Articleshttps://doi.org/10.1038/s41565-020-0747-9

Structured nanoscale metallic glass fibres with extreme aspect ratiosWei Yan   1,7, Inès Richard1, Güven Kurtuldu2, Nicholas D. James3,4, Giuseppe Schiavone   5, Jordan W. Squair   3,4, Tung Nguyen‐Dang   1, Tapajyoti Das Gupta   1, Yunpeng Qu1, Jake D. Cao2, Reinis Ignatans6, Stéphanie P. Lacour   5, Vasiliki Tileli   6, Grégoire Courtine   3,4, Jörg F. Löffler   2 and Fabien Sorin   1 ✉

1Laboratory of Photonic Materials and Fibre Devices (FIMAP), Institute of Materials, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. 2Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, Zurich, Switzerland. 3Center for Neuroprosthetics and Brain Mind Institute, School of Life Sciences, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. 4Defitech Center for Interventional Neurotherapies (NeuroRestore), University Hospital Lausanne (CHUV), University of Lausanne (UNIL) and École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. 5Bertarelli Foundation Chair in Neuroprosthetic Technology, Laboratory for Soft Bioelectronic Interfaces, Institute of Microengineering, Institute of Bioengineering, Centre for Neuroprosthetics, École Polytechnique Fédérale de Lausanne, Geneva, Switzerland. 6Institute of Materials, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. 7Present address: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA. ✉e-mail: fabien.sorin@epfl.ch

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1

Supplementary Information

Structured nanoscale metallic glass fibres with extreme aspect ratios

Wei Yan, Inès Richard, Güven Kurtuldu, Nicholas D. James, Giuseppe Schiavone, Jordan W.

Squair, Tung Nguyen-Dang, Tapajyoti Das Gupta, Yunpeng Qu, Jake D. Cao, Reinis Ignatans,

Stéphanie P. Lacour, Vasiliki Tileli, Grégoire Courtine, Jörg F. Löffler, Fabien Sorin

2

Contents:

1. Viscosity of typical thermoplastics and metallic glasses

2. Thermoanalysis and Time-Temperature-Transformation diagram of Pt57.5Cu14.7Ni5.3P22.5 bulk

metallic glass

3. TEM characterization of the MG ribbon drawn at each step

4. Scalability of thermal drawing of the MG ribbon

5. Thermal drawing of PMMA and Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass

6. Modeling of the axial velocity and diameter of a MG ribbon in the neck-down region

7. Modeling of the instability time of a MG ribbon in the neck-down region

8. Polar transformation of SAED patterns

9. Preparation of complex and structured MGs

10. Comparison of optoelectronic properties

11. Typical fibre implant

12. In vivo and in vitro electrochemical properties of in-fibre MG electrodes along with typical in-

fibre crystalline metallic electrodes and conductive polymeric nanocomposites

3

Supplementary Note 1: Viscosities of typical thermoplastics and bulk metallic glasses

Viscosity measurements of typical thermoplastics: The rheological properties of typical

thermoplastics (polyetherimide (PEI), polysulfone (PSU), poly(methyl methacrylate) (PMMA),

polycarbonate (PC)) were measured with a rheometer (TA AR2000) in flow mode, with shear rates

varying from 1 s−1 to 2.5 s−1, so that the polymer flow was in the Newtonian regime.

Supplementary Fig. 1. Temperature dependence of viscosities for typical thermoplastics PEI,

PSU, PMMA and PC used as claddings for thermal drawing, and some typical bulk metallic glasses

(BMGs) (Zr41.2Ti13.8Cu12.5Ni10Be22.5, Pd43Cu27Ni10P20, Pt57.5Cu14.7Ni5.3P22.5, Au49Ag5.5-

Pd2.3Cu26.9Si16.3, and Zr35Ti30Cu8.25Be26.75). The data on these BMGs are taken from Ref. 1.

