we met in Berkeley and we’re both
scientists and one of the kind of
occupational hazards of being the child
of two scientists like me and my wife is
that your parents like to perform little
science experiments on you from time to
time and I’ll show you a couple of those
today if you’ll let me and I’ll
demonstrate one of them first this one I
did a few days ago with my son they
involved these two cubes shown here you
can see that they’re similar size
similar shape similar colors to them
right and I handed the first one to my
son the sleep fine into his hand he was
like okay dad what are you doing and I
handed the second one to him that sleep
and he went well geez and to prove it to
you we took a little photo of him dick
using a little teeter totter with them
what he realized was that the one on the
right there
even though identical to the other one
is tremendously more heavy than the
first it’s almost ten times heavier if
any of y’all don’t believe me after the
show tonight you’re welcome to come up
and lift these for yourself it’s not the
kind of thing that makes a good visual
demo but trust me and so he sense that
and in looking at these cubes he
realized that a whole new window and
opened up on the world tour in some way
that you could have two objects that
look so similar to each other in every
describable way and yet when you feel
them when you pick them up you can tell
something’s wrong something’s very
different about this one versus this one
what is that thing what is this like
secret invisible world of objects that
we’ve never really seen before this
point I loved watching him have that
kind of little Epiphany because it was
really a lot like what I had about five
or six years earlier I was a postdoc in
MIT at the time and I was in a lab where
we were trying to measure single cells
and in particular trying to see is there
any way you could tell the difference
between a healthy cell and a six cell
based on these measurements and you know
just like these cubes often these cells
would look very similar to each other in
terms of their size and their shape and
their appearance and we needed to find
ways properties of these cells that you
could measure and use to distinguish a
healthy cell from a six cell what could
that be well turns out in the story I’m
here to tell you tonight is that the
very property of these two cubes that
makes one so ridiculously having the
other one feels so light the very
property of these cubes is the same
property we found of living cells that
lets us tell whether a cell is sick or
healthy and that property its density
do you remember density right if you’re
like I was five or six years ago you’ve
learned it in high school it hadn’t
thought much about it since then right
well hopefully by the end of tonight I
can show you why it’s the most awesome
physical property and you should really
appreciate and love density so you might
remember that density is the function of
two other physical properties of an
object namely their mass how much in
object weighs and its volume how much
space a little object occupies and
you’ll agree that both of those
measurements mass and volume they’re
just ways of telling you how much of an
object you have how big is it but when
you divide the two by each other when
you take the mass and divide it by the
volume you get the density and that’s a
whole other thing it tells you so much
more than either of mass or volume can
tell you on their own I’ll try to give
you a few examples of that today so
first we’re gonna talk about density for
a few minutes we might as well acquaint
ourselves with the range of densities
that we encounter here on earth they
range from 0 grams per milliliter all
the way up to about 22 or 23 for the
most dense elements osmium the nuridium
and so we can look at a few spots on
this little number line and see what the
densities of a few things are on the far
left hand side here we see the density
of gases like helium point zero zero
zero two grams per milliliter and air is
point zero zero one three grams per
milliliter these are all very low
densities but you notice helium is just
a little less dense than air and that is
why when we fill a balloon with helium
and let go of it it goes up so sort of
the first example of density giving us
something meaningful in our
interpretation of the world around us in
this example what determines when I let
something go whether it goes down or up
its density if it’s more dense than
what’s around it it’ll go down if it’s
less dense than what’s around it like
the balloon it’ll go up so a little
example of density telling us something
about the world around us let’s go up
the scale a little bit further around
2.7 we encounter aluminum nice metal and
then way up at nineteen point two on the
density scale we encountered tungsten
and that is what these two blocks are
made of and that difference the
difference in density between two point
seven aluminum aluminum can very light
and tungsten one of the densest elements
that’s known is the reason why my son
Rhee
did to these and being so different and
so there’s another insight that density
can give us I could walk up on those on
these blocks and by looking at them I
couldn’t tell you which is tungsten
which is aluminum but if you let me pick
them up and if I know these density
numbers then it’s very easy for me to
tell that this is tungsten and that’s
aluminum so here density a physical
property is giving us a way to identify
chemically what material and objects
made of pretty neat that idea of using
density to identify a substance it’s not
an OL and not a new one in fact over
2,000 years ago Archimedes was asked in
the story by the King to determine
whether one of the Kings crowns was made
of pure gold or not and so Archimedes
was a smart man he realized if he could
measure the density of the crown and
compare that to the known density of
pure gold which is 19.3 very dense if
they equaled then it proved that the
crown was made of pure gold but if they
were different it meant that the King
