“Spleen-on-a-chip” gives an inside view of sickle cell disease

Original paper: Microfluidic study of retention and elimination of abnormal red blood cells by human spleen with implications for sickle cell disease 

Content review: Arthur Michaut
Style review: Arthur Michaut


Though they may not realise it, anyone who’s taken the subway at rush hour knows how a red blood cell feels passing through the human spleen. Almost home now, just need to get through the gates; but wait, someone’s ticket isn’t working, the crowd is starting to push, the gates are getting jammed… Maybe you should have called a cab.

Figure 1. Left: in vivo filtration of red blood cells through the slits of the spleen. Right: in vitro filtration of red blood cells through silicone slits.

Much like turnstiles validating your ticket, the human spleen contains narrow slits that red blood cells must squeeze through to prove they are fit to navigate tight blood vessels while carrying oxygen around the body. The spleen is tasked with ensuring our blood contains the right amount of red blood cells and removing any cells that aren’t in tip-top shape. Shape, however, can be a problem the spleen can’t handle.

People with sickle cell disease produce “sickle” shaped red blood cells due to an inherited genetic mutation. Sickle cell disease patients can experience episodes of acute splenic sequestration, or “spleen crisis” where these misshapen cells clog the slits of the spleen, depriving it of oxygen and leading to swelling that can become life-threatening. Because this process is difficult to observe and monitor in the body, there is little understanding of how and why a spleen crisis might occur. When a spleen crisis occurs, an urgent blood transfusion is needed to treat the problem, however, in some cases, it becomes necessary to surgically remove the spleen.

A group of scientists spanning the USA, France, and Singapore have teamed up to understand how sickle cell disease can become life-threatening. Their device, a silicone model spleen, could be used to predict complications of the disease as well as develop new treatments.

Using a soft material classically used in microfabrication, named PDMS, to cast a mold with slits similar to those in the human spleen (Figures 1 & 2i), researchers were able to directly observe the processes that lead to spleen crises. The group analysed flows of red blood cells from both healthy and sickle cell disease patients through the silicone spleen, simulating splenic blood flow rate and oxygen levels, and measured retention of cells in the slits of the device.

The flow of red blood cells from healthy and sickle cell disease patients through the silicone spleen.
Figure 2. (i) The biomimetic “spleen-on-a-chip”, scale bar 10 µm (with higher magnifications outlined in red, right). The majority of red blood cells from healthy patients (ii) pass through the device, however, cells from sickle cell disease patients (iii) are more frequently retained at the slits of the device, and when deprived of oxygen (iv) these cells block the device entirely.

After one minute of blood flow, red blood cells from sickle cell disease patients blocked more than double the number of slits as healthy red blood cells (Figures 3i & 3ii), demonstrating how the spleen can become blocked and swollen in sickle cell disease patients. When the silicone spleen was deprived of oxygen, as occurs during a spleen crisis, red blood cells from sickle cell disease patients became stiffer, more viscous, and more frequently sickled (Figure 3iii). Under this condition, sickled red blood cells completely blocked all of the slits. Upon reintroducing oxygen into the system, many cells return to their normal shape and blockages begin to open up again within seconds (Figures 3iv & 4).

The effects of de- and re-oxygenation on red blood cells from healthy and sickle cell disease patients and the unblocking of the device following reoxygenation.
Figure 4. Left: red blood cells from healthy (AA) and sickle cell disease (SS) patients under oxygenation and deoxygenation conditions (scale bar 10 µm). Right: the percentage of open slits in the device quickly increases when oxygen is reintroduced into the blocked system.

The results uncover a viscous cycle leading to spleen crises in sickle cell disease patients. Sickled cells cause blockages in the spleen, which deprives the spleen of oxygen, which in turn causes more cells to become sickled. Additionally, it was shown that the addition of oxygen caused cells to “unsickle” and rapidly cleared blockages in the slits, revealing why immediate blood transfusion can alleviate spleen crises. The researchers speculate that a sudden burst of oxygen-rich red blood cells via transfusion has the same effect as when they reintroduced oxygen to their blocked system.

Researchers believe the spleen-on-a-chip could become a new tool to allow sickle cell disease patients to monitor their condition and allow for early diagnosis of spleen crises, as well as providing a testing ground for new treatments against the disease. Safe travels red blood cells!

Disclosure:  The author declares no competing interest.

Trichoplax adhaerens: tropical sea-dweller, microscopic contortionist, and biomechanical marvel

Original paper: Motility-induced fracture reveals a ductile-to-brittle crossover in a simple animal’s epithelia

Content review: Heather Hamilton
Style review: Pierre Lehéricey


Figure 1: The dynamic range of T. adhaerens with size ranging from 100 microns to 10 millimeters. Snapshots taken from live imaging. Images courtesy of the original article.