150 200 250 300 350 400 4501E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

Temperature (C)

Vis

co

sity (

Pa

s)

Zr41.2-MG

Pd43-MG

Pt57.5-MG

Au49-MG

Zr35-MG

PEI

PSU

PMMA

PC

4

Supplementary Note 2: Thermal analyses of Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass

Differential Scanning Calorimetry characterization: Differential scanning calorimetry (DSC)

measurements of PEI and Pt57.5Cu14.7Ni5.3P22.5 were performed using a Mettler-Toledo DSC 1/700

under 30 ml min-1 Ar flow. Their heating curves at a rate of 20 K min-1 presented in Supplementary

Fig. 2 demonstrate the onsets of the glass transition (Tg) and crystallization (Tx) temperatures

Time-Temperature-Transformation (TTT) diagram of Pt57.5Cu14.7Ni5.3P22.5 bulk metallic

glass: DSC measurements were also performed to construct the low-temperature part of a TTT

diagram (Supplementary Fig. 3) for Pt57.5Cu14.7Ni5.3P22.5. The temperature of the metallic glass

(MG) specimens was increased from room temperature to isothermal treatment temperatures in

the supercooled region (250, 260, 270, 280 and 290 °C). They were held at these temperatures

until they were fully crystallized. The time required for 5% and 95% crystal phase fractions were

determined for each isothermal temperature using the total enthalpy of crystallization.

Supplementary Fig. 2. DSC heating curves of PEI and Pt57.5Cu14.7Ni5.3P22.5 BMG at a rate of 20

K min-1. The onsets of the glass transition and crystallization temperatures are indicated by arrows.

These experiments were performed once for each sample and corresponds to literature data.

150 200 250 300 350

Tx= 311°CT

g= 228°C

PEI

Exo

MG

Temperature (C)

He

at

flo

w (

a.u

.)

20 K/min

Tg= 214°C

5

Supplementary Fig. 3. Time-Temperature-Transformation (TTT) diagram of the MG alloy

Pt57.5Cu14.7Ni5.3P22.5. For the DSC measurements, specimens with a mass between 5 and 20 mg

were cut from glassy rods. 5 specimens were tested at each temperature and the mean value is

plotted (n=5). Standard deviations of the measured time for 5% and 95% crystallization fractions

are shown by horizontal error bars, and the ±2°C accuracy of the DSC by vertical error bars.

100 1,000 10,000240

250

260

270

280

290

300

Te

mp

era

ture

(°C

)

Time (s)

5% crystallization

95% crystallization

6

Supplementary Note 3: TEM characterization of the MG ribbon drawn at each step

TEM sample preparation and characterization: The TEM samples of the MG ribbon drawn

once and twice were prepared by embedding the MG fibre in epoxy resin, followed by sectioning

thin slices (~70 nm), which were then transferred onto a carbon/Cu grid support (300 mesh). The

selected-area electron diffraction (SAED) patterns were taken using a Talos F200X operating at

200 kV. The TEM sample of the MG ribbon drawn three times was prepared by dissolving the PEI

cladding using N-Methyl-2-pyrrolidone (NMP), followed by 5 times cleaning with ethanol before

the ribbon was transferred onto a microelectromechanical system (MEMS) chip (commercially

available Wildfire, through hole, DENSsolutions). The SAED patterns were taken using a

ThermoScientific Titan Themis operated at 300 kV.

Supplementary Fig. 4.a-c, SAED patterns of the MG ribbon drawn once (thickness: 2.5 µm),

twice (thickness: 500 nm), and three times (thickness: 50 nm), respectively. Similar SAED patterns

were obtained in different region of each sample. Each characterization was repeated on 5 samples

giving the same results.

20 1/nm5 1/nm5 1/nm

a b c

7

Supplementary Note 4: Scalability of thermal drawing of the MG ribbon

Supplementary Fig. 5. a-c, Photographs of several-meter long MG-based fibres drawn once (2.5

μm ribbon thickness), twice (500 nm ribbon thickness), and three times (50 nm ribbon thickness),

respectively.

2.5 μm 500 nm 50 nm

a b c

8

Supplementary Note 5: Thermal drawing of PMMA and Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk

metallic glass

According to the viscosity measurements (Supplementary Fig. 1), PMMA and

Au49Ag5.5Pd2.3Cu26.9Si16.3 MG have a similar viscosity between 150°C and 175°C and could be

another pair of suitable materials to be co-drawn. We have successfully drawn a few meters of this

Au-based MG ribbon within a PMMA cladding while maintaining the ribbon integrity.

Supplementary Fig. 6 shows the processed fibre, with an SEM micrograph (cross-sectional view)

of the ribbon shown in the inset. Note that the roughness of the drawn ribbon comes from the initial

ribbon roughness and could be prevented by drawing ribbons with higher qualities.

