won |last8=Sebastianelli |first8=Frank M. |last9=Taylor |first9=Elizabeth M. |last10=Trinh |first10=Greenfield T. |last11=Cheung |first11=Kenneth C. |date=2024-01-17 |title=Ultralight, strong, and self-reprogrammable mechanical metamaterials |url=https://www.science.org/doi/10.1126/scirobotics.adi2746 |journal=Science Robotics |language=en |volume=9 |issue=86 |doi=10.1126/scirobotics.adi2746 |pmid=38232146 |issn=2470-9476|url-access=subscription }}{{Cite journal |last1=Liu |first1=Chenyang |last2=Zhang |first2=Xi |last3=Chang |first3=Jiahui |last4=Lyu |first4=You |last5=Zhao |first5=Jianan |last6=Qiu |first6=Song |date=2024-03-20 |title=Programmable mechanical metamaterials: basic concepts, types, construction strategies—a review |journal=Frontiers in Materials |language=English |volume=11 |doi=10.3389/fmats.2024.1361408 |doi-access=free |bibcode=2024FrMat..1161408L |issn=2296-8016}}{{Cite journal |last1=Rafsanjani |first1=Ahmad |last2=Bertoldi |first2=Katia |last3=Studart |first3=André R. |date=2019-04-10 |title=Programming soft robots with flexible mechanical metamaterials |url=http://dx.doi.org/10.1126/scirobotics.aav7874 |journal=Science Robotics |volume=4 |issue=29 |doi=10.1126/scirobotics.aav7874 |pmid=33137714 |arxiv=1906.00306 |issn=2470-9476}} Electrical responsiveness is an important functionality for designing adaptive, actuating, and autonomous mechanical metamaterials. {{Cite journal |last1=Chen |first1=Tian |last2=Pauly |first2=Mark |last3=Reis |first3=Pedro M. |date=January 2021 |title=A reprogrammable mechanical metamaterial with stable memory |url=https://www.nature.com/articles/s41586-020-03123-5 |journal=Nature |language=en |volume=589 |issue=7842 |pages=386–390 |doi=10.1038/s41586-020-03123-5 |pmid=33473228 |bibcode=2021Natur.589..386C |issn=1476-4687|url-access=subscription }}{{Cite journal |last1=Mei |first1=Tie |last2=Meng |first2=Zhiqiang |last3=Zhao |first3=Kejie |last4=Chen |first4=Chang Qing |date=2021-12-13 |title=A mechanical metamaterial with reprogrammable logical functions |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=7234 |doi=10.1038/s41467-021-27608-7 |issn=2041-1723 |pmc=8668933 |pmid=34903754|bibcode=2021NatCo..12.7234M }} For example, research ideas have been opened by active and adaptive mechanical metamaterials that design electrical materials into the microstructural units of metamaterials to autonomously convert mechanical-strain input into electrical-signal output.{{Cite journal |last1=Qi |first1=Jixiang |last2=Chen |first2=Zihao |last3=Jiang |first3=Peng |last4=Hu |first4=Wenxia |last5=Wang |first5=Yonghuan |last6=Zhao |first6=Zeang |last7=Cao |first7=Xiaofei |last8=Zhang |first8=Shushan |last9=Tao |first9=Ran |last10=Li |first10=Ying |last11=Fang |first11=Daining |date=January 2022 |title=Recent Progress in Active Mechanical Metamaterials and Construction Principles |journal=Advanced Science |language=en |volume=9 |issue=1 |pages=e2102662 |doi=10.1002/advs.202102662 |issn=2198-3844 |pmc=8728820 |pmid=34716676}}

