Soft colloidal microgels as bio-functional materials: From mouth to gut

Dr. Anwesha SarkarAssociate Professor of Food Colloids

Soft colloidal microgels

Karg, Ritchering et al. (2019) Langmuir, Volume 35, Pages 6231-6255.

Thorne et al. (2011) Coloid Polm Sci Volume 289, Pages 625-646

Heyes & Brańka (2009). Soft Matter, Volume 5, Pages 2681-2685.

• Cross-linked polymeric discrete gel particle

• Diameter: nm to few μm

• Swollen by a solvent

• Physical nature fall between hard spheres and ultra-soft solids (dilute polymer solutions)

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Soft colloidal microgels - applications

Andablo-Reyes, Sarkar et al. (2019), Soft Matter, Volume 15, Pages 9614-9624

Karg, Ritchering et al. (2019) Langmuir, Volume 35, Pages 6231-6255.

• Fat replacement – lubricants/ viscosity modifiers

• Saturated fat replacement

• Act as Pickering stabilizers

• Ability to delay fat digestion

nm to µm

Water

Biopolymer (protein, starch)

Microgel paticles in food applications

Emulsion droplets

Microgel particles

Emulsion microgel particles

3

Case studies on microgels: mouth to gut

Whey protein microgelparticles (WPM)

1

Emulsion microgelparticles (starch)

2

Fusion of WPM at the interface

3

4

Sarkar, Andablo-Reyes et al. (2019) Current Opinion in Colloid and Interface Science, Volume 39, Pages 61-75.

Laguna and Sarkar (2017). Tribology – Materials, Surfaces & Interfaces, Volume 11, Pages 116-123.

Stokes et al. (2013). Current Opinion in Colloids and Interface Science, , Volume 18, Pages 349-359.

In mouth lubrication

TribologyRheology

Bulk property

Surface property

0.1-100 µm µm-nm>100 µm

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Why oral lubrication?

• Oral lubrication (1/μ) has been linked to creamy, smooth, slippery perception

• μ has been linked to ‘rough’, astringent perception

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How is oral lubrication measured?

Friction force (F) = μ × L Boundary

regimeFr

icti

on

co

effi

cien

t (μ

)

Viscosity (η) × Speed (V) × Load (L)

Stribeck curve

Hydrodynamic

regime

Mixed regime

regime

L

Disc

Ball

Tribometer (ball-on-disc)

Food

Upper palate

Tongue

Tongue-palate

V (Speed)

L (Load)

Sarkar, Andablo-Reyes et al. (2019) Current Opinion in Colloid and Interface Science, Volume 39, Pages 61-75.

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Bio-relevance in tribology?Dry tongue Wet tongue

Soft: PDMS surface

Wettability

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PDMS – HB/ HL+M

HB = 108°

HL+M = 47°

HL = 63°(after O2 plasma treatment,

3 days)

PDMS ball

(Ra < 50 nm, E=2.4 MPa)

PDMS tribopairsBovine submaxillary

mucin

PDMS disc

Laguna and Sarkar, (2017). Food and Function; Tribology - Materials, Surfaces & Interfaces, Volume 11, Pages 116-123

Sarkar et al., (2019). Advances in Colloid and Interface Science, Volume 273, Article Number 101034

1

2

Designing bio-relevant surfaces

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Torres … Sarkar (2018). ACS Applied materials & Interfaces, Volume 10, Pages 26893-26905

Sarkar et al. (2017). Langmuir, Volume 33, 51, Pages 14699-14708

Microgels: Oral Lubrication mechanism?

Whey protein microgelparticles (WPM)

1

Emulsion microgelparticles (starch)

2

10

shear rate/ s-1

10-4 10-3 10-2 10-1 100 101 102 103

/ P

a s

10-3

10-2

10-1

100

101

102

103

104

Glycerol

f = 80 vol%, ▼ 25 ○C, ∆ 37 ○C

f = 10 vol%, + 25 ○C, × 37 ○C

Viscosity of WPMWPM EMP

Whey protein microgel particle (WPM)

(Dh ~ 365 nm, -36.5 mV, pH 7)

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Shear rate increasing from 0.1 to 50 s-1

Shear rate decreasing from 50 to 0.1 s-1

• WPM particles - wide ranging h values as a function of shear history

• High values of h persist after subjection to fairly high shear rates (50 s-1)even though the systems are highly shear thinning

• Thus, the particles may aggregate or interpenetrate as a function of shearand volume fraction, but they are certainly not destroyed – high resilience

