The achievement matters because it resolves a tension that has defined boron chemistry for more than half a century. The known allotropes, including the familiar beta-rhombohedral phase and the high-pressure gamma-B28 form, are built from B12 icosahedra, cage-like clusters of twelve boron atoms that lock the structure into rigid, three-dimensional networks. That architecture delivers hardness and, in some phases, semiconducting behavior, but it also means the material has no easy way to relieve stress. When a conventional boron crystal is loaded past its elastic limit, it cracks rather than bends. Achieving a single boron phase that is simultaneously a good electrical conductor and capable of plastic flow has therefore been an explicit goal, and until now an unmet one, in the field.

The route to Imma-B60 is as interesting as the material itself. Rather than trying to crystallize pure boron directly into the desired framework, the team used a two-step precursor strategy. First, they synthesized a sodium boride, Na4B60, under high-pressure and high-temperature conditions, growing crystals large enough for single-crystal X-ray diffraction. In this compound, sodium atoms occupy channels within a boron framework, acting as a kind of structural scaffold. In the second step, the researchers removed the sodium by degassing the crystals at around 900 degrees Celsius under high vacuum for 48 hours. What remained was a pure boron framework that inherited the geometry of its sodium-containing parent, an approach the authors describe as a powerful way to access metastable materials that cannot be reached by direct synthesis.

The structure that survives the sodium extraction is remarkable in its own right. Imma-B60 adopts an orthorhombic open framework in which B12 icosahedra are interconnected by triangular B3 units through a combination of two-center and three-center sigma bonds. This is not the dense, space-filling arrangement of ordinary boron; it is a porous, cage-like architecture more reminiscent of the open-framework clathrate forms of silicon, which themselves were synthesized by a comparable precursor-removal strategy. The team confirmed the structure through a combination of single-crystal and powder X-ray diffraction, Rietveld refinement, micro-electron diffraction, and scanning transmission electron microscopy, and they benchmarked the diffraction pattern against every reported and predicted boron phase to show that Imma-B60 is genuinely new.

The electronic properties are where the material begins to look transformative. Calculations and measurements show that Imma-B60 has a narrow bandgap of less than 0.2 electronvolts, far smaller than the gap in conventional boron phases. The measured electrical conductivity is approximately 9 times 10 to the 2 siemens per meter, which the authors note is seven orders of magnitude higher than that of beta-boron. In practical terms, a material that was previously an electrical near-insulator has been restructured, without changing its chemical composition, into something approaching a conducting semiconductor. The bonding analysis, performed with a solid-state adaptation of adaptive natural density partitioning, confirms that the framework is held together by predominantly sigma-type bonding, with the open topology leaving the electronic structure in a regime where charge transport becomes facile.

Equally striking is the mechanical behavior. Using in situ uniaxial compression of nanopillars carved from the crystals, oriented along a specific crystallographic direction, the researchers observed that Imma-B60 deforms plastically to strains of about 23 percent. The deformation proceeds through a dislocation-mediated slip mechanism, meaning that planes of atoms slide past one another along defined crystallographic pathways rather than the material shattering. Video recordings of the compression experiments show the nanopillars bending and shearing in a way that looks more like a metal than a covalent nonmetal. Nanoindentation measurements on both the precursor Na4B60 and the final B60 phase provided complementary hardness and elastic modulus data, placing the new material well outside the superhard-but-brittle envelope that has characterized boron to date.

The precursor strategy itself may prove to be the study’s most consequential contribution. The idea of using a guest species, in this case sodium, to template a framework and then removing it to leave a metastable elemental structure echoes the celebrated synthesis of open-framework silicon from sodium silicide clathrates, and it has been proposed as a route to exotic phases of other elements, including polymeric nitrogen and superconducting allotropes of silicon and carbon. By demonstrating the approach for boron, the Yanshan University team adds a new entry to a growing catalog of metastable materials that are accessible only through chemical templating. The sodium diffusion pathway and its energy barrier were mapped computationally, showing how the guest atoms can migrate out of the framework without collapsing it.

The work also required considerable experimental ingenuity at the growth stage. The team found that inserting zinc interlayers between the sodium and boron starting layers in the high-pressure assembly dramatically improved crystal size and quality, a detail they verified with control experiments and scanning electron microscopy of hundreds of crystals. Residual sodium content after degassing was tracked with energy-dispersive spectroscopy and electron energy-loss spectroscopy, and the analysis revealed that lower degassing temperatures or argon-flow conditions left more sodium behind, underscoring how sensitive the final purity, and therefore the properties, are to the extraction protocol. Subtle defect motifs associated with interstitial boron sites were identified in the refined structures, and the authors show that these defects, rather than secondary phases, explain weak additional diffraction features.