Meet Trichoplax adhaerens, a microscopic marine animal from one of the oldest known branches of the evolutionary tree. It looks like a microscopic cell sandwich: two layers of epithelial cells (which make up the surfaces of our organs), with a layer of fibre cells in between. As depicted in Figure 1, T. adhaerens takes a wide variety of shapes from disks to loops to noodles and more. Oddly,  T. adhaerens ruptures when it moves around, a self-induced fracture behavior that has recently captured the attention of physicists and engineers. Fracture is the technical term describing the process by which an object breaks into distinct pieces due to stress. These animals push their epithelial tissue to the breaking point, forming incredible and extreme shapes before separating altogether. This is a surprising behavior for epithelia, which usually prefer to maintain their integrity.  By modeling how T. adhaerens rips itself apart when moving, we can improve our understanding of how soft materials and especially biological tissues behave on the verge of breaking.

Prakash, Bull, and Prakash conducted a two-pronged analysis of fracture in T. adhaerens:  live imaging to record the fracturing in real time and computational modeling to simulate the response of the tissue when stretched too far. The drastic mechanical behavior in question also motivated the researchers to perform a more general inquiry into the competition between flow and fracture in materials that are dramatically deformed relatively quickly. Flow is like stretching out a piece of chewing gum, whereas fracture is like snapping the gum in two. The computational model proposed by the authors helped paint a clearer picture of what happens when T. adhaerens rips apart.

Figure 2: Model tissue is described as a collection of balls and springs. Balls represent cells, and springs represent the sticky adhesion between cells. Springs apply restorative forces to the cells, but can break if stretched too far. This model was used to study the ventral (bottom) epithelial layer, which consists of epithelial cells (green) and larger lipophil cells (red). Figure courtesy of the original article (Extended Data).

The computational model that the researchers used is based on a sticky ball and spring model, as shown in Figure 2, where each ball represents a cell and each spring represents the sticky junctions that cells use to adhere to one another.  The springs break if the balls move too far away from each other, which represents cells being unstuck from their neighbors.  Two cell types are represented in the model epithelial layer in Figure 2: epithelial cells, which are small and comprise the bulk of the tissue, and lipophil cells, which are larger and less common.  Using this model for living tissue, the authors conducted computational simulations where the tissue was stretched to a breaking point. They found that there are three possible tissue behaviors that depend on the strength of the driving force applied to the simulated tissue. For weak forcing (low stress), the tissue behaved elastically and so responded in such a way that it could recover its original shape. For intermediate forcing (medium stress), the tissue underwent a “yielding transition” where the material transitioned from elastic response to plastic response. During plastic response, permanent distortions occurred in the material, and the material could not recover its original shape. In this case, the tissue is ductile and undergoes local changes, like cells interchanging with neighboring cells, to relax some of the pent-up stress. For stronger forcing (high stress), the tissue undergoes brittle fracture where the bonds between cells break with little opportunity for relaxation. The three behaviors in the model represent a transition from elastic to ductile to brittle responses. Using this model of tissue response to applied force, the authors mapped the conditions that lead to different tissue behaviors, as sketched in Figure 3.

Figure 3: Tissue phase diagram (elastic-ductile-brittle) generated by the tissue simulations. The elastic regime (i) implies that bonds do not break, and neighbors are not exchanged. Above the yield transition (blue line), cells undergo local relaxations and flow in the ductile yielding regime. To the left of the red line, cell bonds tend to break and form gaps between cells, demarcating the brittle fracture regime.  Figure courtesy of the original article.

Guided by a better understanding of tissue mechanics thanks to the computer model, the authors experimentally measured the brittle and ductile responses in T. adhaerens. They found that both material responses can occur in our microscopic friend. The ability to access both regimes is important because the ductile response yields by flowing (helping form the longer shapes in T. adhaerens) whereas the fracture response accounts for asexual reproduction by splitting into two separate new individuals. The authors’ combined approach of experimental data that motivated the development of a computer model, which in turn guided further experimental inquiry, is an important modern scientific paradigm. Both approaches are incredibly important tools in the biological and soft matter sciences’ toolkit. Joint application of these tools lets us draw general conclusions from specific experiments as well as apply those general conclusions back to answer specific questions – like explaining how T. adhaerens achieves the diversity of shapes in Figure 1 and how this relates to its hardiness and evolutionary goal of reproduction.  Further, the epithelial layer computational modeling technique generalizes this tissue mechanics study to help us describe fracture versus flow in any living tissue, including our own.

Who needs polymer physics when you can get worms drunk instead?

