An interactive smartphone-controlled molecular model of tartaric acid to visualize stereochemistry and (a)chirality

Edit 04.01.2025:

• Modified base and lid to support wemos d1 v.4 board and increased wall thickness of the base (also added WiFi information and carbon atom labeling to the side)
• Adapted instructions and printable files for magnetic connectors (it is recommended to use non-magnetic connectors for C=O and O-H bonds)
• Slightly improved the code
• Uploaded CAD (step and f3d) files

General Idea

Visualising the principles of stereochemistry and chirality can be hard. Usually an analogy (based on human hands) is used to explain the correlation between images and mirror-images, which is the basis of stereoisomerism.
With the help of a 3D-printer and some microelectronics, it is possible to achieve a more representative haptic model that is controlled via smartphone and can be used by students and teachers alike. There is even the possibility to self-test the assignent of the CIP configuration (correct configuration is displayed on request in the web interface) and observe how the physical and optical properties change for different isomers.

Bonus: When disassembled, the complete set fits into an empty box of prusament; exlcuding the phone ;)

Controls (Web interface)

On the main page there are two types of buttons: 

• Toggle Servo (changes the configuration of selected carbon)
• Show Data (reveals further information)

When pressing the “Show Data” button, the following data is shown:

• Current configurations according to CIP for both stereocenters
• Density of current isomer
• Melting point of current isomer
• Solubility in water of current isomer
• Specific rotation of current isomer
• Structural formula of current isomer including dashed/filled wedged bonds
• Bonus: For the meso-isomer, a Reveal Symmetry button appears that displays the mirror plane in the isomer that makes it achiral.

It is still possible to toggle the servos and observe the properties change according to the configuration of the stereocenters 

Building Instructions

parts (total price <20€):

• Wemos d1 mini (ideally without pins and with holes in the PCB)
• 2 x SG51R servo (SG90 is too large!)
• 56 x 10x2 mm cylindrical magnets (optonal; for magnetic bonds)
• 6 x Jump wire (DuPont wire) with at least one male tip per cable
• 4 x M2x5 screws
   
• 3D-printed parts (PLA)
  • base
  • lid
  • mount
  • 2 x Carbon-sp2 (black/grey)
  • 6 x Hydrogen (white)
  • 4 x Oxygen_H (red)
  • 2 x Oxygen_noH (red)
  • 2 x Carbon_SG51_A (black/grey)
  • 2 x Carbon SG51-B (black/grey)
  • connectors → see under Molecule Assembly below

Code:

The code is provided here as .ino file. Online, there are several tutorials on how to install the Arduino IDE, add the wemos d1 mini board and flash the firmware.

Printing instructions:

3MF files are available with the correct alignment and number of parts (presliced for MK3S+ printer). Everything is printed from PLA. All atoms except the chiral carbons should be printed with 0% infill. Support is needed for most parts (we used organic supports).

Electronics:

1) Remove one connector of the DuPont cables, leaving a male connector on the other side; it is used to connect to the female parts on the servos later.
Remove around 5mm of the isolation from the wires (we had best results just using some sharp pliers). Color coded are used here (white = GND, red = 5V, orange/green = signal).

2) Twist the GND and 5V wires together that they can fit through the holes on the wemos board. Tip: Insert wires into neighboring pads to avoid “jamming” them with solder splashes

3) This is how it should look like before applying solder! Pay attention to the orientation of the board (wires go in from the top)

4) After soldering the GND and 5V wires, proceed with the two signal wires on D1 and D2; then clip of the remaining wire with sharp pliers or scissors. Be careful with the wires soldered as they can be very delicate!

Testing the servos:

Connect the wires to the servo plugs:

• 5V to the red pin 
• GND to the brown pin
• D1, D2 to the respective yellow pin 

Flash the software to the wemos board using the Arduino IDE; The IP-Adress of the Server is displayed in the serial monitor in the arduino IDE (usually it is 192.168.4.1)

Then connect to the WiFi of the board (PW: 12345678) and verify that the servos rotate when the respective button is pressed in the interface (Navigate to http://192.168.4.1 in the browser when connected to the wemos board). 
If something is not working as expected check the solder connections and/or test the servo using a servo-tester.

Done! Next insert the two servos into the printed parts. 

6) Carefully guide the wire through the hole in the part. Then carefully press the servo down and softly pull on the wires to straighten them.

7) Align the servo (push it away from the opening for the cables) and attach one M2x5 screw in the back (see image). Also glue the servo horns to the other half of the carbon atoms. Pay attention on the correct alignment of the horns.

8) Guide the cables to the mounting part (that also resembles the C-C bond), attach the half-spheres to the part (some force might be required, consider using some sandpaper when too much force is required). Do not worry if the connectors do not come out of the bottom yet. We will pull them out later.

9) Verfiy that the lid of the box attaches to the bottom of the part; also use some sandpaper if neccessary

10) Mount the board into the box as shown using the two remaining M2x5 screws.

11) Guide the cables for each servo through a seperate hole in the lid as depicted.

12) and 13) !Carefully! pull the connectors out of the cylinders and attach the male DuPont cables (as already described during the testing of the servos)

14) Carefully push the cables into the cylinders until you feel a resistance (around 10 cm per cyclinder in our case) and attach the lid to the base. 

Great! The hardest part is over! :)

Molecule Assembly:

You now have to choose whether you want magnetic bonds or not;

•  if not, simply print the connectors_simple.3mf (14 x connector_simple for the connection of the atoms); attach the balls using those cylinders in the following paragraph → continue with step 18
   
• if you want the bonds to be magnetic (recommended only for rotatable bonds; for O-H and C=O bonds I advise to use the simple connectors) print the connectors_magnetic.3mf file (12 x  connector_double_magnet + 8 connector_simple)

15) Attach the magnets in pairs of 4
16) Then press-fit one connector part from above on the magnets 
17) Turn around and press-fit the second one on top; this is now a connector pair

Repeat until you have assembled all 12 connectors = 6 bonds.

18) Attach the connector (pair) to the remaining hole on the servo-containing carbon.
19) Insert the free part of the connector (pair) into one of the holes of the sp2-carbon atom.
20) Fill one of the other holes of each sp2-carbon atom with a connector (pair) and attach the sp3-oxygen atom
21) Fill the remaining holes with simple connectors

22) For the upper part of the sp3 carbon add a connector (pair) for the oxygen atoms and a simple connector for the hydrogen atoms and attach them.

Lastly, plug in the USB-cable, log into the webpage and press start; then make sure that the CIP configuration is right! This is achieved - after pluggin in the USB - by mounting the upper part of the sp3 carbon atom with the oxygen atoms pointing inward. Also make sure that the right carbon atom moves when toggeling the servo in the web interface (there are C1 and C2 labelings on the side of the case; if inversed just rotate the assembly except the case by 180°)

Done! Now the model is complete :)

For stability, I used glue to ensure a safe connection between the non-magnetic bonds and the lid of the case and the large C-C bond.

Other molecules containing chlorine, sulfur and other atoms can be built accordingly. Just modify the code with the respective physical properties.
 
 In case any questions arise just contact me on printables or via email: [email protected]