(Basar et al., 2012; Mandsberg et al., 2020; Min et al., 2020; Toennies et al., 2010). For example, the pill-shaped PillCam™ provides access to areas of the GI tract which are challenging or infeasible via endoscopic procedures.(Min et al., 2020) However, the size of an ingestible device is fundamentally constrained to enable swallowing (e.g.PillCam™ SB 3 has a diameter of 11.4 mm, and a length of 26.2 mm) and to mitigate the risks of unexpected retention (1.4% for conventional capsule endoscopes)(Li et al., 2008) or intestinal obstruction which requires surgical interventions.(Steiger et al., 2019) The limitation in size constrains the possible functionalities that can be integrated into an ingestible system, especially since active components such as microelectronics are rigid and planar parts that have to be assembled into the system. For example, most ingestible electronics do not have the ability to be actively transported towards target regions of interest.(Min et al., 2020)
Indeed, integrating functionalities into ingestible, untethered robots with active locomotion capabilities can enable a broader range of surgical-free diagnostic and treatment strategies.(Sitti et al., 2015) Earlier research has demonstrated a wide range of locomotion strategies for small-scale robots, including legged,(Quirini et al., 2008, magnetic legged Metin Sitti robot) rolling,(D’Argentre et al., 2018; Formosa et al., 2020; Miyashita et al., 2016; Yim and Sitti, 2012) peristaltic (i.e., earthworm-like),(Heung et al., 2016; Onal et al., 2013; Phee et al., 2002; Rafsanjani et al., 2018; Wang et al., 2013, 2020; Ze et al., 2022) undulatory,(Gilbertson et al., 2017; Rogóż et al., 2016; Xin et al., 2020) crawling,(Bhattacharjee et al., 2020; Elder et al., 2021; Erina B. Joyee and Pan, 2019; Erina Baynojir Joyee and Pan, 2019; Kim et al., 2013; Koh and Cho, 2013; Li et al., 2019; Pham et al., 2020; Pham and Abbott, 2018; Steiner et al., 2019, 2022; Wu et al., 2019) and other motions.(Basar et al., 2012; Nelson et al., 2010; Ng et al., 2021; Runciman et al., 2019; Venkiteswaran et al., 2019) Among the demonstrated mechanisms, magnetically-controlled actuation is particularly promising because it does not require onboard power or control systems,(Nelson et al., 2010) 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(Ze et al., 2022) 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 without rotation. Other recent works demonstrated entirely-soft crawlers with impressive multi-gait bending locomotion that could transport objects by gripping and direct attachment,(Hu et al., 2018; Wu et al., 2022a; Xu et al., 2022) including cargos 20 times their mass and three times their volume.(Wu et al., 2022b) 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.(Pham et al., 2020; Pham and Abbott, 2018; Steiner et al., 2022, 2019) 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.(Pham and Abbott, 2018) Nevertheless, the crawler lacked a centralized space necessary for functional integration without disrupting the robot’s locomotion.
Here, we demonstrate the creation of a centralized compartment for functional integration by localizing body flexibility of a flexible magnetic crawler in the previous work (Pham et al., 2020; Pham and Abbott, 2018; Steiner et al., 2022) 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.