3D Printing For Facial Application

(1)

3D Printing For Facial Application

Christa Low

Erika Lee

Methodist Girls’ Secondary School

Associate Prof. Yeong Wai Yee

Dr. Lee Jia Min

Mr. Sean Suen

Nanyang Technological University

1. ABSTRACT

In order to 3D print a hydrogel that would display the properties of a facial mask, this study has integrated the serums provided into the hydrogels formulated, to find an ideal ratio of serum to hydrogel such that the printed end-product would display the physical properties of an off-the-shelf fabric facial mask. An arbitrary ratio of hydrogel to serum was first derived, and its physical properties were tested by extruding the hydrogel-serum mixture through a 5-millilitre syringe. From this, the ratio of serum to hydrogel could be tweaked so as to obtain an ideal end-product. Once an optimum formulation was obtained, its physical properties were tested by running the hydrogel-serum mixture through a Nordson EFD PRO4 printer with Red C2 printing tips of diameter 0.25 mm, as well as a rheological test.

2. INTRODUCTION

With the ever-growing global market for skincare products, many people have turned to fabric facial masks as a solution to wrinkles and other problems. The limitation of such facial masks is that fabrics, such as cotton, can only absorb and transport a limited amount of serum, which inadvertently leads to serum being left in the packaging of the facial mask, rather than inside the fabric facial mask. As such, the aim of this study was to find a way in which the serum could be integrated within the material of the facial mask itself, so as to allow the user to receive the maximum amount of serum through transdermal absorption into their skin.

Two assumptions were made in this study. Firstly, that the serum, when integrated into the

hydrogel and placed onto skin, would be able to be absorbed by the skin through transdermal delivery, and secondly, that the ideal end-product would be a hydrogel-serum mixture, that when cooled to room temperature, can be extruded smoothly, can be picked up without breakage, and retains its shape when printed within the geometric parameters.

There are several key terms in this report which will be defined here. A ‘hydrogel’ is defined as a network of hydrophilic (attracted to water) polymer chains that can be chemically crosslinked or physically bonded, generally in the presence of water. For the purpose of this report, ‘homogenous’ will be defined as when there are no observable distinct separation of the components within a mixture, and ‘room temperature’ will be defined as a range of temperatures from 23 to 26 degrees Celsius. This study defines ‘printability’ if: the hydrogel: can be lifted up without breaking, integrates well with the serum, can be extruded smoothly, and exhibits the physical properties of a facial mask.

‘Rheology’, or rheological characterisation, is the study of deformation and flow of matter.. A concept this study is based upon is thermal-gelation, or physical bonding within hydrogels, in which physical (hydro)gels are formed through ionic bonding, hydrogen bonding, hydrophobic association, etc. Most physical hydrogels exhibit reversibility, in that the gel can exist in a liquid state or a solid state according to the conditions such as temperature, pH, solvent, and light.

This study covers the integration of the serums provided into the hydrogels that have been

(2)

formulated and selected as ideal for the desired end-product, and rheological characterisation and properties of the end-products. There are six parts in this report: aims and objectives, methodology, results, conclusion, acknowledgments, and references.

3. AIMS/OBJECTIVES

The aim of this work is to successfully integrate the formulated hydrogels and serums into one product with ideal physical properties. Through this research, the understanding of the process of thermal-gelation and the integration of serums into the hydrogel will improve the capacity of the printing of (different) hydrogels. In addition, this work aims to understand the rheological properties of the hydrogels.

In particular, the objectives of this study are to physically crosslink the hydrogels such that they are suitable for the integration of the serums; To successfully integrate the serums into the previously formulated hydrogels so as to create a product with ideal physical properties that match the original aim(i.e. to create a hydrogel that can be used as the material for a facial mask); “Ideal physical properties” being: a) Printable (as defined earlier); b) Able to be extruded smoothly; c) Able to retain its shape when being lifted; AND d) Able to remain combined with the serum as a gel;

AND to obtain a rheological characterisation of the product by running rheological testing.