Viscosity of WPM

shear rate/ s-1

10-4 10-3 10-2 10-1 100 101 102 103

/ P

a s

10-3

10-2

10-1

100

101

102

103

104

𝜂 = 𝐾𝑑𝛾

𝑑𝑡

𝑛−1

10 vol%, ×

20 vol%, ○

50 vol%, ∆

60 vol%, ◊

75 vol%, □

80 vol%, ∆

WPM EMP

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f = 10 vol%

Smooth HB PDMS tribopairs (●)

HL+M-coated PDMS tribopairs (□)

Stribeck curve of phosphate buffer is represented by ▲

f = 80 vol%

Friction coefficients of WPMWPM EMP

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Boundary, U=3 mm/s (●)

Mixed regime, U= 100 mm/s (○)

Friction force in HB surfaces

Upper palate

Tongue

• Spherical WPM particles - aqueous “ball bearings” similar to oil droplets

• Tribology - strongly dictated by the volume fraction entrained within contacts

Sarkar et al., (2017). Langmuir

Liu et al., (2016). Food Hydrocolloids

Gabriele et al. (2010). Soft Matter

WPM EMP

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Boundary, U=3 mm/s (■)

Mixed regime, U= 100 mm/s (□)

Friction force in HL+M surfaces

30 μm 3 μm

Dh= 365 nm

After tribology

Dh= 380 nm

WPM EMP

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• WPM particles shear thin and show good lubricating performance in the boundary as well as mixed lubrication regimes

• Hydrophobic moieties of WPM particles -effective adsorption to HB PDMS surfaces

• Hydrophilic moieties formed a true hydration layer i.e. “surface separators”.

• Potential Applications: Fat mimetics

f ≥ 65%

WPM

Hydrophobic

Hydrophobic

Hydrophilic

f ≈ 10%

f ≈ 10%

WPM as a potential fat replacerWPM EMP

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Microgels: Oral Lubrication mechanism?

Whey protein microgelparticles (WPM)

1

Emulsion microgelparticles (starch)

2

Torres … Sarkar (2018). ACS Applied materials & Interfaces, Volume 10, Pages 26893-26905

Sarkar et al. (2017). Langmuir, Volume 33, 51, Pages 14699-14708

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Torres … Sarkar (2018). ACS Applied materials & Interfaces, Volume 10, Pages 26893-26905

Torres……Sarkar et al., (2017). Carbohydrate Polymers, Volume 178, Pages 86-94

Torres……Sarkar et al., (2017). Food Hydrocolloids, Volume 71, Pages 47-59

Torres……Sarkar et al., (2016). Trends in Food Science and technology, Volume 55, Pages 98-108

EMP for fat reductionEmulsion microgel particles (EMP)

Oil droplets stabilised

by OSA starch

Starch gel particle

(Young modulus ~ 1 kPa)

Oil droplets

WPM EMP

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Lubrication - emulsion dropletsArtificial saliva: salivary buffer (pH 6.8) + 75 U mL-1 α-amylase Time 0 s

Emulsion (20 wt%oil) Emulsion (20 wt% oil) + salivaoil

Emulsion + saliva

Emulsion

Larger extent of droplet coalescence

lower friction

Emulsion destabilisation

WPM EMP

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EMP on amylase additionArtificial saliva: salivary buffer (pH 6.8) + 75 U mL-1 α-amylase Time 0 s

AA

BB

CC

DD

0 s α-amylase 60 s α-amylase

60 vol%

emulsion microgel particles

No free oil or cream layer

+buffer (30 vol%)

+ α-amylase

WPM EMP

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Lubrication on shear & enzyme

Without saliva With saliva

0 wt% oil 10 wt% oil5 wt% oil 15 wt% oil

WPM EMP

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Theory behind lubrication

+ α-amylase

𝛿

𝑅=

𝑎

𝑅

2−

4

3𝜋 1−𝜐2𝑎

𝑅

3𝑓

𝑎

𝑅with 𝑓

𝑎

𝑅=

2 1+𝜐

4+𝑎

𝑅

2 3/2 +1−𝜐2

4+𝑎

𝑅

2 1/2

Lubricant typeR

(μm)

WL

(%)

𝜹

𝑹∗

η at

0.01 s-1

(Pa s)

Wp

(N)

Fd

(N)

