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Nanotribology of Skin Cream Using AFM and Nanoindentation Prof. Bharat Bhushan Ohio Eminent Scholar and Howard D Winbigler Professor Ohio Eminent Scholar and Howard D. Winbigler Professor and Director NLBB [email protected] Nanotribology Laboratory for Information Storage and MEMS/NEMS © B. Bhushan Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 1

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Page 1: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Nanotribology of Skin Cream gyUsing AFM and Nanoindentation

Prof. Bharat BhushanOhio Eminent Scholar and Howard D Winbigler ProfessorOhio Eminent Scholar and Howard D. Winbigler Professor

and Director [email protected]

Nanotribology Laboratory for Information Storage and MEMS/NEMS

© B. BhushanNanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 1

Page 2: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Micro/nanoscale studies

Bio/nanotribology Bio/nanomechanics Biomimetics

Materials sci biomedical eng physics & physical chem

Materials/DeviceStudies

• Materials/coatings

Techniques

Materials sci., biomedical eng., physics & physical chem.

NanoindentorMicrotriboapparatusAFM/STM

• SAM/PFPE/Ionic liquids• Biomolecular films• CNTs

• Micro/nanofabricationNanoindentorMicrotriboapparatusAFM/STM

Collaborations Applications

Numerical modeling and simulation

Collaborations

• MEMS/NEMS• BioMEMS/NEMS• Superhydrophobic surfaces• Reversible adhesion

Applications

• Reversible adhesion• Beauty care products• Probe-based data storage

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 2

Page 3: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Laboratory Facilities5700 sq. ft. of lab space (T&H controlled; part lab – class 100 clean room)

Major Equipment• Scanning tunneling microscope & five Atomic force/Friction force

microscopesp• Noncontact optical profiler and stylus profiler• Microhardness and Nanohardness testers• High vacuum tribotest apparatus equipped with mass spectrometer and

Augerg• Microtriboapparatus for Micro/Nanoscale Devices• Pin-on-disk type continuous/Reciprocating sliding test apparatuses• Vapor and liquid deposition facilities for self-assembled monolayers and

liquid lubricationq• Scanning ellipsometer• Particle counters/Optical microscopes/Microbalance/Environmental

chambers• Micro/nanofabrication facilities for bio/nanotechnology and biomimetics gy

research• Access to various physical and chemical analyses facilities at OSU

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 3

Page 4: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 4

Page 5: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Director: Prof. Bharat BhushanSenior/Visiting Scientists2005- Dr. Manual L. B. Palacio, “Nanoindentation,” Ph. D. Materials Science and Eng.

2009- Dr. Hyungoo Lee, “AFM Nanotribology,” Ph.D. Mechanical Engineeringy g gy g g

2010- Usama Heiba, “Nanotribology of Contact Switches,” Ph.D. Mechanical Engineering

2010- Dr. Monalisa Mazumder, “AFM Characterization of Biologically Active Electrokinetically-Altered Fluids ” Ph D Chemical EngineeringFluids, Ph.D. Chemical Engineering

Ph.D. Students2006- Shrikant C. Nagpure, “Multiscale Characterization of Aging Phenomena in Li-Ion Batteries,”

Department of Mechanical Engineering

2010- Daniel R. Ebert, “Biomimetics,” Department of Mechanical Engineering

M.S. Student2009- Brian Dean, “Biomimetics,” Department of Mechanical Engineering2009 Brian Dean, Biomimetics, Department of Mechanical Engineering

2010- James Hunt, “Adaptive Optics for Microprojectors,” Department of Mechanical Engineering.

2010- Steven Englehaupt, “Tribology of Skin Cream,” Department of Mechanical Engineering.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 5

Page 6: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Sponsors

IndustrialCorning

RevalesioMicroMedMicroMedTechnova

Government SponsorsGovernment SponsorsNational Science FoundationNational Institute of Health

Institute for Materials ResearchEuropean Unionp

Instrumentation SupportVeeco Instruments

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 6

Page 7: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Industrial Membership Fees

• Annual membership fee is an unrestricted annual grant to NLBB

• Three levels of membershipMember $ 25,000Advisory Member $ 50,000Senior Member $ 75 000Senior Member $ 75,000Corporate Member $100,000

• Members will have limited access to lab facilities AdvisoryMembers will have limited access to lab facilities. Advisory, senior and corporate members can initiate and guide research project.