For applications, the combination of high conductivity and ductility in a light-element framework opens doors that conventional boron could never approach. Thermoelectric devices, which depend on materials with unusual electronic transport and low thermal conductivity, are an obvious candidate given the narrow bandgap and porous structure; theoretical work has long suggested that doping boron icosahedra could even produce superconductivity. Flexible or deformable electronics, lightweight electrodes, and mechanically resilient semiconducting components are other directions the open framework invites. More broadly, the result is a reminder that the periodic table still holds surprises in its simplest entries: a single element, boron, whose known phases were cataloged over decades, has just yielded a form that is simultaneously transparent to none of its usual weaknesses and rich in properties its usual phases never displayed. The team, led by Xiang-Feng Zhou and Yongjun Tian, with Feng Chen, Lianfeng Zou, and Penghui Li as co-first authors, suggests that the precursor-based strategy can now be extended to hunt for further metastable frameworks, meaning Imma-B60 may be less an endpoint than a template for what comes next.

Subject of Research: Synthesis of a conductive and plastic open-framework boron allotrope via a high-pressure precursor route

Article Title: An open-framework boron allotrope exhibiting high conductivity and plasticity

Article References: Chen, F., Zou, L., Li, P., Fan, C., Liu, Y., Xu, B., Shao, X., Zhang, H., He, X.-L., Huang, M.-X., Chen, F., Nie, A., Ke, F., Zhao, Z., Dong, H., Yang, Y., Weng, X.-J., Wang, H.-T., Zhou, X.-F., & Tian, Y. (2026). An open-framework boron allotrope exhibiting high conductivity and plasticity. Nature Chemistry. https://doi.org/10.1038/s41557-026-02267-7

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02267-7

Keywords: boron, allotrope, open framework, high pressure, plasticity, electrical conductivity, B12 icosahedra, sodium boride, dislocation slip, metastable materials, Nature Chemistry, bandgap

Perineuronal nets, or PNNs, are specialized matrix structures composed of chondroitin sulfate proteoglycans, hyaluronan, and link proteins that encase the cell bodies and proximal dendrites of certain classes of neurons, most prominently fast-spiking parvalbumin interneurons. These nets are far more than passive scaffolding. They are thought to regulate the balance of excitation and inhibition in neural circuits and to govern the closure of critical periods of plasticity, the windows during which the brain is exquisitely malleable. When that balance is disturbed, the consequences can be profound: altered excitation-inhibition ratios and disrupted PNN formation have both been implicated in neurodevelopmental disorders, including autism spectrum disorder, a condition diagnosed substantially more often in biologically male children and adults. Understanding when and how PNNs normally appear, therefore, is not merely an exercise in descriptive neuroanatomy; it could illuminate when the developing brain is most vulnerable to insult.

Yet most of what scientists know about PNNs comes from studies of adolescent and adult animals, long after the nets have consolidated. Very little research has charted the typical, day-by-day emergence of PNNs and the parvalbumin interneurons they envelop during the earlier stretches of development. The research team, led by Abigail W. Hamilton and Susan L. Zup, set out to fill that gap by building a precise timeline of PNN onset in juvenile Sprague Dawley rats across a narrow but crucial developmental span: postnatal days 14, 16, 18, and 22. They focused on the dorsal hippocampus, a region central to learning, memory, and stress regulation, examining five distinct subregions: the subiculum, cornu ammonis 1 (CA1), CA2, CA3, and the dentate gyrus.

The methodology was straightforward but demanding in its precision. Rats at each of the four ages were transcardially perfused, and their brains were sectioned and processed for immunofluorescence staining targeting both perineuronal nets and parvalbumin interneurons. By labeling these two features simultaneously, the researchers could count net-bearing neurons in each hippocampal subregion at each time point and, crucially, compare counts between males and females. This fine-grained sampling at two-day intervals allowed the team to capture developmental dynamics that coarser, week-long sampling intervals might have smoothed away entirely.

The results upended the intuitive assumption that PNN numbers climb steadily as the juvenile brain matures. Instead, the researchers found that PNN counts fluctuated across the p14 to p22 window in a consistent pattern of decline followed by rebound, rather than increasing linearly. Remarkably, male and female rats followed the same general trajectory, but on shifted schedules. Female rats showed a dip in perineuronal net number at postnatal day 16 before the counts rebounded, whereas males exhibited a similarly steep decrease two days later, at postnatal day 18, before their own recovery. In other words, the female developmental program appeared to run ahead of the male one, completing the same sequence of net loss and net gain several days earlier.

Across subregions of the hippocampus, the study also documented a broader sex difference: males generally had fewer perineuronal nets than females. This asymmetry is particularly intriguing in light of the clinical epidemiology of autism spectrum disorder, which is more prevalent in biologically male children and adults. Because PNNs help regulate the excitation-inhibition balance whose disruption is associated with the disorder, the authors suggest that differences in the timing and density of net formation between the sexes could contribute to differential vulnerability. A male brain whose PNN maturation lags behind a female’s may spend a longer stretch of development in a state where its inhibitory circuitry and plasticity controls are still in flux, opening more or longer windows during which environmental, genetic, or biochemical insults could derail typical trajectories.