been swindled by the crown maker I guess
and so by measuring density he knew he
could easily tell the difference between
pure gold in 19.3 and something like
iron pyrite Fool’s Gold at 5.0 but
Archimedes faced a challenge how do you
measure the density of an object like a
crown right I told you density is mass
divided by volume now getting the mass
of the crown is easy enough you just
weigh it but how do you measure the
volume of the crown without destroying
it or melting it down in a way he needed
a way to measure the density of the
crown without destroying it what he came
up with was really one of the most I
think brilliant experimental ideas in
the history of science and this is what
it is he realized that he could weigh
the crown under water if you’ve ever
been in a pool you know you weigh a
little less under water than you do
standing on dry land right well how much
less tea away well the way the math
works out the amount that different that
you weigh in the water is a function of
two things the density of the water
which is 1 we know that and the density
of you the object that we’re weighing or
the crown so if we weigh an object like
a crown in two different fluids in this
example a water and air from those two
weight measurements we can calculate the
density of the crown that’s exactly what
Archimedes did he was able to determine
what the crown was made on so now
amazingly 2,000 years later we still do
exactly what Archimedes described so
long ago in this example this athlete
here is being weighed underneath the
water and by combining that measurement
with this
and measurement of her weight on on dry
land we’d be able to calculate her
density why do you need to know a
person’s density well it turns out along
with telling you what a cube is and all
these different things it can tell you
if you’re fat or thin you’ve heard the
expression fat floats and it does if you
have a lot of fat in your body that
reduces your net density down to about
1.0 1 or so if you have a lot less fat
bone and muscle and things are more
dense so that makes your body more dense
and you reach about 1.07 or 1.08 so
here’s our first example of density
remember a physical property telling us
something very biological very health in
nature in this case how fat or how thin
a person could be all from density so
back five or six years ago as a postdoc
we looked at this and saw all the things
density could tell you and we had a
thought what if it worked just as well
on single cells what if just like I
could measure the density of a human
body and tell things about the health of
the body could I possibly measure single
cells and infer things like the
difference between a healthy cell and a
six cell or a cancerous cell based on
this density measurement so we set out
to try to do that and the first
challenge was what we need to measure
the density of a cell which hadn’t
really been done before and we realized
we could use the method that Archimedes
described the idea of weighing it in two
fluids a different density but how do
you weigh a cell you might first turn to
this if it’s in your kitchen but I can
tell you cells are a whole lot too small
to weigh on this and tiny living cells
don’t take too kindly to being plunked
down onto a little kitchen scale so we
needed a way to measure the extremely
tiny mass of cells in order to figure
out what their density was how do we do
that well luckily the lab I joined as a
as a postdoc already had a little math
sensor that they worked on that was
perfect for this job and to explain to
you how it works
I’ll show you another experiment I
performed on one of my children this
this is Jack he’s a little jerky up
there but I feel a lot smoother than
that in real life jack is it looks like
he’s on a baby bouncer and he is on a
baby bouncer but it’s also a
sophisticated mass sensor believe it or
not you see the frequency at which he’s
bouncing up and down as he kicks his
legs there he can’t just control that
frequency at will that’s actually a
fixed frequency it’s a function of a few
things that determine it but crucially
it’s a function of his mass so if he
were to say sit there and eat a
hamburger
that frequency would slow down because
his mass would have gone up how it works
so my wife and I again and this in the
spirit of experimenting on our children
realized we had an unexpected mass since
they’re here and we decided to calibrate
it first so we put some weights of known
weights on it from our weight set
ranging from this perfectly normal
waging from five pounds which had the
highest frequency of oscillation 149
beats per minute all the way to the
highest weight 20 pounds on the bouncer
which had the slowest frequency of
oscillation only 88 beats per minute now
that we calibrated it like a good
scientist if you’re if you’re preparing
lab notebooks for any of your lab
courses remember got a good calibration
curve show my data etc you’ve done you
calibrated our mass sensor now we’re
able to put jack back in it and measure
his frequency and determine what his
weight is 20 pounds and then do the same
for his identical twin brother Henry and
see that he has a slightly lower
frequency which means he has a slightly
higher mass and indeed that’s exactly
true so here you see this since our so
fantastic we have a baby bouncer this
capable of distinguishing two identical
twins from each other even I can’t do
that most days and I’m their father
so this measurement principle is really
quite good as a way to weigh something
it turns out you’ll never look at a baby
bouncer again the same way I hope so
here’s the here’s the point if you take
that baby bouncer and scale it down I
mean way way down so that the bouncing
part is about the size of a human hair
at that point you’re able to put a cell
on it instead of Jack or Henry and do
the same measurement and weigh them and
that’s what this tool here does this was
developed at MIT in the lab before I
joined it they call it a suspended micro
channel resonator and at the heart of it
is a tiny little bouncer a little diving