• Contract research can also be initiated. The cost usually runs from $60 to $100 k per year.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 7

Page 8: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Sponsor Benefits

• Access to all laboratory facilities

• Initiate and guide a research project (Advisory Senior and• Initiate and guide a research project (Advisory, Senior and Corporate members)

• Receive all written reports before they are submitted for external p ypublication or are available for general distribution

• Attend Industry/University Technical Exchange meetings held once per yearonce per year

• May send one representative to any short courses offered by NLBB

• May place a visiting Industrial Fellow for any period

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

• List of students available for employment

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Page 9: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Ongoing Research Projects

Nanotribology and Nanomechanics• Influence of Water on the Interaction Between Graphite and Carbon Nanotubes• Nanotribological and Electrical Characterization of Ultrathin Wear-Resistance Ionic Liquid Films

Bio- & Nanotechnology• AFM Characterization of Biologically Active Electrokinetically-Altered Fluids• Nanolubrication of Sliding Components in Adaptive Optics Used in Microprojectors• Charging/Discharging in Capacitive RF-MEMS Switches Using an AFMg g g g p g• Morphology and Mechanical Properties of Block Copolymer Films for Bone Regeneration Applications (with

Prof. S. Schricker, College of Dentistry)

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 9

Page 10: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Biomimetics• Lotus Effect: Roughness-Induced Superhydrophobic Surfaces• Mechanically Durable Superhydrophobic, Self-Cleaning, and Low-Drag Surfaces with Hierarchical Structure• Shark-Skin Inspired Structures for Low Drag• Gecko-Inspired Hierarchical Structures for Reversible Adhesion

Beauty Care Products• Adhesion, Friction, and Wear Characterization of Skin and Skin Cream Using Atomic Force Microscopy

Batteries Aging• Aging Mechanisms in Lithium-ion Batteries (with Profs. S. Babu, G. Rizzoni, and Y. Guezennec, College of

Eng.)

htt // h d / lbb/http://www.mecheng.osu.edu/nlbb/

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Page 11: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Nanotribology of Skin Cream

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Page 12: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

MotivationIntroduction

• Skin cream improves skin health and creates a smooth, soft, and moist perception by altering the surface roughness, friction, adhesion, elasticity, and surface charging characteristics of skin surface.Th t til ti f ki t t i di t d b ki ib ti hi h• The tactile perception of skin texture is mediated by skin vibrations which are highly dependent on the friction properties of the surface.

• A detailed study should include both friction and resulting skin vibrations.

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Schematics of the application process of skin cream and the interaction between the skin vibration and the brain perception of the texture

Page 13: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Backgroundg• Many studies have focused on the friction and triboelectrical properties of

skin and skin cream on the macroscale.• The study of the coefficient of friction, adhesive force, and surface potential

on the nanoscale provides a fundamental understanding of the mechanisms behind how skin cream interacts with skin. Little data exists.

Engineering Interface Tip - based microscope allows simulation of a

• Viscosity of very thin liquid films increases exponentially with a decrease in thickness.

single asperity contact

During rubbing of the cream over skin surface, it gets thinner, the person starts to apply it at increasing high interfacial speeds. This should lead to high friction towards the end due to high viscosity and high viscous shear rates Understanding of the effect of thickness and shear rates is cr cialrates. Understanding of the effect of thickness and shear rates is crucial.

• Skin is the outermost organ of the body and is easily affected by the environment (humidity, temperature, and surface charge).

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Page 14: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Project PlansPart Ⅰ: Adhesion friction and wear study of virgin skin and commonPart Ⅰ: Adhesion, friction and wear study of virgin skin and common

cream treated skin- Surface roughness and friction force - Effect of film thickness, velocity and normal load on friction andEffect of film thickness, velocity and normal load on friction and

adhesion at nanoscale- Effect of relative humidity and temperature- Durability study

- Effect of film thickness, velocity and normal load on adhesion and friction, and durability at macroscale

Part Ⅱ: Adhesion, friction and wear study of various creams treated skin and effect of humidity

Film thickness adhesion Young’s modulus maps at various- Film thickness, adhesion, Young s modulus maps at various humidities

