Multifunctional Cellular Solids

Modern materials science and engineering have improved our understanding of materials and allowed us to enhance and modify their physical properties to satisfy our ever-expanding technological needs. Modern materials, however, lack the complex functionalities common in biological systems, such as the self-healing that our skin and bones perform so efficiently.

The Pikul Research Group seeks to develop multifunctional materials systems to achieve mechanical and chemical properties not available in natural or man-made engineering materials. Our toolkit includes expertise in electrochemistry, bottom-up fabrication methods, and the characterization of materials from the mesoscale to the nanoscale.

1. Low-energy room-temperature healing of cellular metals

Structural materials in living organisms can heal fractures at or near room temperature, whereas healing fractured metal requires high temperatures and large energy inputs. By taking inspiration from the transport-mediated healing of bone, we demonstrated rapid, effective, and low-energy healing of metal at room temperature using the electrochemical transport of metal ions within a metallic cellular material.

To realize electrochemical healing, cellular nickel was conformally coated with an insulating and chemically-inert polymer, which allowed healing only at fracture sites where the polymer coating cracked and exposed the underlying nickel. After immersing the cellular nickel into an external electrolyte nickel ions could be transported from an anode to the fractured nickel struts. The combination of ion migration, fast ion diffusion, and the cellular structure enabled 100% strength recovery of 1.6 mm thick fractured samples after as little as 1500 J and 4 h of potentiostatic healing at room temperature. Healed samples fully recovered their strength after being loaded to within 1% strain of total failure, which corresponded to a 350% increase in the fractured nickel strength. In addition, applying an electric current to the samples before catastrophic failure would strengthen the areas which has the largest stress concentrations and increase the samples resistance to subsequent failure.

Healing of cellular nickel with scission damage. Photograph a) and SEM micrograph b) of cellular nickel with F2 failure before healing. c) SEM micrograph of healed cellular nickel showing nickel deposits isolated to the scission vicinity. d–f) Stress–strain data of F2 cellular nickel healed with 0, 500, and 1500 J of electrical energy. Data from a pristine cellular nickel is included in (d) for reference. g) Strength and toughness healing efficiency, eσ and eu, plotted versus electrical energy input. h) The probability of attaining a target strength healing efficiency plotted versus electrical energy input. Connected lines correspond to 50%, 80%, and 100% healing efficiency. i) Fraction of samples that fractured outside the healed scission (B samples) compared to samples fractured at the scission (A samples) as a function of electrical energy input.

In addition, electrochemical healing of cellular nickel at room temperature requires lower energy input than most metal healing techniques. The samples presented here could be healed 50-100 times with an average cell phone battery.

Electrochemistry can heal metals at lower temperature and with less energy than other healing techniques. Temperature during healing plotted versus healing energy input per mm crack length for our work, different reports of metal healing (15–21,24) and two welding methods (31–33). The numbers for each data range correspond to the references which can be found in paper 3 (see publications below).

2. High-strength nanostructured cellular solids

We have developed a nickel-based cellular material which has the strength of titanium and the density of water. The material’s strength arises from size-dependent strengthening of load-bearing nickel struts whose diameter is as small as 17 nm and whose 8 GPa yield strength exceeds that of bulk nickel by up to 4X. The mechanical properties of this material can be controlled by varying the nanometer-scale geometry, with strength varying over the range 90–880 MPa, modulus varying over the range 14–116 GPa, and density varying over the range 880–14500 kg/m3. We refer to this material as a “metallic wood,” because it has the high mechanical strength and chemical stability of metal, as well as a density close to that of natural materials such as wood. In addition, its cellular nature allows for future multifunctional material integration.

Figure 1

Overview of metallic wood. (a) The fabrication process for a unit cell of the nickel inverse opal material. (bg) Cross section SEM images of nickel inverse opal material. (b,c) A nickel inverse opal with no coating. (d,e) A nickel inverse opal material with a 21 nm coating of additional electrodeposited nickel. (f) A nickel inverse opal material with a 25 nm coating of additional electrodeposited rhenium-nickel. (g) A closer image of one of the struts in (f). (h) A 2 cm2 nickel inverse opal material with 500 nm pores and 15 µm thickness grown on a gold/chromium coated glass slide. (i) A nickel inverse opal material with 300 nm pores grown on gold/chromium coated 20 µm thick polyimide.

Figure 3

Mechanical measurements. (a) SEM images of inverse opal micropillars with 500 nm pores, before and after compression testing. The uncoated sample is 84% porous and the 19 nm nickel coated sample is 58% porous. Failure occurs in the [111] direction, parallel to the compression axis. Scale bars are 3 µm. (b) Engineering stress, σ*, versus engineering strain for micropillars under compression. The nickel sample is bulk electroplated nickel. (c) Strut yield strength, σy, as a function of strut diameter, d, for nickel inverse opal materials with 260, 500, and 930 nm pores. Blue data are nickel inverse opals measured with micropillar compression tests. Green data are nickel inverse opals measured with nanoindentation. Orange data are 500 nm pore nickel inverse opals coated with 19 and 33 nm of additional nickel. Shaded regions represent standard deviations. The fit line was only applied to nickel samples with no coatings.

Metallic wood compared to other engineering materials. Ashby plot of material strength, calculated at a 0.2% strain offset, versus density for the fabricated materials and several reference materials. For comparison, common Ti, Al, Ni, and Fe high strength alloys are labeled as follows 1 – CP Ti, 2 – 2024-T4, 3 – Inconel 718, 4 – 7075-T6, 5 – HSSS steel, 6 – Ti-6Al-4V, 7 – Ti-10V-2Fe-3Al. Error bars show standard deviations.


[5] Zhimin Jiang, Zakaria Hsain, and James H. Pikul. “Thick free-standing metallic inverse opals enabled by new insights into the fracture of drying particle films” Langmuir, 2020.

[4] James H. Pikul, Jeffrey W. Long, “Architected Materials for Advanced Electrochemical Systems”, MRS Bulletin, vol. 44, no. 10, October 2019.

[3] Zakaria Hsain, James H. Pikul, “Low-energy Room-temperature Healing of Cellular Metals”, Advanced Functional Materials, in press, August 2019.

[2] James H. Pikul, Sezer Özerinç, Burigede Liu, Runyu Zhang, Paul V. Braun, Vikram S. Deshpande, William P. King, ” High strength metallic wood from nanostructured nickel inverse opal materials”,  Scientific Reports, vol. 9, 2019

[1] James H. Pikul, Sezer Ozerinc, Runyu Zhang, Paul V. Braun, and William P. King, “Micro architected porous material with high strength and controllable stiffness”, IEEE Micro Electro Mechanical Systems Conference 2016, Shanghai.