Supplementary Fig. 6. Photograph of a drawn Au49Ag5.5Pd2.3Cu26.9Si16.3 MG ribbon with a

PMMA cladding. Inset: SEM micrograph of the fibre cross-section. Two fibres were prepared by

ultramicrotomie and showed similar cross-sections.

1 µm

10 µm 1 µm

2 µm

9

Supplementary Note 6: Modeling of the axial velocity and diameter of a MG ribbon in the

neck-down region

The tensile stress applied on the preform that is heated above its glass transition temperature Tg

leads to thermoplastic deformation of the preform. The deformation creates a neck-down region,

as shown in Supplementary Fig. 7a. The axial velocity of one domain in this region along the

drawing direction z can be expressed by2:

𝜈𝑧 = 𝑒𝑥𝑝 (𝑙𝑛𝜈f +∫

𝑑𝑧

𝜂(𝑧)

𝑧0

∫𝑑𝑧

𝜂(𝑧)

𝐿0

𝑙𝑛𝑣d

𝑣f), (Suppl. Eq.1)

where 𝑣f, 𝑣d are the feeding speed of the preform and the drawing speed of the fibre, respectively;

L is the length of the neck-down region, where the temperature is above the Tg of the MG so that

it can deform; ( )z is the viscosity of the Pt57.5Cu14.7Ni5.3P22.5 MG and we assume that it is only

temperature dependent (Newtonian regime) so that it can be obtained from Supplementary Fig. 1.

Assuming the conservation of mass for the system, the relation between the radius along the

drawing direction z and the initial radius can be expressed by:

( )

12

00

z

z

R z R

=

, (Suppl. Eq.2)

where 0R is the initial radius, ( )R z is the radius along the z axis, and 0z is the initial velocity.

In order to get the viscosities of the MG core, we precisely measured the temperature

distribution along the preform when it was soaked in the furnace of the fibre-drawing tower using

a thermocouple. The measurements were fitted by the quadratic function 𝑇(𝑧) =𝑇g−𝑇max

(𝐿 2⁄ )2 𝑧2 +

𝑇max, with 𝑇max = 258 °C, 𝑇g = 228 °C and 𝐿 = 6 cm. The result is plotted in Supplementary Fig.

7b, where the position having the highest temperature is set as the origin.

We consider that a 900-nm-diameter MG rod cladded by a PEI cladding is drawn into a 30-

nm-diameter MG fibre with a scale-down ratio of 30, with a typical 𝑣f= 1 mm min-1 and𝑣𝑑=

0.9 m min-1. Based on Suppl. Eq. 1, we obtain the velocity profile in the drawing direction

(Supplementary Fig. 7c) and based on Suppl. Eq.2, we obtain the diameter of the MG rod in the

neck-down region (Supplementary Fig. 7d) and the profile of the MG section (Supplementary Fig.

7a).

10

Supplementary Fig. 7. a, The profile of a MG section. b, Temperature distribution along the MG

section in the neck-down region. c, Velocity profile in the neck-down region. d, Diameter of the

MG section in the neck-down region.

Neck-down region

Ø = 30 nm

Ø =

90

0 n

m

a b

c d

-3 -2 -1 0 1 2 3

0.0

0.2

0.4

0.6

0.8

1.0

Ve

locity v

(z)

(m/m

in)

Position (cm)

-3 -2 -1 0 1 2 3

0

200

400

600

800

1000

Dia

me

ter

(nm

)

Position (cm)

-3 -2 -1 0 1 2 3225

230

235

240

245

250

255

260

Tem

pe

ratu

re (

°C)

Position (cm)

11

Supplementary Note 7: Modeling of the instability time of a MG ribbon in the neck-down

region

Classical Plateau-Rayleigh instability theory points out that when a cylindrical column of liquid is

surrounded by another substance, a sinusoidal perturbation wave may appear at the heterogeneous

interfaces between the two materials and finally lead to the breakup of the liquid. Here, we consider

the MG core heated at a temperature T as a viscous cylindrical thread (diameter 2R and viscosity

𝜂core) encased by an infinite viscous cladding PEI (viscosity 𝜂clad). We further consider that the

core is incompressible, Newtonian and isotropic. The time scale determining the growth of the

perturbation can be obtained using Tomotika’s linear theory3:

