**4. Particle fabrication**

Traditionally, implantable medical devices for neurostimulation have been built with CMOS processes (like other electronic devices). This approach is not scalable to nano-sized products that are needed for noninvasive access. Most of the magnetic particle literature was contributed in the field of bioassays, where particle uniformity is not critical. However, for medical applications (e.g., magnetic particle imaging), lack of particle uniformity is often a limiting factor [21].

**Figure 2.** Example of template-guided shape-engineered synthesis of magnetic particles. Particles are made via sequential processing of polycarbonate track etched (PCTE) membrane films. PCTE films have pores extending through the thickness of the film. Templates (A) are first partially sealed on one surface with a conductive layer (B), followed by deposition of a polymer (e.g. poly-lactic-co-glycolic acid) shell inside the pores of the PCTE (C). Selectively etching the partially sealing conductive layer (D) and replacing it with a completely sealing conductive layer (E) allows for deposition of a conformal gold layer (F), after which a payload (e.g. liquid-crystal-magnetic composite) can be deposited by vacuum impregnation into the sealed pores of the PCTE film (G). Deposition of a final sealing layer (H), followed by selective etching of the conductive sealing layer (I) and removal of PCTE film (J) results in free-floating particles.

We have used template-guided methods to build shape-engineered highly uniform magnetizable particles with features important for transport and effectiveness [22] (**Figure 2**). For example, different sections of the particles can be built with aspect ratios that favor a particular magnetization direction. With appropriate use of precessing magnetic fields, the particles can be drilled through tissue [14]. The template-guided methods are also economical: it is possible to fabricate micromolar quantities of particles for less than \$20 in raw materials. We have evaluated nanoscale spintronic devices for voltage sensing and stimulation, which have very tight tolerances. Transitioning these devices in their current morphologies to templateguided manufacturing (with tolerances of a few nm) may be challenging and may require device redesign.