Original paper: Rheology of Entangled Active Polymer-Like T. Tubifex Worms (arXiv here)


If you speak to a soft matter physicist these days, within a few minutes the term “active matter” is bound to come up. A material is considered “active” when it burns energy to produce work, just like all sorts of molecular motors, proteins, and enzymes do inside your body. In this study, the scientists are focusing specifically on active polymers. These are long molecules which can burn energy to do physical work. Much of biological active matter is in the form of polymers (DNA or actin-myosin systems for example), and understanding them better would give direct insight into biophysics of all kinds. But polymers are microscopic objects with complex interactions, making them difficult to manipulate directly. To make matters worse, physicists have yet to fundamentally understand the behaviors of active materials, since they do not fit into our existing theories of so-called “passive” systems. In this study, Deblais and colleagues decided to entirely circumvent this problem by working with a much larger and easier-to-study system that behaves similarly to a polymer solution: a mixture of squirming worms in water.

The researchers focused on the viscous properties of this living material, which behaves somewhat like a fluid. Viscosity is a measure of a fluid’s resistance to gradients in the flow. Polymer fluids are highly viscous because the long molecules in a polymeric liquid get tangled up in one another. Physical descriptions of most fluids assume that viscosity is a constant (so called Newtonian fluids), but many materials exhibit what is called shear thinning. This is when a fluid flows more easily as one applies an increasing shear force, that is, a force pulling the system apart. We encounter shear thinning at the dinner table all the time when struggling to pour ketchup, another polymeric fluid, out of a bottle. If the bottle is shaken fast enough, increasing the shear force applied, the ketchup flows smoothly like a liquid. In polymer systems (like xanthan gum in the ketchup) shear thinning happens when polymers are pulled apart fast enough that they tend to align together, which loosens the entanglements that held the system together before. 

In this study, the researchers asked: how does shear thinning behavior change if the polymers in question were alive? To answer this question, they set out to measure the shear thinning properties of a mixture of worms at various levels of worm activity. Here, “worm activity” refers to how fast the worm is wriggling, which is calculated by measuring how quickly the distance between the two ends of a given worm changes. This leads to two logistical questions: how is the level of worm activity being modified, and how is the viscosity being measured?

Figure 1. This movie shows two worms, one in water (left) and one in a water + alcohol mixture (right). The worm on the right shows a decrease in activity when they are exposed to alcohol, which is one of the two ways the researchers modified worm activity in this study. Video taken from the original article.

The answer to the first question should be familiar to many humans. To make the worms less active, they were put into a solution containing water and a small amount of ethanol, the same type of alcohol found in beer, wine, and spirits. Once the worms were nice and drunk, the researchers noticed that they squirmed about more slowly, as shown in Figure 1. Thankfully, when the ethanol was removed, the worms returned to their previous level of activity! To make sure the alcohol wasn’t doing anything funny to the worms, they found a second way to reduce the activity — by reducing the temperature of the worm solution. Colder temperatures made for more chilled out worms, no pun intended.

Figure 2. This movie shows the functioning of the rheometer. The worms are placed inside a chamber between two plates. The top plate rotates with respect to the bottom plate, and the response of the material is measured. Video taken from the original article.

The researchers used a device called a parallel-plate rheometer to understand the shear thinning behavior of this living polymer system. As seen in Figure 2, a parallel-plate rheometer sandwiches a sample in between two flat plates and viscosity is measured by determining how much force is necessary to rotate the top plate, effectively pushing the material by twisting its surface. The viscosity of the worm mixtures was first determined at three different temperatures, and for worms drunk on ethanol. The results were surprising! The rheological behaviour of the low-activity worm mixtures matched with theories of polymer shear thinning quite well. It seems the worms have the same alignment properties as passive polymer solutions under shear!

So what happens when the worms are sober, more active, and wriggling around? They saw that the required twisting rate needed to thin the mixture decreased. In this case, the worm activity allowed for easier and quicker rearrangement while the mixture was pulled apart by the rheometer’s twisting motion. One can imagine that instead of needing to pull all the worms to the point of alignment, it may have been enough to nudge them in that direction and their wriggling did the rest. We can now imagine that the same thing might be true for non-living polymers: if a polymer material with shear thinning behavior is given an extra source of activity, then its thinning behavior may become more significant. 

The lesson to be learned here is partly about worms, polymers, and the adverse effects of ethanol, but really this experiment is a testament to the power and generality of physical descriptions. This study teaches us about the possible behavior of an active polymer system with processes that are relevant on the scale of a few micrometers, by studying real life worms that you can see with the naked eye! In general, it is usually possible to find analog systems that have the desired properties for your study, but which are easier to manipulate. Physics then gives you the bridge between the system of interest and your simpler analog, allowing you to harness the power of interdisciplinary science to ask questions previously unanswerable.

Featured image for the article is taken from the original article.