A Constructive Chemical Conversation

Alison Grinthal, Wim L Noorduin, Joanna Aizenberg. American Scientist. Volume 104, Issue 4. Jul/Aug 2016.

Long before life covered the Earth with intricately patterned forms, its precursor materials started talking. Showering each other with synthesis byproducts and confronting each other with changing surfaces, they fueled, slowed, blocked, diverted, shaped each other’s growth-answering to each other at every turn as, for better and for worse, they began to shape and orient as one. Generations later, bacteria inherited the dialog, their inner skeletons and outer skins trading signals in winding paths over their growing surfaces as they curved into boomerangs, stars, and corkscrews. But as they multiplied, their diffusing signals transformed their sculpting into a throbbing social scene-the whole group twisting and elongating in sync, sprouting into fractals and labyrinths. As life diversified, packs of nerves, streamlined by continuous dialog with their sheaths, followed their noses in and out of bundles as crosstalk generated landscapes of signals at their tips. Spiraling signal networks, feeding into and out of a chorus of intracellular dialogs, wound plant cells into finely graded spiraling lattices. The entire living world still talks nonstop, spinning live chemical maps of conversations-sketching, navigating, and negotiating their architectural plans in front of themselves as they grow.

What are they talking about? The weather, food-news from the environment is continually taken in, shuttled through the pinball of signals, and channeled into the living map. The feedback network creates a mind-boggling diversity of options at every step, but at the same time imposes checks and balances that keep the system from randomly running wild. As the environment changes-altering growth rates, turning up and down responses-the dialogs recalibrate, reconfigure the landscape of signals and gradients, and invoke hidden worlds of subtle to radical alternatives. Nerves that avoided each other suddenly become smitten; short, fat bacteria shoot into long filaments or sprout two heads; tree cells meander in gradually shifting turns and waves. The environment becomes an inseparable participant in the conversation: Patterns, networks, and communities become stories of their encounters with changing oceans or atmospheres.

Building in the nonliving world is a different story, usually requiring chisels, hammers, and molds. But at the microscale, where our sculpting tools begin to fall short, dialog becomes highly effective. Materials that form together from the start create precise, versatile feedback systems, addressing errors as they happen, coaxing rigid crystals into winding curves, producing complex shapes that neither part could make alone. Yet life suggests that the behaviors we see may be only half the conversation; such systems could be brewing with possibilities and choices unfolding all the time. Landscape architect Anne Spirn has proposed, “To design wisely is to read ongoing dialogues in a place … and to imagine how to join the conversation.” To explore, we started talking.

The periodic table is itself a fascinating monument to the many dialogs that can happen just among the elements, so we started small, with a pair of chemical reactions in a covered beaker. The beaker contained a high-pH solution of sodium metasilicate (Na2SiO3) and barium chloride (BaCI2), with a flat piece of metal-coated glass inside for crystals to grow on (above left). When we opened the lid and let in carbon dioxide (CO2) from the surrounding air, it reacted with barium ions and water and started growing barium carbonate (BaCO3) crystals (above middle, top). The reaction also surrounds the crystals with a halo of acid formed by freed hydrogen ions, which slows the BaCO3 crystals’ growth but triggers silica (SiO2) to deposit on them via a second reaction-which in turn consumes the acid and revives the first reaction (above middle, bottom).

This reciprocal exchange was known to generate various curving shapes, but we envisioned that it would create a much more complex evolution of gradients and rates as acid diffuses within the emerging microscape. The deposition of each material from solution is tuned to pH, but in different ways (barium carbonate precipitates more with increasing pH, but silica prefers a specific range of pH, as shown in the graph above), and at the same time their growth changes the pH around them. A dialog of dialogs–local acid production and consumption, plus exchange between neighboring shapes–would turn the growing surface into a dynamic pH map, navigated at every point by adjusting the balance of reaction rates. Where SiO2 forms, it blocks further growth; where the pH drops too low, both reactions fizzle out; everywhere else, the party would continue with an updated map.

