Supersonic Cluster Beam Implantation : a new process for biocompatible and stretchable metallization of polymers

The interest for micro- and nanomanufacturing of polymeric materials is continuously increasing driven by different fields such as bioelectronics, flexible optoelectronics and microfluidics for biomedical and chemical analysis systems. The need of polymer-based microdevices incorporating sensing, signal conditioning and actuating functions require the ability to integrate on polymer substrates metallic micrometric electrodes, contacts, wires, circuits and interconnections. The standard approach used for producing such structures is atomic physical vapor deposition of noble metals; this method, although cheap and easily scalable, has poor performances in terms of layer adhesion and attainable lateral resolution. Furthermore such process causes a considerable heating of the sample during the deposition and require the use of pre-treatments of the polymer surface (as for instance the deposition of a Cr layer) in order to promote the adhesion of the metal layer [1]: both these processes can alter the properties (as biocompatibility) of the polymeric substrate. Recently we developed a new method for polymer metallization: the implantation of neutral metal cluster in a polymer substrate. The clusters are produced in the form of a Supersonic Cluster Beam by a Pulsed Microplasma Cluster Source (PMCS) [2] and, thanks to the cluster’s inertia, they are implanted at room temperature in the polymer substrate forming a metal-polymer nanocomposite layer [3]. Unlike atomic physical vapor deposition, we did not alter the polymer surface with chemical or physical treatments in order to improve the adhesion of deposited metal clusters. Furthermore, neither sample heating nor sample charging was induced by the Supersonic Cluster Beam Implantation (SCBI) process [3]. Here we present the application of this process for the fabrication of a biocompatible elastomer-based nanocomposite materials made by gold clusters implanted in a polydimethylsiloxane (PDMS) matrix. The cluster implantation process was monitored during the deposition by measuring the evolution of the electrical properties of the nanocomposite as a function of the amount of deposited clusters [3]. After the deposition, we studied the electro-mechanical performances of the nanocomposite, by measuring the variation of its resistance during uniaxial stretching cycles. Remarkably, the conducting elastomers subjected to 40% strain cycles show finite and reproducible electrical resistance over thousands of cycles (up to 50000). Furthermore, the resistance measured at the point of maximum elongation of the polymer decreases as the number of cycles increases, at odd with what happens in similar experiments on metal coating deposited on the surface of elastomers [1]. All the obtained results give clear evidences of a high adhesion between the implanted conducting traces and the polymer substrate. Next, we carried out preliminary biocompatibility tests: neuronal cells were cultured in vitro both on Au-PDMS nanocomposites and on bare PDMS films used as reference samples. The results have shown that cell adhesion and vitality improve on the nanocomposites in respect to the reference samples (that are already biocompatible), proving the high biocompatibility of this novel material. Finally, we have demonstrated the possibility to use SCBI to pattern high-resolution features on soft and stretchable substrates. Thanks to the very low divergence of the cluster beam produced by the PMCS source (below 1° [2]), we were able to produce on the PDMS substrates conductive patterns with micrometric resolution through standard stencil mask techniques, as for example gold dots with high packing density (dot radius of 15 μm and inter-dot distance of 12 μm). These results indicate that SCBI can be considered a promising tool for the fabrication of conducing patterns on flexible and untreated elastomer substrates, it is compliant with biomechanical and micropatterning constraints and it is capable to assure high biocompatibility to the produced materials, as needed by new classes of stretchable bioelectronic devices. [1] I. M. Graz, D. P. J. Cotton and S. P. Lacour, Appl. Phys. Lett. 94 (2009): 071902. [2] K. Wegner, P. Piseri, H. V. Tafreshi and P. Milani, J. Phys. D: Appl. Phys. 39 (2006): R439–R459. [3] L. Ravagnan, G. Divitini, S. Rebasti, M. Marelli, P. Piseri, P. Milani. J. Phys. D: Appl. Phys. 42 (2009) 082002.