Category: Recent news

Fearsome flaviviruses: A very short note in a recent issue of Science (Vol. 359: 530, 2 Feb. 2018) cites a study from the journal Science Translational Medicine.  Recall the dramatic effects of the zika virus that can infect the developing brain tissue in embryos and fetuses, causing death or, heartbreakingly, brain malformations, notably microcephaly.  Now it appears that West Nile virus (already present here in the US) and Powassan virus may have similar capabilities.  They can grow in the tissues taken from the mother or the fetus.

Establishment of a reservoir of these viruses in a geographic area such as ours is often conditioned on having their insect vectors –  especially mosquitoes – sharing the virus between humans and some forest animals – that is, a sylvatic cycle.  I have more information on this, provided by virus researcher Prof. Kathryn Hanley, in a recording of her visit to me in the KTAL LP FM studio here in Las Cruces, NM.  I made it into a YouTube video.

Densified wood: In a very recent issue of Nature (Vol 554: 224-228), authors Jianwei Song and others reported that they were able to make wood into a very dense, very strong and tough material.  They removed some of the lignin polymer and carefully crushed the remainder, mostly cellulose.  The density increased from 0.42 grams per cubic centimeter to about three times that, denser than water.  Basically, they collapsed the open conduits of wood, the xylem vessels that carry water and nutrients upward from the soil.  They were able to layer it in alternating directions of the former grain, like plywood.  Its strength (stress needed to break it) exceeds that of even high-strength steel .  Interestingly, its toughness also increased (this is the energy or work needed to break it); ordinarily, toughness goes down as strength increases (e.g., see my post about spider silk).  They had an interesting demo of toughness with a projectile shot into it.

It’s premature to say this will replace steel as a structural material in many applications.  For one, densified wood swells alarmingly at high humidity, by 8.4% after 128 hours in 95% relative humidity.  It’s not dimensionally stable then.  One topic I didn’t see addressed is its anisotropy – its properties vary with the direction of applied stress.  Even layered in alternating directions like plywood, in the third dimension, parallel to the original grains, I’d expect it to be easier to disrupt – to delaminate, as it were.

Densified wood also is not more resilient than steel.  After hitting the highest stress that it can tolerate, it breaks down bit by bit at higher strains (relative extension).

Stay tuned.

 

Mutations in our brains as they develop in our time as fetuses: In a very recent issue of Science (Vol. 359: 550-555; 2 Feb. 2018), authors Taejeong Bae and others reported that we accumulate a lot of genetic mutations in our individual brain cells as we develop.   They found different mutations in each cell, and 200 to 400 of them, on average, accumulating over the age of the fetus. They looked for very basic types of mutations, changes of one DNA base for a different one, termed single-nucleotide variants.  (That is, they did not look at mutations that deleted or inserted stretches of DNA.) Nearly half of the mutations occurred in parts of our DNA related to brain function (vs. other organs, though the functions overlapped). Clearly, we still function with these changes in all our neurons.  Granted, many variations in DNA bases don’t affect a protein that the cells make (the genetic code is redundant – several different sets of three bases specify the same amino acid), or, for stretches of DNA that don’t make proteins but interact with genes to regulate their degree of expression, they many not change that regulatory function much.

Still, these mutations are quite abundant – 50 times more per cell than in our adult cells of the liver, colon, and intestine, and almost 1000 times more than in our germ cells (eggs and sperm).  Of course, the latter resistance to mutation is a good thing.  While mutations that don’t disable us or kill us are the source of our evolution of function, including our oversized brains themselves, too many mutations reduce our biological fitness.

The mutations in our young, developing brains resemble those in cancers.  The authors take this to indicate that these “normal” (my word) mutations are part of the background for cancer.  They attribute the high rate of mutation to oxidative stress and to a high rate of cell division (faster is sloppier in copying DNA, then) during the stages called pregastrulation and neurogenesis.

The variations between cells that must occur remind me of quips about the UNIX and Linux operating systems – everyone comes with a different version and claims they’re all equivalent.  I wonder how non-equivalances among neurons affect how we think.

The variations also remind me of the wonder that our extremely complex bodies with so many controls to go awry (hormones, nerves, enzyme complements, basic development) almost always function well or even very well.  I have to skip over the 2/3 or so of conceptions that lead to death of the embryo – some errors are just too big.  We’re the lucky ones.

Recently, astronomers using the European Southern Observatory telescope in northern Chile, detected a planet orbiting the star nearest to our solar system, Proxima Centauri.  The way they detected the planet, by the tiny wobble it causes the star along our direction of view, is very interesting, though not the topic I’m following up here. The star and its planet are 4.25 light-years away, so, still an extreme distance for any probe to reach that can hit even the highest speeds our spacecraft attain.  Nonetheless, astronomers and their biology/physics colleagues are wondering if this planet can sustain life.

The star is cool but the planet orbits close to it, so that the planet might attain an average temperature near the range in which water is liquid.  Of course, there are many other requirements for being amenable to life, even if only microbial life.  Here is a very detailed write-up, running to 16 pages.