4. METHODOLOGY/MATERIALS

4.1 INTEGRATING THE SERUM WITHIN THE HYDROGELS

1.5% Gellan Gum + 1.5% Methylcellulose Hydrogel

In a bottle, mix the Methylcellulose and water. Stir the mixture at 80°C until the Methylcellulose is fully dissolved in the water. In another bottle, mix Gellan Gum and water. Mix until the contents are thoroughly combined. Add the Gellan Gum mixture to the Methylcellulose mixture, and stir the combined mixture at 80°C

until the mixture is free from clumps and homogenous. Use a pipette controller and pipette equal amounts of RADIANCE ESSENCE and SMOOTHENING HYDRATOR into the Gellan Gum and Methylcellulose mixture. Continuing stirring the mixture at 80°C until the mixture is homogeneous (should appear opaque white). This is the hydrogel. Once the mixture is fully liquid, transfer the hydrogel into a syringe using a micropipette. Let the syringe cool to room temperature before attempting to extrude the hydrogel.

0.5% Gellan Gum + 10% Gelatin Hydrogel In a bottle, add Gelatin and water. Stir the mixture at 60°C until the gelatin is fully dissolved. In another bottle, mix Gellan Gum and water. Mix until the contents are thoroughly combined. Add the Gellan Gum mixture to the Methylcellulose mixture, and stir at 60°C until the mixture is clear and free from clumps. Using a micropipette, add equal amounts of RADIANCE ESSENCE and SMOOTHENING HYDRATOR to the mixture and stir in a water bath at 60°C. Continuing stirring the mixture at 80°C until the mixture is homogeneous (should appear clear). This is the hydrogel. Once the hydrogel is clear and fully liquid, transfer the hydrogel into a syringe using a micropipette. Let the syringe cool to room temperature before attempting to extrude the hydrogel.

There are several variables that should be kept constant throughout the experiments conducted. Firstly, the room temperature at which the hydrogel is extruded from the syringe. Temperature is a factor that affects the physical properties of the hydrogel (thermal-gelation), and to ensure that the physical properties of the hydrogel does not change randomly, the room temperature should be kept at 23°C-26°C. Secondly, the amount of serum that is added to each hydrogel formulation should also be kept constant. Different amounts of serum will cause

(3)

the hydrogels to have a higher/lower viscosity than expected.

Thirdly, the equipment (weighing scales, 3D printers, rheometers, etc.) used throughout the study should be the same to ensure that the results measured are the same, and do not differ.

Fourthly, the recovery period for each hydrogel, after undergoing shear stress, should remain constant throughout the experiment. Having varying recovery periods (allowing some samples of hydrogels to recover for a longer time than others) would affect the hydrogel’s physical properties and printability. This can be minimised by attempting to pick up the hydrogel immediately after it has been extruded, or ensuring that after being pneumatically extruded, the hydrogels should all be left to sit for the same amount of time.

Fifthly, inexact measurements of the components of the hydrogels can be minimised by using an accurate electronic weighing balance that measures to four decimal places. Lastly, the period of time that the hydrogel is left idle before being extruded should be constant. Leaving the hydrogel idle for too long could lead them to become too viscous to be extruded, altering its printability. Therefore, it is important to print the hydrogel as soon as possible after formulation.

4.2 EXTRUSION

Fig 4.1: Methylcellulose and Gellan Gum

Fig 4.2: Gelatin and Gellan Gum

In order to determine which hydrogels should be used to integrate the serums into as the base material for facial masks, they are firstly extruded through a syringe by hand (refer to _Fig 4.1 and _{Fig 4.2}) onto sheets of tissue. Different hydrogels displayed different results. For example, a less viscous hydrogel with a longer recovery period might get absorbed into the tissue, and thus would not be able to be picked up; while some hydrogels may be too viscous, and would not be able to be extruded at all by hand. From these, the printability of the hydrogels can be observed and chosen for the integration of the serum.