For our first encounter, we provided steady CO2 and discovered a forest of sticks growing in synchrony (above left). This scene was most telling for what we never saw: Sticks never grew toward one another, and they were never fatter or thinner than a standardized width. The first result suggested they were directing growth by talking amongst themselves-exchanging acid, speeding up silica production on their sides, leaving no choice but the express route up. The second suggested the work of an inner dialog-that BaCO3’s and Si02’s requirement to be near each other, in order to clean up and replenish acid, sets strict limits on how wide they can grow.

We took what we never saw as our cue. If we intervened and temporarily altered the balance, the feedback would kick in and restore order, potentially by finding a new form. So we “talked” by opening the lid wider than its usual crack and supplying a burst of CO2, which triggered a spurt of BaCO3 growth at the top of each stick (above middle). We expected that rapidly widening the diameter would create a steep pH gradient across the top surface, with the center stranded far from the SiO2 cleanup mechanism and building up acid faster than it could diffuse away. This picture matched what happened after we returned the lid to its original position: The tops stopped growing in the center and proceeded as rings, recovering regulation thickness all around the rim, with silica forming on the inner and outer surfaces (above right). They continued growing upward, but now each had to contend with acid from inside its own ring as well as from its neighbors, so the shape widened into a vase as it found its new balance.

We could also widen the shapes earlier on by growing them farther apart, and found that they not only responded similarly but produced a range of patterns depending on how nearby their neighbors were, from narrow vases in the suburbs to big corals full of wavy walls in the boonies (right; first and second images from top). The restored thickness was identical everywhere, indicating that the new growth fronts were just as fastidiously kept below the acid gradient threshold as the original. If we gave the growing vases even a small pulse of CO2, enough to introduce a ripple in their width, the feedback immediately slimmed them back all around as soon as we stopped. If we gave a pulse again, the vase rings slimmed themselves again, and we developed a rhythm, sculpting ridged patterns by subtly varying the length and time intervals of our interventions (far right, top image).

Talking to a wall was more complicated. Giving corals the same small pulses uncovered a new set of dialogs, between the ends of each wall (nearly surrounded by SiO2 and built for robust acid turnover) and the main length (with SiO2 on only two sides). This distinction is usually subtle enough to keep the wall advancing as a united front, but even a small CO2 pulse can trigger a feedback cascade in which the rich get richer and the poor get poorer. The ends, better able to weather the acid from increased BaCO3 production, kept growing upward, while the adjacent regions, burdened by extra acid from the faster-growing ends, grew even more slowly. Ultimately the maze of walls turned into a dense field of SiO2-coated spikes, and we grew a porcupine. Once transformed, the porcupine was amenable to rhythmic rippling with subsequent small pulses (far right, second image from top).

Building new forms and patterns would no doubt introduce still more hidden dialogs, so we wanted a precise way to pick and choose, combine, and expand our range-we needed a volume knob. Temperature gave us a way to adjust both our talking and the system’s response threshold. Cooling preloads the solution with CO2 by increasing its solubility, and adjusting the temperature after that lets us modulate its delivery, from slowly over time to fast and concentrated. Cooling also slows down the reactions, giving acid more time to diffuse away without sharp gradients building up. This control allowed us to smoothly widen every feature of the landscape, irregular walls and all (below, second from bottom), giving us more freedom for direct sculpting. Turning up the temperature evoked a grand fireworks of multiple dialogs from even a simple ring-splitting it into concentric rings, suppressing the inner one, and breaking the outer one into a crown of spikes by amplifying even slight asymmetries in the acid fields (below, bottom).

