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1. Stronger Artificial Muscles, with a Twist

   Tawfick, S and (July 2019, Science)

   Heat engines have long been used for transportation, but electric motors, which deliver clean mechanical actuation and come in a wide range of sizes, have enabled the spread of automated motion used, for example, in pumps, compressors, fans, and escalators. Nonetheless, natural muscles, their biological counterparts, are far more ubiquitous. Human bodies have more than 600 muscles that drive functions such as heartbeat, facial expressions, and locomotion. There are many opportunities for expanding the use of actuators that mimic muscles by directly using electric, thermal, or chemical energy to generate motion and enable more pervasive automation. In this issue, on pages 150, 155, and 145, Mu et al. (1), Yuan et al. (2), and Kanik et al. (3), respectively, describe new types of fiber-shaped artificial muscles that exploit advantages derived from the mechanics of twisted and coiled geometries. 

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2. What determines the rest frame of bubble nucleation?

   https://doi.org/10.1088/1475-7516/2023/11/084  

   Chen, Yilin; Vilenkin, Alexander (November 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract We revisit the question addressed in recent papers by Garriga et al.: what determines the rest frame of pair nucleation in a constant electric field? The conclusion reached in these papers is that pairs are observed to nucleate at rest in the rest frame of the detector which is used to detect the pairs. A similar conclusion should apply to bubble nucleation in a false vacuum. This conclusion however is subject to doubt due to the unphysical nature of the model of a constant eternal electric field that was used by Garriga et al. The number density of pairs in such a field would be infinite at any finite time. Here we address the same question in a more realistic model where the electric field is turned on at a finite timet₀in the past. The process of turning on the field breaks the Lorentz invariance of the model and could in principle influence the frame of pair nucleation. We find however that the conclusion of Garriga et al. still holds in the limitt₀ → -∞. This shows that the setup process of the electric field does not have a lasting effect on the observed rest frame of pair nucleation. On the other hand, the electric current and charge density due to the pairs are determined by the way in which the electric field was turned on. 

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3. Variability in locomotor dynamics reveals the critical role of feedback in task control

   https://doi.org/10.7554/eLife.51219  

   Uyanik, Ismail; Sefati, Shahin; Stamper, Sarah A; Cho, Kyoung-A; Ankarali, M Mert; Fortune, Eric S; Cowan, Noah J (January 2020, eLife)

   null (Ed.)

   People come in different shapes and sizes, but most will perform similarly well if asked to complete a task requiring fine manual dexterity – such as holding a pen or picking up a single grape. How can different individuals, with different sized hands and muscles, produce such similar movements? One explanation is that an individual’s brain and nervous system become precisely tuned to mechanics of the body’s muscles and skeleton. An alternative explanation is that brain and nervous system use a more “robust” control policy that can compensate for differences in the body by relying on feedback from the senses to guide the movements. To distinguish between these two explanations, Uyanik et al. turned to weakly electric freshwater fish known as glass knifefish. These fish seek refuge within root systems, reed grass and among other objects in the water. They swim backwards and forwards to stay hidden despite constantly changing currents. Each fish shuttles back and forth by moving a long ribbon-like fin on the underside of its body. Uyanik et al. measured the movements of the ribbon fin under controlled conditions in the laboratory, and then used the data to create computer models of the brain and body of each fish. The models of each fish’s brain and body were quite different. To study how the brain interacts with the body, Uyanik et al. then conducted experiments reminiscent of those described in the story of Frankenstein and transplanted the brain from each computer model into the body of different model fish. These “brain swaps” had almost no effect on the model’s simulated swimming behavior. Instead, these “Frankenfish” used sensory feedback to compensate for any mismatch between their brain and body. This suggests that, for some behaviors, an animal’s brain does not need to be precisely tuned to the specific characteristics of its body. Instead, robust control of movement relies on many seemingly redundant systems that provide sensory feedback. This has implications for the field of robotics. It further suggests that when designing robots, engineers should prioritize enabling the robots to use sensory feedback to cope with unexpected events, a well-known idea in control engineering. 