After determining which hydrogels should be integrated with the serums based on our printing parameters, the resulting hydrogels were mixed with the provided serums. This was with done 20ml and 10ml of serum and the resulting hydrogels were similarly extruded onto another sheet of tissue paper.

(4)

Fig 4.3: Nordson EFD PRO4 3D printer

Fig 4.4: Optimum tip height

After a hydrogel with ideal physical properties (as defined above) was derived, all hydrogels with the serums were extruded through Red C2 0.25mm printing tips with a Nordson EFD PRO4 3D printer (refer to _{Fig 4.3}) via pneumatic extrusion, in which air pressure is used to force the hydrogel from the syringe barrel onto the substrate.

The height of the printing tip (tip height) from the substrate has to be 60 to 70 percent in length of the diameter of the nozzle (refer to _{Fig 4.4}). In this study, the tip height established was approximately 0.167mm. If the tip height is too high, the extrusion will clump up;if the tip height is too low, the extrusion will not be smooth or not be able to be extruded at all. As such, the tip height must be calibrated and then recalibrated as needed to compensate for slight variations in height that occur when changes are made to the system, primarily nozzle or tip change-out.

4.3 RHEOLOGY

Samples were to be measured through both rotational and oscillatory mode; and are run through the Anton Paar MCR 302 rheometer

5. RESULTS/DISCUSSION

5.1: FORMULATION OF BASE MATERIALS Hydro

gels

Methylc ellulose

Gela tin

Gellan Gum

Water (75%)

Serum (25%)

Total /ml

T1 1.5%

(0.3g)

- 1.5% (0.3g)

20ml - 20

T2 1.5%

(0.3g)

- 1.5% (0.3g)

15ml 5ml 20

T3 - 10% 0.5% 20ml - 20

T4 - 10% 0.5% 15ml 5ml 20

The concentration of the hydrogels (a solute dissolved in a liquid) is expressed in weight by volume (w/v). To find the weight by volume of the base materials, the mass in grams of the dissolved solute is divided by the volume in milliliters of the entire solution, and expressed as a percentage.

All chemicals used were from Sigma Aldrich. Those used in this study are: Methylcellulose (M7027, BioReagent), Gelatin from porcine skin (G1890, BioReagent), and Gelzan™ CM (G1910). The serum used is a mixture of equal parts of SMOOTHENING HYDRATORS and RADIANCE ESSENCE.

The base materials are all thermal reactive, and have varying setting and melting points as stated below in _{Fig 5.2}.

(5)

Base material Setting point Melting point

Methylcellulo se

50°C 80°C

Gellan Gum 40°C-50°C 80°C-90°C

Gelatin 15°C 60°C

Fig 5.2

At the setting point (and below), the hydrogel (made up of the base material and water) exists in a solid state, and is very viscous. However, at the melting point (and higher), the hydrogel exists in a liquid state, its viscosity is lower.

As these materials are thermoreversible, they exhibit different properties at different temperatures. These gels exhibit reversibility in that the gel can exist in a liquid state or a solid state, responding according to the external stimuli (temperature). In this case, the lower the temperature, the higher the viscosity of the hydrogel. The greater the temperature, the lower the viscosity of the hydrogel. These materials (methylcellulose, gelatin, gellan gum) undergo physical crosslinking.

The molecular chains of the hydrogel aggregate at lower temperatures, and undergo physical entanglement. The thermal energy (when the hydrogels are being heated up in the water bath) causes the hydrogel to behave like a liquid as there is enough energy for the polymer chains to separate. However, at room temperature, the polymer chains don’t have enough thermal energy to move freely, and it gets entangled into a network, trapping the water molecules.

5.2: EXTRUSION

When the hydrogels were pneumatically extruded through the NORDSON EFD PRO4 machine, they were printed in the shape of a circle.Fig 5.3 and Fig 5.4 display the commands keyed in to print 6 successive circles on the substrate (a glass slide), with a radius of 1.25cm.