Time for a brief intermission. From flat glass we now had reactions dancing on a stage of ridges, peaks, valleys, and chasms–what alternate stories might play out if we could start a new act on this emerging stage? To change scenes, we ushered the actors out by replacing the solution, which cleared away the acid chatter and supplied reagents to start again (above). Now when we opened the lid and lowered the temperature, a rose blossomed on a cluster of leaves-free of checks and balances, it unfurled its own broad segments while suppressing growth around it (above right, and below). It almost always sprang from the leaf tops, where BaCO3 had just been growing, but a bit of behind-the-curtain work revealed a world of unseen growth sites tucked away inside the crevices. If we ended the first scene by closing the lid, BaC03 growth slowed as CO2 ran out, SiO2 covered the tops as the last acid was produced, and all went quiet (opposite page, top). Upon reopening, stems took root deep in the nooks and crannies—finding the abandoned BaC03 sites where acid buildup had stopped all growth when the structure split into walls or rings.

Stems emerging from solitary caverns find themselves in situations unlike any they encountered when all arose in synchrony. The topography of a leaf cluster or coral shapes a 3D neighborhood, orienting them in all different directions free of any initial feedback (above). By providing a burst of CO2 after they came out, we could now evoke rings of petals, shaped by the intricate acid patterns that arise when they all start talking (right, inset). A single stem in a vase, with neighbors only in other vases, is free to expand spontaneously when it comes out. By meeting it with a CO2 pulse, we could synergize and grow a two-tiered flower (right). A lone stem coming out of a coral-both grown at a low density-brings its established axial symmetry to a boundless open space, enabling us to create a jellyfish with CO2 pulses that successively build flaps, mouth, and tentacles (above right).

We discovered that structures-inprogress can even start growing on and inside each other: When we lowered the pH, the entire scene crossed “through the looking-glass” and neighbors started heading toward each other instead of away (left, top). They still made and navigated acid gradients as before, but superimposing the maps on a higher acid backdrop created an inverse steering situation. Now SiO2 outpaced BaCO3 on the outwardfacing sides but was completely inhibited on the interfacial side, letting BaCO3 grow there even with its reduced rate. This mechanism led neighbors to approach each other’s growth fronts, but rarely with exact aim. Curving was a delicate balance of inner and outer dialogs, with BaCO3 tracking both acid-consuming SiO2 and the neighbor’s SiO2-inhibiting acid. So rather than colliding head-on, the growth fronts almost always climbed and spiraled around each other before finally merging (/eft, inset). After that, their joint growth front continued winding and crawling, following its own echo over the surface.

This twisting growth not only gave us a way to build elaborate sprawling architectures from multiple converging small ones, but also wove unique chemical surface patterns. As BaCO3 walks its fine line between burying itself and following SiO2, it can leave trails of abandoned sites wound in shallow grooves. When we replaced the solution and restored the original pH, now new forms could sprout not only from the tips but also-especially where tips had fused and become covered by SiO2-along the grooves. With successive pH shifts, we could braid a bed of twisted vines and stud them with blossoms (opposite page, top image), many unfolding around and harboring snails-or pattern a crustacean’s segments with curved paddles, tapered plates, and spines- their spacing and shaping precisely controlled by the pitch of the spiral grooves (opposite page, bottom).

The more we explore, the more the chemical dialog turns out to be beautiful in itself: We’re now modeling the coevolution of shapes and diffusion fields, examining how BaCO3 and SiO2 direct each other at the nanoscale, and seeing what changes if we use different elements and reactions. At the same time, fields and navigation come in many forms, some just beginning to be recognized, across all kinds of materials and systems. Chemical diffusion can interact with mechanical stress fields that are generated and relieved as forms grow. Reverse reactions or elasticity introduce local regions of disassembly into the map. Reactions and stress embedded in fully formed materials can even create, bend, and twist microshapes as environmental changes alter the fields. The micro-porcupines, sea monsters, and flowers show us that if we learn to participate in the mapmaking, the creation of microscale shapes and patterns becomes a process of continuously exploring yet never being lost. Ultimately, spires and chambers we create can become a playground for light, fluids, heat, current, vapor-the intricate architectures that make up every inch of us and the rest of life orchestrate the infinitely diverse ways in which we sense, respond to, and harvest energy from the environment. When it comes to designing future materials, discovering and participating in microscale dialogs may be the “disruptive” idea that helps us bring creativity and design to our evolving dialogs in the larger landscape.