- Durability study

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Page 15: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Part Ⅲ: Triboelectrification of virgin skin and common cream treated skin- Effect of Velocity, normal load, rubbing time, and relative

humidity on electrostatic charging of virgin skin and treated skin

Part Ⅳ : Nanomechanical properties of skin and skin creamN t h- Nanoscratch

- Nanoindentation- In situ tensile properties

Part Ⅴ: Nanoscale characterization of synthetic skins for cosmetic science

- Surface textureSurface texture- Film thickness and adhesive force maps - Friction force - Nanoindentation

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Page 16: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Experimental Samples

• Category of skin and skin creams

- Rat skin with common cream (Vaseline Intensive Care)

- Rat skin, pure lanolin, pure petroleum jelly, aqueous glycerin (weight fraction = 1/4), common cream and oil free cream

- Two synthetic skins and rat skin, common cream

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Page 17: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Experimental Techniqus

• Nanoscale measurements were conducted using the AFM and nanoindenter.

• A homemade humidity control chamber and a temperature control system were used to study the effect of relative humidity and temperature.

• Macroscale tests were conducted to compare with nanoscale tests by using a ball-on-flat tribometer.

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Page 18: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

AFM

Schematic of a small sample AFM/FFM

Schematic of a large sample AFM/FFM

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Page 19: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Three-sided pyramidal (natural) diamond tip

Square pyramidal silicon nitride tip

Square pyramidal Single-crystal silicon tip

Carbon nanotube tip

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Various AFM tips19

Page 20: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Optical micrographs of standard tip and microtips of different radii

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Page 21: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Various AFM operating modes

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Tapping mode (constant amplitude) is used for roughness measurements. Contact mode is used for adhesion, friction and durability studies. Torsional resonance (TR) mode (constant load) is used to measure in-plane (lateral) heterogeneity (elastic and viscoelastic properties).

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Page 22: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

• Film thickness, Fad and Young’s modulus mapping of various creams , ad g pp gtreated skin was obtained using the force calibration plot technique (force-volume mode.- The film thickness is the sum of the travel distance of the piezo (h1)

d th d fl ti f til (h )and the deflection of cantilever (h2).

- Fad is calculated from the force calibration plot by multiplying the spring constant with the vertical distance between point B and Fspring constant with the vertical distance between point B and F.

- Young’s modulus can be determined using Hertz analysis:

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3aF F RE z+ = Δ

A typical force distance curve for cream treated skin

Page 23: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

• Surface potential was measured using Kelvin probe method with the p g pMultimode III AFM.

• Skin samples were rubbed with polystyrene as it is known that it creates a charge on skin surface. They were rubbed on the microscale with 45 µm φ polystyrene microsphere mounted on the AFM tip and on the macroscale with a polystyrene plate in a tribometer. Surface potential map of skin samples (electrically i l t d f d t t i k di h ) bt i disolated from ground to prevent quick discharge) were obtained.

–Before rubbing–After rubbing

Before rubbing, skin surface is negatively charged because the epithelial cells in skin carry a negative charge

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cells in skin carry a negative charge.

Page 24: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

In-Situ Tensile Measuring SetupS tu e s e easu g Setup

• In situ tensile measurements are made using a custom built tensile

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• In situ tensile measurements are made using a custom-built tensile stage attached to the AFM base and uses a linear stepper motor to load a sample in tension.

Page 25: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

NanoindenterNanoindenter

SEM imagingg g

Philips XL-30 ESEM

Hair sample sputtered with thin gold coating prior to SEM measurementscoating prior to SEM measurements

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Page 26: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Tribometer with an environmental chamber

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Page 27: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Part Ⅰ : Adhesion, friction and wear study of virgin skin and common cream treated skin

Surface roughness and friction force on virgin skin and common cream treated skinvirgin skin and common cream treated skin

at nanoscale

Surface roughness statistics and contact angleSurface roughness statistics and contact angle

Skin type

Surface roughness statisticscontact angle

(°)RMS (nm) P-V distance (nm)(nm)

Virgin skin 160 ± 28 983 ± 156 66 ± 6

Cream treated skin 119 ± 20 730 ± 125 40 ± 5

• After the application of skin cream, the surface roughness and contact angle decrease and friction force increases.