𝜏 =2𝑅𝜂clad

𝛾 𝑚𝑎𝑥[(1−𝑥2)𝛷(𝑥,𝜂core𝜂clad

)], (Suppl. Eq.3)

where is the interfacial energy between the cladding and the core metal, 𝑥 =2𝜋𝑅

𝜆 with 𝜆 the

wavelength of the perturbation, and an explicitly known function3:

𝛷(𝑥) =𝑁(𝑥)

𝐷(𝑥), (Suppl. Eq.4)

with

𝑁(𝑥) ≡ 𝐼1(𝑥)𝛥1 − {𝑥𝐼0(𝑥) − 𝐼1(𝑥)}𝛥2, (Suppl. Eq.5)

𝐷(𝑥) = (𝜂core

𝜂clad) {𝑥𝐼0(𝑥) − 𝐼1(𝑥)}𝛥1 − (

𝜂core

𝜂clad) {(𝑥2 + 1)𝐼1(𝑥) − 𝑥𝐼0(𝑥)}𝛥2

− {𝑥𝐾0(𝑥) + 𝐾1(𝑥)}𝛥3 − {(𝑥2 + 1)𝐾1(𝑥) + 𝑥𝐾0(𝑥)}𝛥4

(Suppl. Eq.6)

where I0(x) and I1(x) are the modified Bessel functions of the 0th and 1st order, respectively.

Furthermore, Δ1, Δ2, Δ3, and Δ4 are functions of x expressed in determinantal forms as follows:

𝛥1 = ||

𝑥𝐼0(𝑥) − 𝐼1(𝑥) 𝐾1(𝑥) −𝑥𝐾0(𝑥) − 𝐾1(𝑥)

𝐼0(𝑥) + 𝑥𝐼1(𝑥) −𝐾0(𝑥) −𝐾0(𝑥) + 𝑥𝐾1(𝑥)

(𝜂core

𝜂clad) 𝑥𝐼0(𝑥) 𝐾1(𝑥) −𝑥𝐾0(𝑥)

||

𝛥2 = ||

𝐼1(𝑥) 𝐾1(𝑥) −𝑥𝐾0(𝑥) − 𝐾1(𝑥)

𝐼0(𝑥) −𝐾0(𝑥) −𝐾0(𝑥) + 𝑥𝐾1(𝑥)

(𝜂core

𝜂clad) 𝐼1(𝑥) 𝐾1(𝑥) −𝑥𝐾0(𝑥)

||

12

𝛥3 = ||

𝐼1(𝑥) 𝑥𝐼0(𝑥) − 𝐼1(𝑥) −𝑥𝐾0(𝑥) − 𝐾1(𝑥)

𝐼0(𝑥) 𝐼0(𝑥) + 𝑥𝐼1(𝑥) −𝐾0(𝑥) + 𝑥𝐾1(𝑥)

(𝜂core

𝜂clad) 𝐼1(𝑥) (

𝜂core

𝜂clad) 𝑥𝐼0(𝑥) −𝑥𝐾0(𝑥)

||

𝛥4 = |

𝐼1(𝑥) 𝑥𝐼0(𝑥) − 𝐼1(𝑥) 𝐾1(𝑥)

𝐼0(𝑥) 𝐼0(𝑥) + 𝑥𝐼1(𝑥) −𝐾0(𝑥)

(𝜂core

𝜂clad) 𝐼1(𝑥) (

𝜂core

𝜂clad) 𝑥𝐼0(𝑥) −𝐾1(𝑥)

|, (Suppl. Eq.7)

The perturbation with the max[(1 − 𝑥2)Φ(𝑥, 𝜂core 𝜂clad⁄ )] will dominate the instability and

generate breakup (which is a very conservative estimate). We first model the instability time of a

typical metal, e.g. Sn, codrawn with a PSU cladding (𝜂core= 10-3 Pa s, 𝜂clad= 105 Pa s, = 0.1 N

m-1). The max[(1 − 𝑥2)Φ(𝑥, 𝜂core 𝜂clad⁄ )] reaches unity when x is around 0.015 (Supplementary

Fig. 8a). The instability time versus radius given by Suppl. Eq. 3 is plotted in Supplementary Fig.