Fig 5.3

(6)

Fig 5.4

The geometrical boundaries are shown below in Fig 5.5, and the indicator of good printability is how far away the hydrogel deviates away from the dimensions of the circle drawn. The closer the hydrogel stays within the geometrical boundaries, the more printable it is. If the hydrogel printed deviates from the geometrical boundaries of the circle, the less printable it is.

Fig 5.5 (From left to right: (Top row) 1.5% MC+GG, 10%G+0.5%GG, (Bottom row) 1.5%MC+1.5%GG+10ml

serum, 1.5%MC+1.5%GG+5ml serum, 10%G+0.5%GG+10ml serum, 10%G+0.5GG+5ml serum)

Fig 5.6 (From left to right: (Top row) 1.5% MC+GG, 10%G+0.5%GG, (Bottom row) 1.5%MC+1.5%GG+10ml

serum, 1.5%MC+1.5%GG+5ml serum, 10%G+0.5%GG+10ml serum, 10%G+0.5GG+5ml serum)

As seen in _{Fig 5.4} above, the only 3 hydrogels that managed to stay within the geometrical boundaries were the 10%G+0.5%GG hydrogel, 10%G+0.5%GG+5ml serum hydrogel, and the 1.5%MC+1.5%GG+5ml serum hydrogel.

5.3: RHEOLOGICAL CHARACTERISATION

Definitions:

Shear stress is the force exerted on a material that is coplanar with its cross-section.

Shear rate is the rate or velocity at which a shearing deformation is applied to a material. The storage modulus G’ is the measure of the elastic response of a material, and relates to the hydrogel’s ability to store energy elastically. The loss modulus G” is the measure of the viscous response of a material, and relates to the hydrogel’s ability to store energy dissipated as heat.

Shear-thinning is the decrease in viscosity of a material with increased shear rate.

Flow curve

A flow curve is a graph showing how the viscosity of a hydrogel changes when it is subjected to a constant shear rate or shear stress.

(7)

Fig 5.6: Flow curve

gelatinGG from 7220.000 Pa.s to 0.126 Pa.s

difference of 7219.874 Pa.s

gelatinGG with serum

from 12160.000 Pa.s to 0.078 Pa.s

mcGG from 1700.000 Pa.s to 0.194 Pa.s

mcGG with serum

from 736.000 Pa.s to 0.346 Pa.s

It can be observed that all four hydrogels tested are non-newtonian fluids, as the curves are all thixotropic in nature; that is, they decrease in viscosity with time under a constant shear rate, as seen in the graphs, which can thus be used to conclude that all four hydrogels possess shear-thinning qualities. A steeper flow curve indicates a less viscous fluid under the same shear rate.

A slight fluctuation in results can be observed in the Gelatin and Gellan Gum hydrogel with 10ml of serum (gelatinGG with serum) (refer to _Fig 5.5), where at the start of the curve there is a slight increase in viscosity at a shear rate of below 1/s.

In the case of the four hydrogels, it can be seen that the Gelatin and Gellan Gum hydrogel with serum (gelatinGG) has the steepest flow curve (it having the largest difference in viscosity under the same shear rate) and thus possesses the most shear thinning characteristics, while the Methylcellulose and Gellan Gum hydrogel with

serum (mcGG with serum) possess the most gradual curve.

Thus, the Methylcellulose and Gellan Gum (mcGG) hydrogel with serum has a better resistance to shear stress than the Gelatin and Gellan Gum hydrogel with serum (gelatinGG with serum), making it the most ideal formulation for the manufacturing of facial masks.

