Scalable Scheme for Spatially Selective Actuation of Living Microrobots, Demonstrated by Colonization of Tumor Spheroids

The ability to minimize off-target effects is a desirable feature for drug delivery platforms. This is often realized by either localizing the accumulation of active compounds to the target site or by selectively activating the portion that arrives in the targeted tissue. In the context of bacterial therapeutics, it is rational to ensure localized delivery since any off-target accumulation may result in complications associated with their toxicity. Simultaneously, due to their tumor tropism, local amplification at the tumor site can act as a form of selective activation. However, targeted accumulation of bacteria equipped with onboard sensing is contingent on the ability of their innate propulsive forces to overcome biological barriers. As a result, strategies for targeted introduction of external energy can potentially offer a much-needed element for enhanced selectivity of these living therapeutics. Magnetotactic bacteria (MTB) that biomineralize iron-rich nanocrystals provides this opportunity owing to their intrinsic responsiveness to magnetic fields.<br/>We report on a spatially selective actuation scheme for the remote manipulation of MTB as a model system for magnetically responsive delivery vehicles. This actuation strategy is realized by superimposing a magnetostatic selection field onto a rotating magnetic field (RMF). The spatially controlled bacterial accumulation in tumor spheroids as a physiologically relevant model of cancer is studied. Human breast cancer cells, MCF-7, are used to form spheroids of roughly 400 in diameter. Spheroids are then placed inside separate wells containing MTB suspension within a 1.5cm × 1.5cm workspace. Since spheroid colonization relies on penetration of bacteria into these 3D cancer models, we first verify spatial selectivity of the actuation of MTB in response to the selection field by measuring their translational velocity inside the wells. Results under 12 mT RMF at 14 Hz superimposed by the gating field from small magnets demonstrate the suppression of torque-driven motion of MTB. The same experiment with the stronger magnetostatic field improves the level of off target suppression but also reduces the extent of actuation within the target well.<br/>While the main resistance force and torque arise primarily from viscous drags for actuation in suspension, the spheroid presents an additional barrier for successful penetration. To account for the added resistance, prepared samples with spheroids are exposed to RMF of 20 mT and 14 Hz for 1 hour. Small NdFeB block magnets are responsible for the gating field in all the spatially selective actuations. Following magnetic actuation, the tumor spheroids are washed thoroughly in culture media and then incubated for 24 hours. Staining bacteria with a proliferative day allows tracking of daughter cells within the course of the experiment.<br/>Quantification of the stained bacteria demonstrates significantly higher accumulation (2.3 fold) when exposed to the RMF compared to control. Most importantly, MTB colonization of the target spheroids is shown to not experience any drop under application of the selection field, resulting in an approximately 3-fold increase compared to the control. However, at off-target sites, a significantly lower accumulation is observed in the presence of a superimposed gating field. The trade-off between the overall suppression and selectivity is caused by the field gradients present in the miniaturized system. While stronger static fields lead to complete suppression of the off-target transport, the associated field gradients cause forces that act against the transport to the target. However, scaled-up setups with larger targeting areas feature lower field gradients which makes this undesired effect less physiologically relevant. Owing to the scalability and selectivity of the proposed control strategy, the progress towards the clinical translation of magnetically based delivery strategies can be facilitated.