Integrating functional materials and mechanical design is an emerging research area to explore responsive mechanical metamaterials. Recent studies explore new classes of mechanical metamaterials that can response to different excitation types such acoustic,{{Cite journal |last1=Li |first1=Feng |last2=Anzel |first2=Paul |last3=Yang |first3=Jinkyu |last4=Kevrekidis |first4=Panayotis G. |last5=Daraio |first5=Chiara |date=2014-10-30 |title=Granular acoustic switches and logic elements |url=https://www.nature.com/articles/ncomms6311 |journal=Nature Communications |language=en |volume=5 |issue=1 |pages=5311 |doi=10.1038/ncomms6311 |pmid=25354587 |bibcode=2014NatCo...5.5311L |issn=2041-1723}} thermophotovoltaic{{Cite journal |title=High-efficiency thermophotovoltaic energy conversion enabled by a metamaterial selective emitter |url=https://opg.optica.org/optica/viewmedia.cfm?uri=optica-5-2-213&html=true |access-date=2024-07-23 |journal=Optica |doi=10.1364/optica.5.000213 | date=2018 | last1=Woolf | first1=David N. | last2=Kadlec | first2=Emil A. | last3=Bethke | first3=Don | last4=Grine | first4=Albert D. | last5=Nogan | first5=John J. | last6=Cederberg | first6=Jeffrey G. | last7=Bruce Burckel | first7=D. | last8=Luk | first8=Ting Shan | last9=Shaner | first9=Eric A. | last10=Hensley | first10=Joel M. | volume=5 | issue=2 | page=213 | bibcode=2018Optic...5..213W }} and magnetic.{{Cite journal |last1=Xie |first1=Yunsong |last2=Fan |first2=Xin |last3=Wilson |first3=Jeffrey D. |last4=Simons |first4=Rainee N. |last5=Chen |first5=Yunpeng |last6=Xiao |first6=John Q. |date=2014-09-09 |title=A universal electromagnetic energy conversion adapter based on a metamaterial absorber |journal=Scientific Reports |language=en |volume=4 |issue=1 |pages=6301 |doi=10.1038/srep06301 |issn=2045-2322 |pmc=4158331 |pmid=25200005|arxiv=1312.0683 |bibcode=2014NatSR...4E6301X }}

Recent studies have explored the integration of sensing and energy harvesting functionalities into the fabric of mechanical metamaterials. Meta-tribomaterials{{Cite journal |last1=Barri |first1=Kaveh |last2=Jiao |first2=Pengcheng |last3=Zhang |first3=Qianyun |last4=Chen |first4=Jun |last5=Wang |first5=Zhong Lin |last6=Alavi |first6=Amir H. |date=2021-08-01 |title=Multifunctional meta-tribomaterial nanogenerators for energy harvesting and active sensing |journal=Nano Energy |language=en |volume=86 |pages=106074 |doi=10.1016/j.nanoen.2021.106074 |issn=2211-2855 |pmc=8423374 |pmid=34504740|bibcode=2021NEne...8606074B }} Alavi A.H., Barri K., “Self-aware composite mechanical metamaterials and method for making same”, U.S. Pat. No. US2022/0011176A1, 2022. proposed in 2021 are a new class of multifunctional composite mechanical metamaterials with intrinsic sensing and energy harvesting functionalities. These material systems are composed of finely tailored and topologically different triboelectric microstructures. Meta-tribomaterials can serve as nanogenerators and sensing media to directly collect information about its operating environment. They naturally inherit the enhanced mechanical properties offered by classical mechanical metamaterials. Under mechanical excitations, meta-tribomaterials generate electrical signals which can be used for active sensing and empowering sensors and embedded electronics.

Electronic mechanical metamaterials{{Cite journal |last1=Zhang |first1=Qianyun |last2=Barri |first2=Kaveh |last3=Jiao |first3=Pengcheng |last4=Lu |first4=Wenyun |last5=Luo |first5=Jianzhe |last6=Meng |first6=Wenxuan |last7=Wang |first7=Jiajun |last8=Hong |first8=Luqin |last9=Mueller |first9=Jochen |last10=Lin Wang |first10=Zhong |last11=Alavi |first11=Amir H. |date=2023-05-01 |title=Meta-mechanotronics for self-powered computation |url=https://www.sciencedirect.com/science/article/pii/S1369702123000974 |journal=Materials Today |language=en |volume=65 |pages=78–89 |doi=10.1016/j.mattod.2023.03.026 |s2cid=258230710 |issn=1369-7021}} are active mechanical metamaterials with digital computing and information storage capabilities. They have built the foundation for a new scientific field of meta-mechanotronics (mechanical metamaterial electronics) proposed in 2023. These material systems are created via integrating mechanical metamaterials, digital electronics and nano energy harvesting (e.g. triboelectric, piezoelectric, pyroelectric) technologies. Electronic mechanical metamaterials hold the potential to function as digital logic gates, paving the way for the development of mechanical metamaterial computers (MMCs) that could complement traditional electronic systems. Such computing metamaterial systems can be particularly useful under extreme loads and harsh environments (e.g. high pressure, high/low temperature and radiation exposure) where traditional semiconductor electronics cannot maintain their designed logical functions.

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Category:Metamaterials