board shaped thing that you can see here
and it oscillates like that if we look
inside of it we don’t actually put the
cells on the outside of it we slide the
cells on the inside and this little blue
channel shown there if you zoom in on it
that’s the sensor it’s oscillating and
when we send a cell like this little red
blood cell through we weigh the cell
basically so by measuring two cell
masses and two fluids of different
density we can calculate the density of
this cell long story short so there
that’s how we do it what is the density
of a cell after all that is this even
interesting and what we found we’ve
measured a bunch of cells and we found
that most
have a density between 1.0 and 1.2 grams
per milliliter and on one hand that’s
not too surprising because water has a
density of 1.0 and cells are mostly
water so it makes sense that we would
see values around that number but not
all cells have the same density and I’ll
give you just a few examples tonight
that’s how the density of a cell can be
used to determine something very
interesting and get some insights about
the cell here’s the first example this
plot here shows each black dot is the
measurement of a single red blood cell
from someone’s blood on the bottom you
can see what the density of those cells
are ranging around one point one oh one
point one two on the side here you can
see the weight of each of those cells in
picograms if you don’t know picograms or
wonder how much it cell weighs a
millionth of a millionth of a gram very
very small weights and so in healthy
blood cells we can see all of those
measurements kind of pile up into that
little oval region and that’s where we
expect them to go but if you do the same
measurement Unseld x’ that part that
some of them are infected with malaria
parasite we see a few cells over there
to the left that are a little less dense
than the healthy cells those are the
cells that are infected with malaria
it’s been known for a while that the
little parasite that is malaria eats up
the hemoglobin in your red blood cells
turns it into a form that’s less dense
and the result of that is an infected
cell becomes less dense and we can see
that and we can identify the infected
cells compared to the healthy cells
based on their density what else here’s
data showing the mass and volume of
cancer cells these are leukemia cells
from a mouse and you’re seeing in the
black points the measurements before we
treat them with a drug and in the red
points the measurements after we treat
them with an anti-cancer drug and if you
look just at the mass and volume of the
cells before and after black and red you
don’t see much of a change and in fact
if this data was all you had to go by
you might not have thought that drug was
any good but like I said density is mass
divided by volume if you do that math
for each one of these points you get
this story over here a very much clearer
picture we see that after treatment just
minutes after we dosed these cells with
a drug we see that a large number of the
cells has increased in their density by
a large statistically significant amount
what this means is that just minutes
after treating these cells with a drug
we can see that yeah those cancer cells
are reacting to that drug we hope that
eventually this could become a drug
screening tool you might be able to use
density change as a way to find new
cancer drugs based on whether or not the
cells density changes when they
encounter the drug and then I can finish
with just a little recent data from my
lab here at UC Riverside this is showing
the a buoyant mass which is a function
of density of single zebra of a single
zebrafish embryo why we’re studying
zebrafish well a few reasons zebrafish
embryos are an aquatic organism and so
if something hurts the zebrafish embryo
like a chemical then there’s reason to
think that it would hurt a lot of other
fish and it’s probably the kind of
chemical you’d want to not have in your
waterways and to a zebrafish embryo
looks a lot like a human embryo so if
something hurts the zebrafish embryo and
is a good reason to think that you
wouldn’t want to have it around any of
your embryos either so this is what the
mass buoyant mass or density of a
healthy zebrafish looks like over time
and then if we do the same measurement
on a sick zebrafish that we’ve exposed
to some chemical that makes it ill we
see a very different reaction we see
little decreases in it’s buoyant mass it
means its density is changing we don’t
know what’s causing this yet but what we
do know is that we’ve tried the same
experiment on a whole bunch of different
sorts of sicknesses different sorts of
toxicants for these zebrafish and we
still see this same signature it’s as if
this change is somehow indicative of a
sick zebrafish a regardless of exactly
what we did to make it ill and so where
we want to see this going in the future
is to familiar with the idea of a canary
in a coalmine right the the bird that
the coal miners brought down to see if
the air was safe to breathe we hope that
the density measurements that we’re
making right now will in the future they
come like a sort of super canary in a
coal mine that by measuring the density
of these organisms of these embryos or
other microorganisms we could have an
instrument that will live and sample the
groundwater and tell you if there’s
something dangerous entering into it or
an instrument that can live in the air
and sample the air around us and tell us
if there’s something dangerous all based
on how the density of these organisms
react to exposure to whatever’s in the
environment that’s one direction we’re
taking it so hopefully today I showed
you and shared with you why I think
density is just the most awesome
physical property that’s out there it’s
something that every object around here
from crowns to cells to fish to you and
me we all have a density we can’t see it
but we can feel it and we can measure
it and if you can measure it accurately
enough you can use it to sort of open up
whole new views on the world around you
thank you [Applause]