www.sciencemag.org/cgi/content/full/334/6058/962/DC1

Supporting Online Material for Ultralight Metallic Microlattices T. A. Schaedler,* A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit, W. B. Carter

*To whom correspondence should be addressed. E-mail: [email protected] Published 18 November 2011, Science 334, 962 (2011) DOI: 10.1126/science.1211649

This PDF file includes: Materials and Methods Fig. S1 Table S1 References

Other Supporting Online Material for this manuscript includes the following: available at www.sciencemag.org/cgi/content/full/334/6058/962/DC1 Movie S1

Supporting Online Material Methods Hollow nickel micro-lattice fabrication: Thiol-ene micro-lattice samples were fabricated from an interconnected pattern of selfpropagating photopolymer waveguides as described in detail elsewhere (12,13). The polymer micro-lattice samples were then used as direct templates for electroless nickel plating using a commercially available process (OM Group Inc., Cleveland, OH). Prior to electroless plating, all samples were thermally post-cured at 120°C in air for 12 hrs. To prepare the surface for electroless deposition, the samples were first immersed in an aqueous solution of potassium permanganate and sodium hydroxide, then palladium catalyst was deposited by immersion in activator solution containing hydrochloric acid and tin(II) chloride (Fidelity 1018, OM Group Inc.), followed by an etch in accelerator solution containing fluoboric acid (Fidelity 1019, OM Group Inc.). The samples were then immersed in electroless nickel plating solution with nickel sulfate as nickel source, sodium hypophosphite as reducing agent, and sodium malate and acetic acid as complexing agents (9026M, OM Group Inc.). The electroless nickel plating bath was kept at pH 4.9 by addition of ammonium hydroxide and plating was performed at 80ºC. Different plating times were chosen to achieve different coating thicknesses as reported in Table I. A wall thickness t of 500 nm was achieved by electroless nickel plating of approximately 3 minutes. After nickel deposition the top and bottom surface of each sample was sanded to expose the underlying polymer at each node. The polymer was then chemically etched in a base solution (3M NaOH at 60°C) for 24 hours, creating the hollow tube nickel micro-lattice samples in figure 1. Samples with wall thickness below ~150 nm could not be removed from the aqueous NaOH solution directly because the capillary forces deformed the lattice. In these cases the samples were freeze dried after exchanging the NaOH solution to deionized water and then to t-butanol.

Characterization: The micro-lattice dimensions and weight were measured with a caliper (Mitutoyo Corp.) with an accuracy of 0.01 mm and a balance (Mettler H54) with an accuracy of 0.01 mg.

To assess the microstructure of the electroless nickel films, site-specific transmission electron microscopy (TEM) was performed on a cross-section of the sample prepared by focused ion beam lift-out technique (FIB, FEI Nova 600) and is reported elsewhere (16). The mechanical properties of the thin films on flat substrates were also characterized by nanoindentation (Nanoindenter G200, Agilent Corp., Oak Ridge, TN, USA) with a Berkovich tip, as well as by individual hollow truss member compressions described in detail elsewhere (16). The bulk-level compression tests were performed on a servo-electric INSTRON 8862 frame, equipped with a FastTrack 8800 controller and a National Instrument SCXI Data Acquisition system. The displacement rate was accurately controlled at 0.01mm/s for all tests. The loads were measured by a SENSOTEC load cell with a range of 250N, and the displacements were measured by the internal LVDT embedded in the frame actuator. The accuracy of the internal LVDT for Young’s modulus measurement was verified with an external LVDT: at the low loads experienced by these samples (of the order of 1-30N), the LVDT readings agree to within less than 10%. Hence the same accuracy is expected for the modulus measurements reported in Fig. 3. Figure S1 shows an ultra-light Ni-P sample with 11.3 mg/cc (L: 4662µm, D: 500µm) being compressed to densification. A pristine sample is loaded, unloaded and reloaded four times, increasing the maximum strain every cycle from 50% to full densification. This demonstrates how the stress increased each time the samples is strained beyond the maximum strain reached in previous compression cycles. Once all damaged truss members have deformed around the previously damaged nodes, they come in contact with one another and begin carrying more load, until undamaged regions start carrying the load and the stress approaches levels required to accumulate further damage. In the fourth compression cycle, the sample was strained to densification, which occurs above 90% strain. Movie S1 shows a compression test in real time and illustrates the recoverable deformation. Table S1: Summary of architecture and properties of fabricated micro-lattices

Cell Average Strut Wall Relative Compressive Length Diameter Angle thickness Density Density Stiffness 3 Sample L (µm) D (µm) θ (º) t (nm) (mg/cm ) (%) (kPa) Notes A 4000 500 60 100 0.9 0.01 freeze dried B 4000 500 60 120 1.0 0.01 0.7 freeze dried C 4000 500 60 1.7 0.02 9 D 1050 135 60 100 3.4 0.04 freeze dried E 4000 500 60 4.5 0.06 78 with face sheet F 4000 500 60 650 6.3 0.08 328 with face sheet G 675 120 60 250 11.2 0.14 181 H 4662 500 60 11.3 0.14 473 I 817 170 50 500 14.1 0.18 624 J 1050 150 60 500 14.4 0.18 336 K 1050 140 60 550 16.2 0.20 706 160 60 550 17.1 0.21 1300 with face sheet L 1050 M 1050 150 60 1400 43.1 0.54 9100 did not recover N 4000 500 60 ~5000 55.0 0.69 6000 did not recover

Fig. S1. Compression of a Ni-P micro-lattice with 11.5mg/cc, cell length L: 4mm, diameter D: 500µm. 1. Cycle 50%, 2. Cycle: 70%, 3. Cycle 90% 4. Cycle: 97.5% strain at 172 kPa.

Movie S1. A Ni-P micro-lattice with hollow tube diameter D: 150 µm and wall thickness t: 500 nm is compressed to 50% strain and subsequently unloaded. The movie shows the deformation in real time and demonstrates almost complete recovery. Local buckling starts in the lower part of the sample due to a slight taper in the tube diameter. A stressstrain curve of the compression is provided in Figure 3a (first cycle).

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