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Representative surface roughness and friction force AFM images

W. Tang and B. Bhushan, Colloid Surface B 76, 1 (2010)

Page 28: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Effect of Film Thickness

• The coefficient of friction (COF) and Fad increase as film thickness increases.

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Page 29: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Effect of Velocity

• For both virgin and treated samples, when the velocity is below 1000 μm/s the COF decreases (decreasing meniscus contribution) with an increase of velocity. When it is higher

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( g ) y gthan 1000 μm/s the COF increases due to increasing asperity deformation and viscous shear (in the case of treated).

• Fad for both decreases with an increase in velocity due to reduced meniscus contribution.

Page 30: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Effect of Normal Load

• The COF increases above a critical load, due to large asperity deformation.• The critical load in the cream treated skin increases from ~ 250 to 500 nN, which

suggests that the cream film serves as a protective layer on the skin surface.• For both samples F increases slightly with load

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 30

• For both samples, Fad increases slightly with load.

Page 31: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Effect of Relative Humidity and Temperature

• For virgin and treated samples, the COF and Fad increase as the relative humidity increases. Adsorbed water molecules increase film thickness leading to higher COF and F d

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 31

Adsorbed water molecules increase film thickness leading to higher COF and Fad.• For both samples, COF and Fad decrease as the temperature increases. Desorption of water

molecules in both is believed to be responsible. Reduction in viscosity at higher temperatures also should lead to a decrease in friction.

Page 32: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Durability studyy y

• For virgin skin, COF and Fad remain constant during the initial few cycles and then increase and decrease, respectively. This is due to removal of the lipid film on virgin p y p gskin.

• For treated skin, COF and Fad decrease with the sliding cycles. This is primarily due to adsorption of the skin cream by skin and more uniform distributionby skin and more uniform distribution.

• Damage observed from AFM profiles of worn area shows scratches created by the sharp tip.

Surface roughness images

32Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Page 33: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Macroscale StudiesFilm thickness, velocity, normal load, and durability study

M it d f COF i hi h th• Magnitude of COF is higher on the macroscale than on the nanoscale.

• COF increases with the cream thickness increasing up to 1 4 μm and then itincreasing up to 1.4 μm, and then it decreases with the further increase of cream thickness.

• Velocity has no effect on COF for virgin skin, but a higher velocity leads to a lower COF for treated skin (shear thinning).

• COF decreases as the normal load increasesincreases.

• COF of virgin skin during sliding remains constant. For treated skin, COF increases when the sliding cycle increases up to about e t e s d g cyc e c eases up to about400, after which it decreases slightly and then remains constant

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Page 34: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Summary• Skin cream reduces the surface roughness and increases the

hydrophilicity of skin.

y

• For a thicker film, the larger meniscus at the contact interface results in larger meniscus forces leading to larger COF and Fad.

• Menisci shear and alignment of molecules affect COF and Fad under lowMenisci shear and alignment of molecules affect COF and Fad under low velocity and viscoelastic shear affect under high velocity.

• Cream film exhibits a larger load carrying capacity and serves as a protective covering to the skin surfaceprotective covering to the skin surface.

• Increase of humidity increases adsorbed water molecules leading to high COF and Fad.

• Desorption of water molecules and reduced viscosity with an increase in temperature is responsible for decrease in COF.

Magnit de of COF is higher on the macroscale than on the nanoscale

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

• Magnitude of COF is higher on the macroscale than on the nanoscale.

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Page 35: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Part Ⅱ: Adhesion, friction and wear study of various creams treated skin

Friction and Adhesion of

• Among the five kinds of skin l li d

various Skin Creams

cream, pure lanolin and pure petroleum jelly have the highest coefficient of friction and adhesive force (highest viscosity) andforce (highest viscosity), and aqueous glycerin has the lowest coefficient of friction and adhesive forceforce.

Friction, adhesion, dynamic viscosity d d bilit f i ki

35Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

and durability of various skin creams

W. Tang, B. Bhushan, and S. Ge, J. Micros. 239, 99 (2010).

Page 36: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

• There is a good correlation b t ffi i t f fi tibetween coefficient of fiction and dynamic viscosity.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 36

Dynamic viscosity measured using a rheometer with parallel plate configuration.