8b. For the MG/PEI system, however, max[(1 − 𝑥2)Φ(𝑥, 𝜂core 𝜂clad⁄ )] is almost three orders of

magnitude lower compared to that of the Sn/PSU system (Supplementary Fig. 8c). This creates a

much longer instability time, as shown in Supplementary Fig. 8d. For example, the instability time

of the MG/PEI system is two orders of magnitude larger than that of the Sn/PSU system when the

diameter is 1 μm. Using the same drawing parameters and temperature profile as in section 3 and

assuming that the interfacial energy is dominated by the surface tension of Pt57.5Cu14.7Ni5.3P22.5

(𝛾 = 1 N mm-1, a typical value for BMGs4, which is another conservative estimate considering

that the interface between MG and PEI is intimate), we obtain the instability time at each position

in the neck-down region (Supplementary Fig. 8e).

In order to determine the stability of the MG rod, it is required to compare the processing time

(the time when the material dwells in the neck-down region before it exits the furnace) with the

capillary instability time. To do so, we use the growth factor Γ defined as5:

( ) ( )

/2

/2Γ

L

L

dz

v z z−= , (Suppl. Eq.8)

where ( )z is the instability time and ( )

dz

v zis the dwelling time at each position in the neck-down

region. Γ ≫ 1 corresponds to break-up while Γ ≪ 1 indicates a stable draw.

With our parameters we obtain

13

Γ = 0.79. (Suppl. Eq.9)

This value does not completely fulfill the criteria of Γ ≪ 1, but we made many conservative

assumptions. Therefore, we can expect a stable draw under these conditions.

Supplementary Fig. 8. a, [(1 − 𝑥2)𝛷 (𝑥,𝜂core

𝜂clad)] versus x for the Sn/PSU system. b, Instability

time versus Sn radius. c, [(1 − 𝑥2)𝛷 (𝑥,𝜂core

𝜂clad)] versus x for the MG/PEI system. d, Instability

time versus MG radius. e, Instability time in the neck-down region of the MG/PEI system.

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0 Sn/PSU

core

cladding

= −

(1-x

2)

(x)

x1E-06 1E-05 1E-04

1

10

100

1

10

100

Sn/PSU

core

cladding

= −

Inta

bili

ty t

ime

(s)

R (m)

-3 -2 -1 0 1 2 310

100

1000

10000

100000

Insta

bili

ty tim

e (

s)

Position (cm)

1E-9 1E-8 1E-7 1E-60.1

1

10

100

MG/PEI

core

cladding

=

= C

Inta

bili

ty tim

e (

s)

R (m)0.0 0.2 0.4 0.6 0.8 1.0

0.000

0.001

0.002

0.003

0.004

0.005

(1-x

2)

(x)

MG/PEI

core

cladding

=

= C

x

a b

c d

e

14

Supplementary Note 8: Polar transformation of SAED patterns

Applying polar transformation to the SAED patterns turns the curved, broad spots in the diffuse

rings to straight, broad lines (see Supplementary Fig. 9). In the polar transformed patterns two

rectangular regions of interest were selected to quantify Tx: one on the straight, broad line and the

other on the region just below the broad line, quantifying the background (the sizes of both regions

were the same).

Supplementary Fig. 9. a, Recorded original SAED pattern of the 45-nm ribbon. b, Polar

transformed SAED pattern. The image contrast in the top rectangular region and the bottom

rectangular region is the intensity of the spot and background, respectively. The temperature was

300 °C. The same results were obtained from 3 samples.

Temperature 300 °C

Temperature 300 °C a

b

15

Supplementary Note 9: Preparation of complex and structured MGs

Various methods were used to make macroscopic preforms from which complex and structured

MG fibres were fabricated. To make the preform shown Fig. Supplementary 10a, ten alternating

layers of MG and PEI were placed in a groove milled in one PEI plate before it was consolidated

via hot pressing with another PEI plate. The thickness of these MGs was ~10 μm, obtained by the

thermal drawing of a ~60 μm-thick MG ribbon. The same approach was used to make the preform

of the fibre shown in Fig. 4b. Here, the stack of alternating layers of MG and PEI was placed on

the edge of two PEI plates. To make the preform shown in Supplementary Fig. 10b, the MG ribbon

with the desired thickness was inserted between PEI thin films that were tightly rolled around a

ceramic rod and can be removed after consolidation in an oven under vacuum (to make the hollow

core fibre shown in Fig. 4c) or a PEI rod (to make the fibre shown in Supplementary Fig. 4d). To

make the preform shown in Supplementary Fig. 10c, an optical waveguide structure consisting of

a PEI core and a polyethersulfone (PES) cladding was first made via a thin-film rolling technique.