Emulsion + buffer

(20 wt% oil)0.08 86 0.72 0.1

1.3

10-8 9.1 10-9

15 wt% starch particles

(30 vol%)15 29 18.7 3.5

1.5

10-4 2.9 10-5

Emulsion microgel

particles (30 vol%)15 86 12.7 200

4.5

10-4 1.7 10-3

a : radius of contact

WL : normal force supported by the lubricant

R* : reduced radius

v : Poisson’s ratio

Wp : normal force per particle

Fd : drag force

η: viscosity

Relative indentation (𝛿

𝑅) in the contact zone at 3 mm s-1 – Hertz Theory

deformation factor

WPM EMP

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EMP for fat reduction purposes

• Starch-based EMP provides excellent lubrication yet protectsthe oil droplets from complete coalescence (via shear and α-amylase).

• The EMPs can serve as unique delivery system for lipophilliccompounds/ saturated fat reduction

WPM EMP

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Surface roughness: 3D Printing

Conclusions from mouth case studies

• Soft tribology offers a great opportunity to understand oral lubrication mechanisms of microgels and other fat mimetic particles

• Microgels shows promise for lowering friction coefficient in soft contacts, highly dependent upon volume fraction of microgels and oil content

• Lubrication is a systems property, so work is ongoing on designing bio-relevant surfaces to better replicate human tongue and palate with accurate surface roughness & modulus

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Final case study on use of microgels - gut

Whey protein microgelparticles (WPM)

1

Emulsion microgelparticles (starch)

2

Fusion of WPM at the interface

3

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Sarkar et al., (2019). Advances in Colloid and Interface Science, Volume 263, Pages 195-211

Mackie et al. (2000), Langmuir, Volume 16, Pages 2242-2247

Orogenic displacement of interfacial film

Bile salts

pH 7

Lipase-colipase,

Trypsin,

Chymotrypsin

Intestine

Lipid digestion is an interfacial process

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oil

water

E =r2 (1-cos)2

e.g. θ ~30○, r =10 nm, =32mN/m

Desorption energy ~ 10, 000kBT

Hypothesis: Microgel particles at O/W interface will not get displaced by bile salts and eventually delay lipid digestion

Binks, (2002). Current Opinion in Colloid & Interface Science, Volume 7, Pages 21-41.

Oil

θ

r

Hypothesis microgels at the interface

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+ 20 wt% sunflower oil

WPM (1 wt%, pH 7.0,

20 mM PBS)

Heat 90○C

Scale 20 µm

Emulsions d43

(μm)

Adsorption

efficiency

(%)

Surface

coverage

(mg/ m2)

WPM 42.9 33 14.0

HT WPM 42.8 55 23.6

HT-WPM emulsion

Oil

HT-WPM emulsion (fused)2

Fused network

WPM emulsion

Oil

WPM emulsion (intact)1

WPM

Interfacial tuning

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0

10

20

30

40

50

0 30 60 90 120 150 180

% F

FA

Digestion time (min)

Lipid

LipidWNaOHNaOH

W

MMVFFA

2100%

tdnd

Dk

dt

d

M

denMax

en

en

w

1

2

6

2

0

0

0

3

0

wenen

Max

en

MnkD

dt

62ln 0

2

021

Intestinal digestion

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Interface k (μmol s-1 m-2) Φmax (%) t1/2

(min)

Protein 46 2.8

WPM 0.62 20 16.52

HT-WPM 0.18 16 44.44WPMBile

Lipase

Pickering emulsion

Oil

Heat treated (fused) Pickering emulsion

HT-WPM

Delaying lipid digestion - tuning interface

WPM: Gap dimension between the WPM particles, arranged on the

triangular lattice, is 3 − 1 𝑑0/2

≈ 110 nm, for particles of size d0 = 300 nm >>> 2.5 nm lipase/colipase

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• Engineering O/W interface with microgel-based Pickering stabilizers and tuning them with thermal treatment has implications on delaying digestion if gastric digestion is bypassed.

• Use of non-proteinaceous biopolymeric particles or suitable coating at interface is needed to avoid digestion during gastric regime.

Conclusions from the gut case study

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• Soft colloidal microgels are unique systems with fascinating bio-functionalities that distinguish them from classical colloids.

• Combination of deformability and penetrability – make them versatile in broad area of biological applications

• Shear thinning properties, lubrication aspects, performance at interface, encapsulating agent: key area for food applications

Overall summary: Microgel Bio-functionality

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The European Research Council is acknowledged for its financial support (Funding scheme, ERC Starting Grant 2017, Project N○ 757993, LubSat) for this work

Acknowledgements

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