Page 37: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Film thickness, Fad and Young’s modulus maps of various creams treated skin at various humidity

• Film thickness and Fad increase as the humidity increases; Young’s modulus decreases as the humidity increases (adsorption leading to compliance).

• The cream film is unevenly distributed on skin surface.

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Page 38: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

• For the five kinds of creams, film thickness and Fad i th h iditincrease as the humidity increases; Young’s modulus decrease as the humidity increasesincreases.

• Relative changes in the film thickness, adhesive force, and effective Young’sand effective Young s modulus of pure lanolin, pure petroleum jelly treated skin, and virgin skin are less than

Relative change (wrto 55% RH) in film thickness, Fad and effective Young’s modulus of various creams at various h idit

gthe oil free cream, common cream, and aqueous glycerin treated skin.

humidity

38Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics

Page 39: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Summary• The cream film unevenly distributes on skin surface.

P l li d t l j ll h hi h d bilit d ti k

Summary

• Pure lanolin and pure petroleum jelly have high durability and sticky tactile perception compared with other skin cream. The higher viscosity results in higher friction (stickiness) and longer durability.

• Adsorption of water molecules increases the film thickness and Fad,softens the skin surface.

• Lanolin and petroleum jelly is less sensitive to the change of humidity compared to others.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 39

Page 40: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Virgin skin and treated skin both before and after rubbing with l t l t (5 5 ) t t l iti

Part Ⅲ: Triboelectrification of virgin skin and common cream treated skin

• Virgin skin has larger absolute surface

polystyrene plate (5 × 5 mm) at two velocities

absolute surface potential compared to cream treated skin.

• For virgin skin, the absolute surface potential decay with time. For cream treated skin, it is more constant.

Samples scanned top to bottom. Relative surface

t ti l i b l t f

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potential is absolute surface potential minus average value.

Page 41: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Virgin skin and treated skin both before and after rubbing with l t i h (d 45 ) t t l itipolystyrene microsphere (d = 45 μm ) at two velocities

• For virgin skin, the absolutethe absolute surface potential increase with time For creamtime. For cream treated skin, it is more constant.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 41

B. Bhushan, W. Tang, and S. Ge, J. Vac. Sci. Technol. A 28, 1018 (2010)

Page 42: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Summary of effect of velocity, normal load, and rubbing time on surface t ti l f i i ki d t t d kipotential of virgin skin and treated skin

• Charge build up in treated skin is less than virgin skin.• The surface potential increases with an increase of velocity normal load and rubbing time

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 42

• The surface potential increases with an increase of velocity, normal load, and rubbing time when skin is rubbed with polystyrene plate and decreases when rubbed with microsphere.

• Compared with virgin skin, the surface potential of cream treated skin does not change much with an increase of velocity, normal load, and rubbing time.

Page 43: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Surface potential of various treated skin at various humidities

• For all skin samples, the surface potential increases at low humidity , decreases at high humidity. (Data shown only for common cream.)

• Compared to oil free cream, common cream, aqueous glycerin, and virgin skin, p re lanolin and p re petrole m jell

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 43

pure lanolin and pure petroleum jelly have lower relative change in surface potential at RH 8%.W. Tang, B. Bhushan, and S. Ge, J. Micro. 239, 99 (2010).

Page 44: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Summary

• Cream treated skin significantly reduces the charge buildup on skin surface.

y

p

• Electrostatic charge on skin can dissipate rapidly. The cream treatment can increase the rate of dissipation.cream treatment can increase the rate of dissipation.

• The increasing of velocity, normal load, and rubbing time increase the electrostatic charging on skin surfaceincrease the electrostatic charging on skin surface.

• Low humidity increases the surface potential, and high humidity increases ithumidity increases it.

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 44

Page 45: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Part Ⅳ: Nanomechanical properties of skin and skin cream

NanoscratchNanoscratch

• Virgin skin could be scratched at a normal load of 3 µN and 15 µcycles. The average scratch depth increases almost linearly with an increase in the normal load.

• For cream treated skin, the scratch depth increases little until a critical normal load of 15 µN isa critical normal load of 15 µN is reached, above which the scratch depth increases rapidly.

AFM topographical images of scratch, and (b) scratch depth as a function of normal load

Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics 45B. Bhushan, W. Tang, and S. Ge, J. Microscopy (in press).

