Indeed, integrating functionalities into ingestible, untethered robots with active locomotion capabilities can enable a broader range of surgical-free diagnostic and treatment strategies \cite{Sitti2015,Abramson2020,Nadeau2017,Kong2017,kong2019transforming}. Earlier research has demonstrated a wide range of locomotion strategies for small-scale robots, including legged \cite{quirini2008feasibility,xu2019millimeter}, rolling \cite{du2018programmable,formosa2019novel,miyashita2016ingestible,yim2011design}, peristaltic (i.e., earthworm-like)\cite{heung2016design,onal2012origami,phee2002innovative,rafsanjani2018kirigami,wang2013micro,wang2020design,Ze2022}, undulatory \cite{Gilbertson2017,Rogoz2016,Xin2020}, crawling \cite{Bhattacharjee2020,Elder2021,Joyee2019a,Joyee2019,Kim2013,Koh2013,Li2019,Pham2018a,Pham2020,Steiner2019,Steiner2021,Wu2019a}, and other motions \cite{Basar2012,Nelson2010,Ng2021,Runciman2019,Venkiteswaran2019,Diller2014,Ren2021,Du2020,Kim2011,Nam2014}. Among the demonstrated mechanisms, magnetically-controlled actuation is particularly promising because it does not require onboard power or control systems \cite{Nelson2010}, freeing critically-needed space for additional functional integration.
Recent advances have demonstrated the ability of miniature magnetic crawlers to actively transport cargo in complex and confined systems, such as the GI tract, by leveraging magnetic fields to induce locomotion. For instance, Zhao et al. demonstrated a magnetic origami robot that crawled by in-plane contraction \cite{Ze2022} where the anisotropic friction on the robot’s feet enabled forward locomotion that can be steered. Nevertheless, the need of anisotropic friction on the feet also precluded bidirectional locomotion in a confined space, such as in a lumen, where reversing direction by turning in place is challenging. Other recent works demonstrated entirely-soft crawlers with impressive multi-gait bending locomotion that could transport objects by gripping and direct attachment \cite{Hu2018,Wu2022,Xu2022}, including cargos 20 times their mass and three times their volume \cite{Wu2022}. Nevertheless, integrating the existing crawlers with modular electronics is challenging due to the planar and rigid nature of electronics that will impede the robot’s bending motions.
Other recent works demonstrated axisymmetric crawler robots with flexible bodies and magnetic feet. Importantly, the robots were capable of bidirectional undulatory or inchworm-like locomotion in confined lumens when actuated by an external rotating magnetic dipole \cite{Pham2018a,Pham2020,Steiner2021}. The nonuniform fields of the actuation mechanism could facilitate clinical use, as utilizing a rotating permanent magnet eliminates the need to surround the patient with coils \cite{Pham2018a}. Nevertheless, the crawler lacked a centralized space necessary for functional integration without disrupting the robot’s locomotion.
Here, we demonstrate a magnetic robot with localized flexibility (MR-LF) which includes a centralized compartment for functional integration. The compartment is created by localizing the body flexibility of a flexible magnetic crawler in the previous work \cite{Pham2018a,Pham2020,Steiner2021}, while preserving the robot's ingestible size and bidirectional locomotion characteristics. The availability of a centralized compartment enables MR-LF to be readily integrated with modular functional components, such as commercial off-the-shelf electronics, and payloads such as medication. Ultimately, we envision that the integration of sensing, actuation, and drug delivery capabilities into an ingestible robot can address a broad range of unmet clinical needs.