Company using microbes to make spider silk garners $123,000,000 in venture capital.

This is a story reported on the Tech Crunch site, about Bolt Threads.  The proposed uses seem to center on wearable textiles (vs. engineering textiles).  Why spider silk?  It’s touted to be stronger than other polymers – stronger than Teflon, in the Tech Crunch page. That’s a mistake, since they meant Kevlar, the amazing polymer used in bulletproof vests.  Spider silk is also claimed to be stronger than steel.  Consider these points:

• “Strength” is one of many properties of a structural material (yes, textiles are structures of a special sort); stiffness and toughness are equally important.
• These properties commonly trade off against each other, and in any application of a material – polymer, steel, carbon fiber, etc. – there are choices to make. Brick is a strong material but it’s very un-tough, as are ceramics in general.  Steels are strong but the strongest steels aren’t the toughest (meaning taking the most energy to break).  I have a quick summary of properties below.
• Many structural materials have properties that vary with the environment. The steel in WW II Liberty ships became brittle at cold North Atlantic Ocean conditions, and they sometimes broke apart under wave stresses.
• Spider silk is notorious for its properties depending strongly on humidity and temperature.

Another property of note is resilience.  When stressed (stretched along its length, for example, some spider silk stays stretched.  Not so good for a tie.  Of course, silkworm silk is good here (doesn’t stretch, is stiff), and some types of spider silk are resilient, and stiff, like silkworm silk.  Pick the right type of spider silk, and perhaps blend several types.

• As many investigators have noted, spider silks vary; some classifications put as many as 27 types in play. Bolt Thread likely will need to make a number of different silks.  Look at orb-weaving spiders, the common web-makers.  They use strong and resilient silk for the main radial lines and nonresilient, sticky silk for the cross lines.  The strong lines keep the web intact, but they would just kick an insect back out rather than trap it.  The sticky lines do the trapping.
• • One use I saw touted the use of spider silk instead of steel cables on aircraft carriers for arresting the landing of an aircraft. Two scenarios seem to have be implied.
    • One is that the trapping type of silk, or flagelliform silk, be used, as if catching a very big fly. As Stephen Vogel notes in his oh-so-readable book, Cats’ Paws and Catapults, spider silk is nonresilient; the energy it would absorb would go into making so much heat that the net would melt. So, that won’t work.
    • The other is using spider silk as the arresting cables, the ones that the aircraft’s tailhook engages, or perhaps the balancing cables that take the shock of stopping a landing aircraft by paying out from a drum with hydraulic damping, like big car shock absorbers. There, the tough and strong silk might be used.  A variant arresting method, for aircraft missing a tailhook, is to put out a big net instead of the arresting cable.  OK, but why go to spider silk, which is likely harder to maintain and in a function where weight is not critical?
  • Spider silk really isn’t stronger than Kevlar. Both (some) spider silk (drag lines) and Kevlar are stronger than steel, and notably better than stell per unit weight, but Kevlar is stronger than spider silk by a factor of 1.5 to 7.  Nothing is better for strength.

  Here’s a rundown of strength, stiffness, toughness, resilience, ductility, and some shape dependence of material failure.   We can conclude at the end that spider silk has a mix of properties that suit it to certain uses, choosing it over Kevlar (or steel, or putty, or ….), but not to other uses.  Using it for high-priced silk ties is an affectation for the rich.  I’ll wait to hear what real uses are in line.

We can look at a graph of the extent to which a material deforms (its fractional change in length) against the stress put on it (the force over an area).  Such a plot is useful for materials that have the same properties in any direction, or isotropic materials, though one can use it for materials whose properties vary by direction, if one specifies the direction.  (There are also differences in stresses applied in one direction, as extension or compression, and shear stresses applied nonuniformly, as in trying to stretch a material into a distorted rectangle or parallelogram.)

Here’s a simple comparison of two materials, well, OK, three.  Material A deforms a lot less than does material B, under the same stress.  Or, you can say that material A takes a lot more stress to deform it by a given amount.  It has greater stiffness.  We can take the slope of stress vs. strain at any point and call it stiffness.  For many materials, stiffness is pretty constant over a range of stress levels – e.g., steel.

 

Strength is the amount of stress needed to make the material break apart.  Material A isn’t as good as B.

 

Toughness is the total energy needed to make a material fail.  It’s the area under the curve from zero stress to failure.  Again, B is better.

 

Resilience is the ability to return to the original shape when stress is relieved.  We tend to like that.  Spring steel needs to be resilient.  Putty is non-resilient, and we like that for keeping it in place once we apply it.  I drew a line on the side of a related but distinct material, A2, indicating what strain remains when the stress goes down.  It ends with a non-zero strain when stress is completely removed.

Material A2 is not fully resilient; it might be resilient, however, from its new, once-stretched state, under later stresses.  If so, we can call the material ductile, able to be deformed and take on a new, resilient state.  It’s what we do in forming metals with dies by stamping, pulling through wire dies, forging, etc.