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4. Trigeminal Ganglion and Trigeminal Nerves in Gbx2 Neo/neo Mouse Mutants

   https://doi.org/10.1002/dvdy.632  

   Ziermann, Janine M; Johnson, Nicholas R; Correa-Alfonzo, Paola; Aquino_Colon, Andres; Waters, Samuel T (July 2023, Developmental Dynamics)

   A key player in brain and neural crest development is the gastrulation-brain-homeobox (Gbx) transcription factor family member, Gbx2. During the early stages of gastrulation, Gbx2 RNA is broadly expressed in the prospective hindbrain and posterior region of the embryo. Later it becomes restricted to a sharp transverse band at the interface between the prospective midbrain and hindbrain, and is maintained in the anterior hindbrain in the developing neuroaxis (Bouillet et al. 1995; Li & Joyner 2001; Martinez-Barbera et al. 2001). Gbx2 regulates diverse developmental processes, including anteroposterior patterning within the mid-/hindbrain boundary and anterior hindbrain (Burroughs-Garcia et al. 2011). Expression of Gbx2 is required for the correct formation of rhombomeres r1-r3 (Wassarman et al. 1997). Loss of Gbx2 function in mouse embryos (Gbx2-/-), results in aberrant neural crest cell patterning leading to defects in neural crest derivatives and to abnormalities in the central nervous system, craniofacial, and cardiovascular components (Byrd & Meyers 2005). Li et al. (2009) demonstrated that Gbx2 is a direct target of the neural crest inducer Wnt, and is essential for neural crest induction. Together, these studies show that Gbx2 resides upstream in the genetic cascade controlling neural crest development and directly regulates the expression of key molecules involved in the migration and survival of neural crest cells that differentiate into neural and other components (e.g., connective tissue) of the head and heart. It was shown that Gbx2neo/neo mouse embryos, in which wild-type levels of Gbx2 expression is reduced to 6-10% of normal, are useful to further elucidate the complexity concerning the role of Gbx2 in anterior hindbrain development (Waters & Lewandoski 2006). Among other malformations, in Gbx2neo/neo embryos the mandibular branch of the trigeminal nerve (CNV3) is absent. CNV3 innervates the muscles of mastication (e.g., pterygoids, masseter, temporalis). However these muscles are needed to suckle and neonate (P0) Gbx2neo/neo mice are not able to suckle and die perinatally (Langenbach & van Eijden 2001). Here we describe the anatomy of the trigeminal ganglion and the trigeminal nerves in neonate Gbx2neo/neo mice and evaluate if there are differences in the muscles of mastication in these mice as compared to wildtype specimens. We expected that we find clear abnormalities in the thickness of the masseter, temporalis, and other muscles innervated by CNV3. However, this is not the case, indicating that the innervation of a muscle is not, as previously thought, needed for the differentiation of the muscles. Histological analyses will give insights into the muscle cell structure and if this is altered in the Gbx2neo/neo mice, which could be related to the loss of motor innervation. The research was funded by NSF EiR HBUC 18-522 awarded to JMZ (#2000005) and STW (#1956450). Bouillet et al. (1995). Dev Dyn, 204: 372-82. Burroughs-Garcia et al. (2011). Dev Dyn, 240: 828-38. Byrd & Meyers (2005). Dev Biol, 284: 233-45. Langenbach & van Eijden(2001). Am Zool, 41: 1338-51. Li et al. (2009). Development, 136: 3267-78. Li & Joyner (2001). Development, 128: 4979-91. Martinez-Barbera et al. (2001). Development, 128: 4789-800. Wassarman et al. (1997). Development, 124: 2923-34. Waters & Lewandoski (2006) Development, 133: 1991-2000. Funding or Support Information: The research was funded by NSF EiR HBUC 18-522 awarded to JMZ (#2000005) and STW(#1956450). 

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5. Two-dimensional ferroelectricity by design

   https://doi.org/10.1126/science.abi7296  

   Tsymbal, Evgeny Y. (June 2021, Science)

   The discovery of ferroelectricity marks its 100th anniversary this year ( 1 ), and this phenomenon continues to enrich our understanding of many fields of physics and material science, as well as creating subfields on its own. All of the ferroelectrics discovered have been limited to those exhibiting a polar space group of the bulk crystal that supports two or more topologically equivalent variants with different orientations of electric polarization. On pages 1458 and 1462 of this issue, Yasuda et al. ( 2 ) and Vizner Stern et al. ( 3 ), respectively, show that ferroelectricity can be engineered by artificially stacking a nonpolar in bulk, two-dimensional (2D) material, boron nitride (BN). A relatively weak van der Waals (vdW) coupling between the adjacent BN monolayers allows their parallel alignment in a metastable non-centrosymmetric coordination supporting 2D ferroelectricity with an out-of-plane electric polarization. These findings open opportunities to design 2D ferroelectrics out of parent nonpolar compounds. 

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