More PEI films were wrapped around the structure before the three MG ribbons with desired

thicknesses were introduced. After consolidating the structure, two pieces of MG rod-based fibres

drawn from a larger MG rod were inserted into the channels of the solid. To make the preform

shown in Supplementary Fig. 10d, four pieces of MG rod-based fibres were inserted into the

channels of the PEI rod. All the consolidations were performed under vacuum at a temperature

above the glass transition temperature of the PEI.

16

Supplementary Fig. 10. a, Schematic of the preform assembly for the fabrication of the

metamaterial fibre shown in Fig. 4a, consisting of ten alternating layers of MG and PEI. The same

approach was exploited to make fibres where the MGs are exposed on the fibre surface (Fig. 4b).

b, Schematic of the preform assembly for the fabrication of fibres containing a hollow air core

surrounded by a MG full cylinder (Fig. 4c) or a MG slotted-cylinder (Fig. 4d). c, Schematic of the

preform assembly for the fabrication of a hybrid fibre that integrates a PEI/PES optical fibre and

three MG ribbons as well as two MG rods (Fig. 4e). d, Schematic of the preform assembly for the

fabrication of a fibre for neural stimulation and recording (see Fig. 6).

Ceramic rod or PEI rod MG ribbon

PEI film

Drawn fibres

PEI rod

MG ribbons

MG cylindrical fibres

PEI/PES optical fibre

Alternating

layers of PEI

and MG

PEI

claddings

consolidation

consolidation

a b

c d

17

Supplementary Note 10: Comparison of optoelectronic properties

Supplementary Table 1 Comparison of performances of MG-based fiber optoelectronics with

previously reported fiber optoelectronics and nanoscale planar devices

R is the responsivity of the device, VSD is the source-to-drain voltage bias. While a direct and

rigorous comparison is not possible since it depends on several parameters (geometry, contacts,

power level, bias etc.), the values we give below are an indication that the MG-based fiber

optoelectronics show an excellent combination of high responsivity and high photosensitivity. In

particular, the photosensitivity is orders of magnitude higher than that of many other devices.

Reference Material Configuration R (A/W) Iph

/Idark

Light

source (nm) V

SD

(V)

This work Se-MG Nanowire 0.0015 12612 532 10

Nature Nano6 Graphene layers 0.0005 0.7 1550 0.004

Small7 Graphene layer 0.0001 1 532 0.1

Advanced Materials8 Se In-fiber nanowires 0.058 420 532 10

Advanced Functional

Materials9 Se Single Microtube 0.019 32 610 5

Advanced Materials10 Ge In-fiber spheres 0.000038 2.3 1550 3

Nano Letter11 CH3NH3PbI3 Nanowires 0.005 633 1

Journal of Materials

Chemistry C12 Sb2Se3 Nanowires 150 10

Advanced Materials13 ZnSe Horizontal

nanowires 1000 405 30

18

Supplementary Note 11: Typical fibre implant

Supplementary Fig. 11. Photograph (total view) of a fibre implant. The inset shows an optical

micrograph of the fibre cross-section containing two electrodes of 30 µm diameter and two of 50

µm diameter. The fibre was cut by ultramicrotome to obtain a high-quality surface. The cross-

section of the 6 implants fibres was observed with an optical microscope and showed similar

features.

100 μm

19

Supplementary Note 12: In vivo and in vitro electrochemical properties of in-fibre MG

electrodes along with typical in-fibre crystalline metallic electrodes and conductive

polymeric nanocomposites

In vitro electrochemical characterization of the fibre electrodes: In vitro measurements were

taken before implantation to verify the functionality of the fibre electrodes. The fibre under test

was immersed in phosphate-buffered saline solution (Gibco PBS, pH 7.4, 1X), together with

counter (platinum rod) and reference (Metrohm, El. Ag/AgCl DJ RN SC: KCl) electrodes. In this

three-electrode configuration, electrochemical impedance spectroscopy (EIS) measurements were

taken using a Gamry 600 potentiostat to characterize the electrochemical properties of the fibre

electrodes. EIS spectra were acquired by injecting sinewave signals of 0.1 V amplitude at the 1 Hz

