© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION 3D Porous Graphene by Low-Temperature Plasma Welding for Bone Implants Dibyendu Chakravarty, Chandra Sekhar Tiwary,* Cristano F. Woellner, Sruthi Radhakrishnan, Soumya Vinod, Sehmus Ozden, Pedro Alves da Silva Autreto, Sanjit Bhowmick, Syed Asif, Sendurai A Mani, Douglas S. Galvao,* and Pulickel M. Ajayan* Dr. D. Chakravarty International Advanced Research Center for Powder Metallurgy and New Materials (ARCI) Balapur P.O., Hyderabad 500005, Telangana, India Dr. C. S. Tiwary, S. Radhakrishnan, Dr. S. Vinod, Dr. S. Ozden, Prof. P. M. Ajayan Department of Materials Science and Nano Engineering Rice University Houston, TX 77005, USA E-mail: cst.iisc@gmail.com; ajayan@rice.edu Dr. C. F. Woellner, Dr. P. A. da S. Autreto, Prof. D. S. Galvao Instituto de Fisica ‘Gleb Wataghin’ Universidade Estadual de Campinas, Unicamp CP 6165, 13083-970, Campinas, Sao Paulo, Brazil E-mail: galvao@ifi.unicamp.br S. Radhakrishnan, Prof. S. A. Mani Department of Translational Molecular Pathology The University of Texas MD Anderson Cancer Center Houston 77005, TX, USA Dr. S. Bhowmick, Dr. S. Asif Hysitron, Inc. Minneapolis, MN 55344, USA Dr. P. A. da S. Autreto Federal University of ABC Center of Natural Human Science Santo Andre, SP, Brazil DOI: 10.1002/adma.201603146 the 3D network, and even create junctions between adjacent graphene sheets or individual CNTs for use in a wide range of applications. In-plane highly covalent bonding in graphene and graphene-based materials yield abnormally high mechan- ical properties; however, a 3D network of the same material shows a marked decrease in properties due to the weak van der Waals and π–π interactions between the sheets. [13] Therefore, it is important to device methods to provide strong bonding between the sheets or layers in graphene-based scaffolds to impart sufficiently high strength, to go along with their high elasticity, porosity, and modulus for use in practical applica- tions. [14] A possible technique of achieving this, by retaining the nanostructure, is by welding of the nanosheets, nanow- ires, or nanotubes by imparting instantaneous high energy through electric field, radiation, heating, or by chemical modi- fications. [15–18] Welding occurs in the micro level at junctions of the individual layers or sheets and gradually builds up the 3D macrostructure. In this work, we report the use of spark plasma sintering (SPS) technique as a possible method for welding graphene sheets into 3D scaffolds for biological applications. SPS is a high pulsed current, low voltage, plasma discharging process which can generate highly localized Joule heating in very short times. The high current density is concentrated at particle necks or joints leading to instantaneous over-shooting of temperatures to several hundred degrees. [19] There are few reports on the con- solidation of CNTs and CNT-composites using SPS; [20–22] how- ever, there are no reports on the use of SPS as a tool for welding graphene sheets into strong 3D porous structures. It is to be mentioned that the CNT-based composites in SPS [20,21] were dense materials for structural applications unlike the current work where we intentionally retained a porous microstructure. CNT interconnects were also fabricated in SPS in the tempera- ture range 1000—1400 °C. [22] It is expected that the high pulsed current during SPS impinge on the graphene sheets and weld them at the junctions thereby developing the 3D network, as shown in the schematic in Figure 1a. The inherent densification mechanisms of the SPS technique enable the sintering process to be completed in ≈2–3 min thereby making it economically viable; besides, complex shapes can be developed by designing requisite graphite molds, paving the way for easy scalability. The strength of the developed 3D network having porosities in excess of 40% was estimated experimentally and validated by molecular dynamic (MD) simulations. The mechanical testing of the welded interfaces on a micro level was performed by in situ nanoindentation technique inside the scanning electron Nanoengineered 3D carbon-based porous scaffolds with unique microstructures garnered visibility in the recent past and have been envisaged for numerous novel applications, primarily in the two most arduous challenges before mankind today, namely, environment and energy, [1,2] besides being used in other fields such as electronics, catalysis, and sensors. [3,4] Mate- rials such as graphene, functionalized graphene oxide, carbon nanotubes (CNTs)) (single and multi-walled), aerogels, amor- phous carbon, etc., have yielded encouraging results for use as 3D macroscopic, low-density, high-porosity units for struc- tural and functional applications. [5–8] Novel 3D shapes have been developed using chemical, mechanical, or second phase reinforcements for use in water purifiers, foams, sensors, etc., using bottom-up methods like solution processing and chem- ical vapor deposition (CVD). [9–12] However, these processes yield products having poor modulus (<10 MPa) and stiffness values (a few hundred Newtons); besides, they are also limited by their cost and scalability. To develop 3D C-based scaffolds it is imperative to create strong intermolecular bonding to enable structural stability in Adv. Mater. 2016, DOI: 10.1002/adma.201603146 www.advmat.de www.MaterialsViews.com