We can also call resilience elasticity.  The opposite of it is plasticity – undergoing plastic deformation.

 

 

I’ve attached another graph with an explicit comparison of spider silk to Kevlar

OK, spider silk is tougher than Kevlar if not stronger and not stiffer (of course, we knew it wasn’t stiffer!).   Its resilience depends on how far you stretch it – probably not as resilient as Kevlar, but close.

 

19 October 2017.  National Geographic misses the math.  In the November 2017 issue you’ll find a two-page spread entitled Kelp is on the Way.  The premise is that, if we begin farming kelp in a big way for food, it “could remove billions of metric tons of carbon dioxide from the atmosphere.”  Here are the real limitations:

• If we eat kelp and metabolize it, all the carbon in the kelp goes back into the atmosphere as CO₂ in our breath.  The only way to keep the carbon sequestered is to prevent it being metabolized.  One scheme touted at least a decade ago is to harvest trees and bury them.  That could hold the carbon back for centuries to millenia, giving us time to fix our energy economy.  Kelp will not.  Rather in reverse, what goes down (C in our food) must come up (in our breath).
• Let’s not forget the fossil fuel usage in harvesting and transporting the kelp or kelp products.   Processing and transport is a big part of all crop production and use.
• The global area suitable for kelp farming is very small, compared even to land-based crop area.  It’s the continental shelves, and not too far out.  Of course, if you want to put kelp on platforms in the open ocean… good luck.
• Will kelp grow without (added) nitrogen and phosphorus fertilizers, or even take up the N and P that we add in sewage or in cropland runoff?  The marine ecologist cited in the entry is indirectly referencing some big inputs, such as the Mississippi River basin.   Runoff of excess fertilizer, mostly, from over 1 million km² of crops, puts enough N and P into the river delta to cause a massive algal bloom each year, over tens of thousands of km².  The dieback of the algae and subsequent decomposition  deoxygenates the water with disastrous consequences for marine life.  OK, but the big N and P inputs are at major river deltas, not the whole set of continental margins.  Sure, many margins have upwelling of nutrient-rich waters, but the coverage is not good.

Kelp might be healthful as a food.  It’s not a CO₂ sink, by any means.

(Photo: National Geographic)

15 October 2017.  The pros and cons of hydrogen-powered cars and trucks have been discussed over the years, so I’m mostly refreshing the discussion, in light of recent news – e.g., in the September 2017 issue of Physics Today is an article entitled “Hydrogen-powered vehicles: A chicken and egg problem.”  Yes, any new technology needs infrastructure to spread and thrive, and insfrastructure goes to a thriving technology.  A delicate balance of hope and hard-headed pragmatism allows growth to start.  Hydrogen-powered vehicles, or HPVs, are now for sale in California.  The Physics Today article reviews the technological challenges and advances, particularly in catalyst formulations.

In that article is a look under the hood of a Hyundai HPV:

What are the pros and cons of HPVs?

Pros:

• No greenhouse gas or pollutant emissions at the vehicle; they occur, at a lesser rate than that of emissions from directly fossil-fueled vehicles, at the plant producing H₂ .
• Potential to produce the energy source, H₂, from non-fossil sources such as solar photochemistry.

Cons:

• There is a relatively low first-law efficiency of electrolyis to split water into H₂ and O₂.  There is an overvoltage, above the 1.2 V (depending upon conditions) as theoretical minimum, that results in about 55% as much enthalpy (heat equivalent, essentially) being embodied in the H₂ product as in the electrical energy input.  Overvoltage is reduced with expensive and limited platinum-group metals.
• Hydrogen can be stored in cars in two ways, each with drawbacks.  Liquefaction of H₂ is costly in energy and money.  Compression is less energy-costly, though still significant in cost, and there is a major limitation on how much fuel can be packed into the space available in a vehicle (H₂ has to be adsorbed into a material that takes up much more space and weight than the H2).  One might also worry about the safety of liquid or of pressurized gas in a collision.  Modern fuel tanks do exceptionally well with flammable fuels, but these have minimal vapor pressure.
• If hydrogen is produced from fossil fuels, there are emissions of CO₂ in the process.  They broadly compare with those from direct fossil-fuel use.  If nuclear fusion becomes a practical source of electrical power, this may be moot, but fusion has proved to be a very refractory problem over half a century.
• One more thing, noted, as I recall, by Ken Caldeira at the Carnegie Institution, is that inevitable leakage of H2 all along the supply and use chain would eventually turn the atmosphere from its current state as oxidative to a state of being reductive.  This would change the natural atmospheric processing of pollutants, in ways not yet predicted.

Battery-powered vehicles, a la Tesla, are more promising, for efficiency, adaptability to current infrastructure, and cost.  They have a different set of pros and cons, including the possibly limited supply of battery materials.  More on that, elsewhere.

12 October 2017: comments on a proposal to farm regular food crops, such as vegetables, underwater.  It’s called Nemo’s Garden.  I give a brief analysis here.  The limitations are very daunting.  Due diligence and doing the math are both challenges.