Wireless capsule endoscopy (WCE) is a powerful tool for medical screening and diagnosis, where a small capsule is swallowed and moved by means of natural peristalsis and gravity through the human gastrointestinal (GI) tract. The camera-integrated capsule allows for visualization of the small intestine, a region which was previously inaccessible to classical flexible endoscopy. As a diagnostic tool, it allows to localize the sources of bleedings in the middle part of the gastrointestinal tract and to identify diseases, such as inflammatory bowel disease (Crohn's disease), polyposis syndrome, and tumors. The screening and diagnostic efficacy of the WCE, especially in the stomach region, is hampered by a variety of technical challenges like the lack of active capsular position and orientation control. Therapeutic functionality is absent in most commercial capsules, due to constraints in capsular volume and energy storage. The possibility of using body-exogenous magnetic fields to guide, orient, power, and operate the capsule and its mechanisms has led to increasing research in Magnetically Guided Capsule Endoscopy (MGCE). This work shortly reviews the history and state-of-art in WCE technology. It highlights the magnetic technologies for advancing diagnostic and therapeutic functionalities of WCE. Not restricting itself to the GI tract, the review further investigates the technological developments in magnetically guided microrobots that can navigate through the various air-and fluid-filled lumina and cavities in the body for minimally invasive medicine.

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The large magnetocaloric effect (MCE), which accompanies the first order ferromagnetic/anti-ferromagnetic transition in CsCl-ordered Fe-Rh alloys, has been investigated by measurements in slowly cycled magnetic fields of up to 2 T in magnitude for a range of temperatures, 300K < T < 350K. A bulk sample with composition Fe 50.4 Rh 49.6 was used and the results were compared with those produced by the ab-initio density functional theory-based disordered local moment (DLM) theory of the MCE. The measurements revealed an irreversibility effect in which the temperature of the material did not return to its initial value following several cycles of the magnetic field. These observations were explained in the framework of the ab-initio theory for the first order transition in which the consequences of the incomplete long range compositional order and small compositional inhomogeneities of the sample were included. The mean value of the long range order parameter S used in the theoretical work was 0.985, close to the value obtained experimentally from XRD measurements. The sample inhomogeneities were modeled by regions in the sample having a distribution of S values with narrow half-width 0.004 about the mean value. The influence of such compositional disorder on both the transition temperature (323.5 K) and MCE adiabatic temperature change (∆Т = 7.5 K) was also studied.

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The current boom of research activity in magnetocaloric materials science is fuelled by the expectation that new advanced refrigerants may be found whose ΔT will significantly surpass that of gadolinium (Gd) metal (2.6–2.9 K/T). Because of this expectation, the main effort in the field has been diverted from the important issues of refrigerator design to the routine characterization of magnetic materials. Estimating the maximum adiabatic temperature change that can be achieved in principle by applying a certain magnetic field, say 1 T, is a matter of priority. In this work the problem of maximum ΔT is approached from general principles. According to the most optimistic estimates, ΔT can never exceed ∼18 K/T, the more realistic upper limit lying somewhere in high single figures. We therefore deem it most unlikely that a refrigerant much better than Gd, in respect of the ΔT value, will ever be found.

Articles you may be interested inComparison of the order of magnetic phase transitions in several magnetocaloric materials using the rescaled universal curve, Banerjee and mean field theory criteria J. Appl. Phys. 117, 17D144 (2015) The field dependence of the adiabatic temperature change ⌬T ad of second order phase transition materials is studied, both theoretically and experimentally. Using scaling laws, it is demonstrated that, at the Curie temperature, the field dependence of ⌬T ad is characterized by H 1/⌬ . Therefore, as the magnetic entropy change ⌬S M follows a H ͑1−␣͒/⌬ power law, these two dependencies coincide only in the case of a mean field model. A phenomenological construction of a universal curve for ⌬T ad is presented, and its theoretical justification is also given. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory ͑extrapolations in field or temperature͒, for enhancing the resolution of the data and as a simple screening procedure for the characterization of materials.

This review of the current state of magnetocalorics is focused on materials exhibiting a giant magnetocaloric response near room temperature. To be economically viable for industrial applications and mass production, materials should have desired useful properties at a reasonable cost and should be safe for humans and the environment during manufacturing, handling, operational use, and after disposal. The discovery of novel materials is followed by a gradual improvement of properties by compositional adjustment and thermal or mechanical treatment. Consequently, with time, good materials become inferior to the best. There are several known classes of inexpensive materials with a giant magnetocaloric effect, and the search continues.

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