The fluctuations themselves carry implications for how neuroscientists should think about early brain development. If PNN numbers can drop sharply within a matter of days and then rebound, the developing hippocampus cannot be viewed as a structure steadily locking in its inhibitory architecture. Instead, it appears to pass through alternating phases in which nets are pruned and re-established, potentially reflecting active remodeling of parvalbumin interneuron circuitry during the juvenile period. Each phase of net reduction may correspond to a period of heightened plasticity, when circuits are more open to modification, and each phase of net re-accumulation to a period in which those modifications are being consolidated. Sex differences in the timing of these phases would mean that males and females are not equally susceptible to the same disruption at the same chronological age.

The study’s authors emphasize that these data point to windows of both increased and decreased vulnerability to neurons and neuronal circuitry during early neurodevelopment, windows that are specific to sex. For researchers modeling neurodevelopmental disorders in animals, the message is that the choice of developmental time point is not neutral. A postnatal day 16 female and a postnatal day 18 male may occupy comparable positions in their respective PNN trajectories, while a male and female of the same chronological age may be at entirely different stages. Experimental interventions, environmental manipulations, or pharmacological challenges delivered on identical schedules could therefore produce systematically different outcomes in the two sexes, a confound that precise developmental timelines like this one can now help investigators control for.

More broadly, the work adds to the growing recognition that sex differences are not peripheral curiosities in neuroscience but fundamental features of brain development that deserve systematic study from the earliest ages. By mapping the fluctuating emergence of perineuronal nets and their parvalbumin interneuron partners across the dorsal hippocampus, the University of Massachusetts Boston team has provided a reference framework against which future studies of atypical development can be measured. The research was funded by the University of Massachusetts Boston’s Program Development Grant and College of Liberal Arts Research Funds, and all experiments were approved by the institution’s Institutional Animal Care and Use Committee. As the field continues to probe the developmental roots of conditions like autism spectrum disorder, this sex- and age-resolved atlas of early hippocampal PNN development offers both a caution against assuming steady linear maturation and a promising map of the moments when the juvenile brain may be most, or least, open to lasting change.

Beyond the immediate findings, the study sits within a broader methodological shift in neuroscience. For decades, preclinical research relied disproportionately on male animals, partly on the assumption that cyclic hormonal variation in females would introduce unwanted variability. That assumption has been repeatedly challenged, and funding agencies now routinely require attention to sex as a biological variable. The present work illustrates why such policies matter: had the investigators examined only one sex, or only a single developmental time point, the fluctuating, sex-shifted trajectory of perineuronal net development would have been invisible. A study sampling at postnatal day 16 alone, for instance, would have concluded that females show reduced net density relative to earlier ages, while a day 18 sample would have suggested the same for males, with neither result revealing the underlying shared pattern.

The choice of the dorsal hippocampus also carries significance for interpretation. This region receives extensive attention in research on learning, memory consolidation, and the physiological stress response, and its parvalbumin interneuron population is known to mature gradually across the juvenile period. Parvalbumin cells are fast-spiking inhibitory neurons that depend on precise excitatory input and on metabolic and structural support from the extracellular matrix that surrounds them. Perineuronal nets are believed to stabilize the synapses formed on these cells and to contribute to the maturation of their characteristic rapid firing properties, so fluctuations in net number during the juvenile window may reflect ongoing reorganization of inhibitory microcircuitry rather than simple cell loss or gain.

It is also worth noting what the study does and does not establish. The work documents correlations between age, sex, and net abundance in a typical laboratory strain; it does not demonstrate a causal link between perineuronal net timing and any disorder. Autism spectrum disorder in humans involves many genetic and environmental influences, and rodent hippocampal development, while homologous in broad outline, cannot be mapped directly onto human clinical outcomes. What the data provide is a normative reference: a detailed baseline against which future studies of genetically modified animals, early-life stress models, or environmental exposures can be compared. Such baselines are essential for detecting whether an experimental manipulation accelerates, delays, or flattens the normal developmental curve in one sex but not the other.

The article itself was published as open access under a Creative Commons Attribution license, making the full dataset and methods available to any laboratory without subscription barriers. The version released in early September 2026 is a citable accepted manuscript carrying a permanent digital object identifier, with a final version of record to follow after routine production edits. For a field increasingly attentive to reproducibility, the combination of fine-grained age sampling, dual immunofluorescence labeling, and sex-balanced design offers a template for how descriptive developmental neuroanatomy can be made both rigorous and broadly reusable.

Subject of Research: Sex differences in the developmental timing of perineuronal nets in the rat hippocampus

Article Title: Sex and age impact the developmental trajectory of perineuronal net number in the rat dorsal hippocampus

Article References: Hamilton, A. W., Pritchard, E. K., Hunter III, R. G., & Zup, S. L. (2026). Sex and age impact the developmental trajectory of perineuronal net number in the rat dorsal hippocampus. Biology of Sex Differences. https://doi.org/10.1186/s13293-026-00961-2

Image Credits: AI Generated

DOI: 10.1186/s13293-026-00961-2

Keywords: perineuronal nets, hippocampus, sex differences, neurodevelopment, parvalbumin interneurons, autism spectrum disorder, rats, excitation-inhibition balance, critical periods, postnatal development, extracellular matrix, plasticity