– 1 MHz frequency range, with 10 data points per decade. Cyclic voltammetry (CV) was

performed in PBS at room temperature. CVs were recorded between voltages of -0.6 and 0.8 V at

a scan rate of 100 mV s-1 after several initial unstable cycles. The cathodic charge-storage capacity

(CSC) was calculated from the integration of current over time. In the same three-electrode setup,

voltage-transient (VT) measurements following constant current pulsing were also taken to assess

the stimulation properties of the fibres. Constant current, symmetric, biphasic, charge-balanced,

cathodic-first pulses were applied between the electrode under test and the counter with an A-M

Systems 2100 Isolated Pulse Stimulator (300 µs per phase-pulse width, 1 s inter-pulse period, 50

µA amplitude), while measuring the voltage across the working and reference electrodes with an

oscilloscope. The VT cycles for all electrodes were performed using the abovementioned pulse at

40 Hz, which is the same frequency used for in vivo stimulation.

Supplementary Fig. 12a reveals a typical capacitive spectrum from low frequency to about

10 kHz, with a less steep slope at frequencies above, indicating, as expected, that the impedance

becomes more dependent on the resistive components of the circuit (wiring, solution resistance) at

high frequencies 14. This is also suggested by the phase plots shown in Supplementary Fig. 12b.

The recorded impedances at 1 kHz is in the range of 34 – 56 kOhm, significantly lower than for

composite electrodes15,16,17,18 and comparable to traditionally used crystalline metallic electrodes15.

20

Supplementary Fig. 12. a, Representative electrochemical impedance spectra (EIS) measured in

vitro on two 30-µm-diameter MG fibre electrodes. b, Impedance phase measured in vitro via EIS

in phosphate-buffered saline solution on two 30-µm-diameter MG electrodes. The plot shows the

transition from a more resistive impedance at high frequency to a capacitive-dominated impedance

towards the low-frequency range. The similar results were obtained from 4 samples.

1 10 100 1k 10k 100k 1M

-80

-60

-40

-20

Impedance p

hase

(deg

rees)

Frequency (Hz)

a b

1 10 100 1k 10k 100k 1M1k

10k

100k

1M

10M

Impedance m

odulu

s (

Ohm

)

Frequency (Hz)

21

Supplementary Table 2 demonstrates that our in-fibre MGs show significantly better charge-

storage capacity (CSC) than that of all existing materials (crystalline metals and polymeric

nanocomposites) in the fibre platform, and exhibit superior CSC compared to commonly reported

values for commercially available probes19,20 and typical crystalline metallic probes21,22,23 in other

platforms.

Supplementary Table 2 | Comparison of the CSC between structured MG electrodes, typical in-

fibre crystalline metal electrodes and conductive nanocomposites, and metallic electrodes in other

platforms. × indicates an unmeasurable CSC and – indicates an extremely low CSC.

Material

Geometrical

surface area

[µm2]

Charge storage

capacity (CSC)

[mC cm-2

]

Scan rate

[mV s-1]

Metallic glasses 986 4.3–11.3 100

In-fibre Sn 986 × 100

In-fibre BiSn 986 × 100

In-fibre CPC 986 –

100

In-fibre CPE 986 –

100

Pt19 7850 1.2 50

PtIr20 17 000 1.2 100

Ti/Pt/Ti21 8400 2.3 50

Sputtered Ir

oxide22 1.4 × 108 2.8 20

Activated Ir

oxide23 37 000 3 100

TiN22 1.4 × 108 0.25 20

22

Ta/Ta2O524 0.0088–0.7

23

Supplementary Fig. 13a-c shows the CV curves of typical in-fibre crystalline metal electrodes

including Sn (pure metal with a high melting point of 230 °C), SnPb (eutectic alloy with an

intermediate melting point of 183 °C), and BiSn (eutectic alloy with a low melting point of 138 °C),

being subject to 5 CV cycles between voltages of -0.2 and 0.2 V at a scan rate of 100 mV s-1. All

crystalline metals exhibit highly unstable CV behaviors, leading to an unmeasurable CSC. After 5

CV cycles, the corresponding SEM micrographs of the fibre cross-sections clearly demonstrate

that these electrodes were entirely damaged after the voltage sweep between -0.2 and 0.2 V

(Supplementary Fig. 13d-f). The poor electrochemical properties of these electrodes may be due

to the fast and drastic oxidation and chemical reactions at the electrodes interface when they were

subjected to a voltage sweep.