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Nanoindentation

• At 1000 nm indent depth, the p ,load on virgin skin is about 35 µN, and load on cream treated skin is about 22 µN.

• The hardness and elastic modulus of treated skin is lower than virgin skin, i di ti th t th kiindicating that the skin cream can moisten and soften the skin surface.

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Representative load–displacement plots of nanoindentations

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In situ tensile properties• During low strain, the stress-strain

curve ascends according to an exponential function. Afterward an almost straight section is reached.

• For virgin skin, the number of patches present on skin surface atpatches present on skin surface at around 10% strain increase as the strain increases.

• For treated skin, there are few t h t ki f tpatches present on skin surface at

around 20% strain.• The patches occur due to interlayer

shear force and consequent s ea o ce a d co seque tseparation between inner and outer keratin layer.

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Stress-strain curves, and (b) AFM topographical images of a control area showing progress of damage with increasing strain

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Elastic modulus, ultimate tensile strength, and ultimate strain, obtained using in situ tensile tester

Virgin skin Cream treated skinElastic modulus (MPa) 31±11 22±7

Ultimate tensileUltimate tensile strength(MPa) 11±2 10±2

Ultimate strain (%) 59±3 62±5

• It shows that there is a slight decrease in the tensile properties of treated skin. The elastic modulus is a little lower than virgin skin, andtreated skin. The elastic modulus is a little lower than virgin skin, and ultimate strain is a little higher than virgin skin.

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Summary• Cream treated skin exhibits better scratch resistance up to a normal

load of 15 µN. Once the normal load exceeds the value of 15 µN, the protection of cream film fails

Summary

the protection of cream film fails.

• The hardness and elastic modulus of cream treated skin is lower than virgin skin indicating that the skin cream can moisten andthan virgin skin, indicating that the skin cream can moisten and soften the skin surface.

• The stress-strain curves show a characteristic shape, which isThe stress strain curves show a characteristic shape, which is related to the deformation of the collagen fibers of the dermis.

• Skin cream moistens and softens the skin surface which increases the extensibility of the keratin layer and reduces the generation of fractions (patches) as the strain increases.

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Page 50: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

Part Ⅴ: Nanoscale characterization of synthetic skins for cosmetic science Preparation Procedure

Synthetic Skin -1 – Dragon Skin®

High performance cast silicone rubber used for applications including

p

High performance cast silicone rubber used for applications including medical prosthetics and cushioning applications.

Synthetic Skin -2Synthetic Skin 2Mixture of gelatin from porcine skin, glycerol, formaldehyde, prolipid, and water cast on a replica of human skin make of silicone (originally developed for dental implants) (Lir et al., 2007). p p ) ( , )

Measure surface roughness, contact angle, adhesive force, friction force, and mechanical properties of the two synthetic skins and compare with rat skin.Apply skin cream to the synthetic skins and compare various properties with that of rat skin.

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W. Tang and B. Bhushan, (submitted).

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Surface texture

• Rat skin surface has some hair follicles• Rat skin surface has some hair follicles and fraction of hairs on the skin surface.

• The surface of the two synthetic skins areThe surface of the two synthetic skins are different with rat skin and there are no hair follicles and hairs present on skin surface.

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SEM images of the surface of virgin synthetic skin-1, synthetic skin-2, and rat skin

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Typical surface roughness AFM images and (b) RMS and P V distance

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Typical surface roughness AFM images, and (b) RMS and P-V distance

• In the case of various skin, before and after treatment, the RMS and P-V distance of the two synthetic skins and rat skin are comparable.

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Contact angleg

Contact angle of virgin skin and cream treated skin• In the case of virgin skins, the contact angle of the two synthetic skins and

rat skin are comparable. • After treatment the contact angles of the two synthetic skins and rat skin

Contact angle of virgin skin and cream treated skin

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• After treatment, the contact angles of the two synthetic skins and rat skin remain comparable and dramatically decrease as a result of treatment, indicating cream improves the hydrophilic properties of skin surface.

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Film thickness and adhesive force mapsp

• In the case of virgin skins, the film thickness and adhesive force of the two synthetic skins and rat skin are comparable.

