A Biodegradable Flexible Porous Silicon Nanoneedles Platform for Topical Delivery of Biologicals

Vertically aligned porous Si nanoneedles (pSi NNs) arrays have attracted great interest in biomedicine and is capable for intracellular and intratissue delivery of biomolecules. Extensive research has focused on the construction of pSi NNs on rigid and opaque Si carrier, which restricts potential benefits and limits the functionality of the NNs array in clinical systems. The rigidity of NNs arrays makes the application to tissue less effective as they cannot conform to the non-planar tissue surface, resulting in a mechanical mismatch. Also, the opacity of NNs arrays restrict the possibility of tracking dynamic cellular processes at the nanoneedle interface and real-time assessment of cell behaviour. Herein, we report a robust, rapid and precise methodology to generate well-ordered vertical pSi NNs on flexible and transparent substrates with desired flexibility, tuneable optical transparency, good cell and tissue compatibility. These devices are manufactured by first forming porous silicon nanoneedles on Si carriers through metal assisted chemical etching (MACE) and reactive ion etching (RIE), followed by the formation of an underlying porous silicon and a detachment layer by electrochemical etching (EC), to transfer NNs onto recipient substrates. We explored the role of synthesis parameters including MACE, electrochemical etching and RIE conditions in controlling the porosity, transparency, and geometry (length, diameter, density) of pSi NNs to understand the relationships among the structure, property and performance of NNs to achieve the desired scalability, stability and controllability. This method can generate pSi NNs onto different receiving platforms to unlock the potential of nanoneedles on arbitrary substrates, while preserving the original spatial arrangement (i.e., vertical orientation, geometry, density), while providing a mechanically elastic interface that compensates the large mismatch in contact with the biological systems, allowing for the interaction of NNs for intracellular drug delivery. The tuneable transparency enables the direct observation of real-time interactions which are hampered by the opaque carrier. <i>In vitro</i> data showed the efficient delivery of biomolecules into living biological cells and tissues, indicating the utility of this flexible pSi NNs platform for therapeutic applications. Comprehensive demonstrations of various recipient substrates (e.g., polydimethylsiloxane, hydrogel, polylactic acid, medical bandages) indicated this proposed method to be readily adaptable for facilitating the fabrication of high-density nanostructures onto diverse substrates, not limit to the flat surface, to shed the lights on the future applications including clinical systems, nanomedicines, drug delivery, tissue engineering and implantable devices.