Flow Recovery Curve

Fig 5.6: Hydrogel extruded through a dispenser

Fig 5.7: Flow recovery curve of mcGG with serum

Fig 5.8: Storage modulus and Loss modulus plotted against Time (in seconds)

(8)

In Fig 5.6, the syringe the polymer chains of the hydrogel (in red) form a temporary network and induce gel-like viscosity. Upon dispensing through a tip (an extruder), it undergoes shear stress. The temporary network is broken up by shear stress and all polymer chains align, reducing the viscosity of the hydrogel. Directly after removal of shear stress (being printed onto the substrate), the temporary network of polymer chains is restored and the gel-like viscosity returns.

Fig 5.7 shows the flow recovery curve of Methylcellulose and Gellan Gum hydrogel, which represents the speed of recovery of a hydrogel after shear stress is removed, in which the hydrogel solidifies as time passes. From the graph, we can note two things; firstly that the hydrogel recovers, and secondly that the hydrogel recovers at an extremely fast rate (as shown in the steep, almost vertical, curve). This quality of the hydrogel possessing a short recovery period further suits it as the base material for a facial mask as it allows the hydrogel to be printed, and then restored to its previous viscosity.

Fig 5.8 shows that after shearing, the storage modulus, G', is more dominant than the loss modulus, G"of the mcGG with serum hydrogel. It can be concluded that the mcGG with serum hydrogel is elastic in nature, and behaves more like a gel than a liquid. The hydrogel’s ability to store elastic energy is greater than its capacity to release energy as heat, causing the mcGG with serum hydrogel to have a better resistance to shear stress.

Amplitude sweep

Measuring Profile: Shear Stress, Amplitude = 1 to 1,000; Angular Frequency = 10 rad/s; Temperature: 25 °C. During an amplitude sweep, the amplitude of the shear stress is varied while the frequency is kept constant. The amplitude is the maximum of the oscillatory (back and forth)

motion. For the analysis, the storage modulus G' is plotted against the shear stress.

Out of all the samples tested, the gelatinGG hydrogel has the highest storage modulus, G’, making it stronger than the other hydrogels. This is seen when attempting to physically manipulate it, it can be picked up and it behaves like a tough agar jelly.

When pneumatically extruded, the gelatinGG hydrogel has the highest yield stress. More stress/force needs to be exerted during printing for the material to start to yield and flow. When attempting to pneumatically extrude the gelatinGG with serum hydrogel, a greater amount of pressure is required for the hydrogel to be extruded.

Fig 5.9: Storage modulus plotted against Shear stress

Frequency sweep Measuring Profile:

Strain = 1 % ; Angular Frequency = 0.1 to 100 rad/s log ; Temperature = 25 °C.

Whichever modulus is dominant at a particular frequency will indicate whether the hydrogel appears to be elastic or viscous, in a process of similar time scale. Elasticity is the tendency of solid materials to return to their original shape after undergoing shear stress. When there is no more shear stress acting on the object, it will return to its initial shape and size. Viscous behavior is referred to as Newtonian behavior, viscous materials resist shear flow and strain linearly with time when a stress is applied. The

(9)

storage modulus G' is more dominant than the loss modulus G" across different frequencies. Therefore, the hydrogel samples exhibit more elastic behaviour, and can be described as a Gel rather than a liquid.

When there is serum in the hydrogel, there is a higher modulus, as observed below. That means that the addition of serum causes the hydrogel to be stronger and have a better resistance to shear stress. Therefore, when the hydrogels containing serum are being pneumatically extruded, they are able to be picked up without breaking.

Fig 5.9.1: Storage modulus G’ plotted against Angular Frequency

Fig 5.9.2: Loss modulus G” plotted against Angular Frequency

Hydrogel s

Can be picked up w/out breaking

Smooth extrusion

Adheres to geometrical boundaries

mcGG

w/serum

^✓ ^✓ ^✓

gelatinG G w/serum

✓

^✕

✓

Fig 5.9.3: Comparison table (hydrogels with serums)

The aim of this research was to formulate a hydrogel suitable for facial applications: able to be picked up without breaking, able to be extruded smoothly and without clumps, and able to conform to the geometrical boundaries we set. In the table above, we compare the 2 hydrogels that have been incorporated with serum. In an ideal facial mask, serum should be inside of the hydrogel. Therefore, the only hydrogel (with serum) that fulfils our definition of printability would be the mcGG with serum hydrogel.