• After treatment, the trends of film thickness and adhesive force

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Film thickness and adhesive forces maps, and (b) film thickness and adhesive forces of virgin skin and cream treated skin.

remain comparable, and both of them increase.

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Friction properties

• In the case of virgin skins, the coefficient g ,of friction of the two synthetic skins and rat skin is on the same order.

• After treatment trends of the friction force are similar but the values increase.

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Typical friction force AFM images of skin samples, and (b) coefficient of friction of skin samples

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Nanoindentation• In case of virgin skins, the hardness of synthetic skin-1 is higher than

synthetic skin-2 and rat skin, indicating its hard texture. The hardness of rat skin and synthetic skin-2 is comparable. The effective Young’s modulus of synthetic skin-2 is lower than synthetic skin-1 and rat skin.

• After treatment with skin cream, hardness and effective Young’s modulus of synthetic skin-1 show a slight decrease.

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Typical load versus displacement plots for the virgin, and (b) hardness and effective Young’s modulus of virgin skin and cream treated skin

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Summary

• Based on surface adhesion and friction properties, synthetic skins are good simulations of rat skin. However, the hardness of synthetic skin-2 i bl t t ki d b b tt i l ti

y

2 is comparable to rat skin and can be a better simulation.

• After treatment with cream, the trends of the properties of the two synthetic skins and rat skin are comparablesynthetic skins and rat skin are comparable.

• The methodology presented here can act as a good reference for researchers to evaluate the surface frictional and mechanicalresearchers to evaluate the surface, frictional, and mechanical properties of synthetic skins.

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Overall Summary and Future PlansOverall Summary• Nanotribology of skin cream and role of operating condition have been

studied using an AFM. • Skin cream reduces the surface roughness and increases the hydrophilic

y

Skin cream reduces the surface roughness and increases the hydrophilic properties of skin.

• The cream film unevenly distributes on skin surface.• The higher viscosity results in higher friction and longer durability• The higher viscosity results in higher friction and longer durability.• Cream treated skin significantly reduces the charge build up on the skin

surface.• Skin cream moistens and softens the skin surface.

• Synthetic skin provides a good simulation for the skin and can be used in nanotribology research.

Future Plans• Study the nanotribology of cream treated damaged skin

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Study the nanotribology of cream treated damaged skin• Develop a AE sensor to measure interface vibrations during sliding on

nanoscale and macroscale. Correlate vibration data to friction

Page 59: Nanoprobe Laboratory for Bio/Nanotechnology and ......Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics Director: Prof. Bharat Bhushan Senior/Visiting Scientists 2005-

References• B. Bhushan and C. LaTorre, “Structural, Nanomechanical, and Nanotribological , , , g

Characterization of Human Hair and Skin using Atomic Force Microscopy and Nanoindentation” in Springer Handbook of Nanotechnology, 3rd Ed., pp. 1055-1170, Springer-Verlag , Heidelberg, Germany, 2010.

• B Bhushan “Nanoscale Characterization of Human Hair and Hair Conditioners”• B. Bhushan, Nanoscale Characterization of Human Hair and Hair Conditioners , Progress Mat. Sci. 53, 585-710 (2008).

• Tang W. and Bhushan B., “Adhesion, friction and wear characterization of skin and skin cream using atomic force microscope,” Colloids Surf. B: Biointerfaces76, 1-15 (2010).

• Tang W., Bhushan B., and Ge S., “Friction, adhesion, and durability and influence of humidity on adhesion and surface charging of skin and various skininfluence of humidity on adhesion and surface charging of skin and various skin creams using atomic force microscopy,” J. Microscopy 239, 99-106 (2010)

• Tang W., Bhushan B., and Ge S., “Triboelectrification studies of skin and skin cream using Kelvin probe microscopy,” J. Vac. Sci. Technol. A 28, 1018-1028 (2010).

• Bhushan B. and Tang W., and Ge S., “Nanomechanical characterization of skin and skin cream,” J. Microscopy, In press.

• Bhushan B. and Tang W., “Surface, Tribological and MechanicalBhushan B. and Tang W., Surface, Tribological and Mechanical Characterization of Synthetic Skins for Tribological Applications in Cosmetic Science,” J. Appl. Poly.Sci. (in press).

http://www.mecheng.osu.edu/nlbb/