6. CONCLUSION

This study has concluded that the hydrogel most suitable for the manufacturing of facial masks is 1.5% Methylcellulose and 1.5% Gellan Gum with 5ml of serum. After attempting to extrude the Methylcellulose and Gellan Gum and serum (mcGG with serum) hydrogel through a 3D printer, this study has shown that the mcGG with serum hydrogel best adheres to the geometrical boundaries, and suits the printing parameters, for the purpose of providing a base material for the fabrication of facial masks, when integrated with the serums. The rheological testing has also shown that the mcGG with serum hydrogel has the best resistance to shear stress. Therefore, it is the best out of the four for manufacturing facial masks, as it can be pneumatically extruded, but the viscosity of the hydrogel won’t decrease when touched by human hands. This is exemplified in picking up

(10)

the mcGG with serum hydrogel without it breaking.

These results/observations ensure that the hydrogel is: able to be printed in the distinct shape of a facial mask, behave like a gel, and thus can be physically picked up and placed on a substrate (the face). This study has also proved that the addition of serum helps improve the rheological properties of the hydrogel to better fit our definition of printability.

As this study only covers three base materials (Gellan Gum, Gelatin, and Methylcellulose) in the formation of hydrogels, and only including the serums provided by NOVU Aesthetics (‘SMOOTHENING HYDRATOR’, ‘RADIANCE ESSENCE’) in the final formulation, this study is restricted to these materials and may overlook other hydrogels as better alternatives to fabric facial masks. This study also does not cover the rate of transdermal absorption of the serums in skin in the presence of the end-product. As such, the scope of study is limited and the end-product is not altogether conclusive. In order to extend this study, more base materials and serums can be researched. In addition, the integration of different hydrogels should be studied to fully utilise the capabilities of 3D printing a facial mask, and be able to print different hydrogels and serums together as one facial mask. The transdermal absorption rate of the hydrogels formulated from this research should also be studied to further reinforce the belief that using a hydrogel facial mask is more beneficial than using a traditional fabric facial mask due to its higher rate of transdermal absorption through the skin.

7. ACKNOWLEDGMENTS

This research was supported by Nanyang Technological University, School of Mechanical

& Aerospace Engineering. We would like to thank Prof. Yeong Wai Yee and Dr. Lee Jia Min for their guidance throughout the course of the research. We are also immensely grateful to our research mentor, Mr. Sean Suen, who provided insight and expertise that greatly assisted the research.

8. REFERENCES

“Biomedical Applications of Hydrogels Handbook”, edited by R. M. Ottenbrite, K. Park, T. Okano. Springer 2010.

J. Malda, J. Visser, F. P. Melchels, T. Jüngst , W. E. Hennink, W. J. A. Dhert, J. Groll, D. W. Hutmacher, “Engineering ^Hydrogels ^for Biofabrication,” Advanced Materials, 25, 5011-5028 (2013).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5 811736

Assoc. Prof Li Lin (2018). “Rheology of hydrogels” [Powerpoint slides].

The Difference Between Elastic Materials and Viscoelastic Materials. (_n.d.). Retrieved from https://www.sorbothane.com/the-difference-betw een-elastic-materials-and-viscoelastic-materials. aspx

Eckelt, A. (n.d.). Retrieved from http://www.wee-solve.de/en/rheology.html Desbrieres, Jacques & Hirrien, M. & Ross-Murphy, S.B.. (2000). Thermogelation of methylcellulose: Rheological considerations.

Polymer. 41. 2451-2461.

10.1016/S0032-3861(99)00413-9.

Basics of rheology :: Anton Paar Wiki. (n.d.) Retrieved from

https://wiki.anton-paar.com/sg-en/basics-of-rheol ogy/

3D Printing For Facial Application