Supplementary Fig. 13. a-c, CV curves of in-fibre Sn, SnPb, and BiSn electrodes, respectively,

between -0.2 and 0.2 V at a scan rate of 100 mV s-1 for 5 cycles. d-f, The corresponding SEM

micrographs of the fibre cross-sections after 5 CV cycles. All characterization were repeated 10

times on 10 samples giving the similar results.

50 μm 100 μm 50 μm

a

d e

c

f

b

24

Supplementary Fig. 14 shows the typical VT curves of in-fibre Sn and BiSn electrodes being

subject to 5000 cycles at a current amplitude of 50 µA and a frequency of 40 Hz, which are similar

conditions as the ones used for the in vivo experiment. It can be seen that the VT curves rapidly

lose stability.

Supplementary Fig. 14. a, VT curves of an in-fibre Sn electrode for 5000 VT cycles. b, VT curves

of an in-fibre BiSn electrode for 5000 VT cycles. The geometrical surface area for both of the

electrodes is 986 µm2. The measurements were repeated twice for each material and similar results

were obtained.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Voltage (

V)

Time (ms)

1st pulse

100 pulse

1000 pulse

5000 pulse

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Voltage (

V)

Time (ms)

1st pulse

100 pulse

1000 pulse

5000 pulse

Sn BiSn

a b

25

Supplementary Fig. 15 shows the CV curves of typical in-fibre conductive polymeric

nanocomposite electrodes including carbon-loaded polycarbonate (CPC) and carbon-loaded

polyethylene (CPE), being subjected to 5 CV cycles between voltages of -0.6 V and 0.8 V at a

scan rate of 100 mV s-1. The CV curve at each cycle is noisy, which might be due to the high

resistivity of the two materials. The CSC is thus very low and could not be measured with the

current instrument.

Supplementary Fig. 15. a and b, CV curves of in-fibre CPC and CPE electrodes, respectively,

between -0.6 and 0.8 V at a scan rate of 100 mV s-1. Both characterization were repeated 10 times

on 10 samples giving similar results.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

5E-11

1E-10

1.5E-10

2E-10

2.5E-10

3E-10

Cu

rren

t (A

)

Voltage (V)

CPC, 5 cycles

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

1E-11

1E-10

Curr

ent (A

)

Voltage (V)

CPE, 5 cycles

a b

26

In vivo electrochemical characterization of the fibre electrodes: Once implanted, the fibre

electrodes were characterized by taking EIS and CV measurements in vivo. The methodology is

similar to that followed in vitro, but using a two-electrode configuration, where one multi-stranded

stainless steel wire (Cooner Wire) attached to a skull-fixed screw acts as both counter and reference

electrode. The measurements were taken using a portable PalmSens potentiostat. The parameters

used are as follows: EIS – signal amplitude 0.1 V, frequency range 1 Hz – 50 kHz, 21 data points

per decade; CV – voltage window -0.6 V – 0.8 V, voltage scan rate 100 mV s-1, 3 cycles per

measurement. The CV scans were used to assess the cathodal CSC of some electrodes in vivo. This

is calculated as the time-integral of the negative-current part of the CV curve, where the time-

voltage relationship is given by the 0.1 V scan rate.

Supplementary Fig. 16a shows the in vivo impedance spectra of two 30 µm-diameter MG

electrodes. The recorded impedances at 1 kHz are in the range of 92 – 185 kOhm, significantly

lower than that of previously reported devices15,18 Supplementary Fig. 16b shows an example of

the CV measurement, with a calculated CSC of about 6 mC cm-2 for a fibre electrode of 30 µm

diameter. The CSC could be certainly improved by optimizing (roughening, structuring, etc.) the

exposed surface of the fibre.

Supplementary Fig. 16. a, Representative electrochemical impedance spectra (EIS) measured in

vivo on two 30-µm-diameter MG fibre electrodes. b, Representative example of a cyclic

voltammetry (CV) curve measured in vivo 1 week post implantation. The plot shows the second

CV cycle in a repeated measurement session. The greyed region is the area used for the calculation

of the cathodal charge storage capacity (CSC). The characterization were repeated 4 times on 4

samples giving the similar results.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-30

-20

-10

0

10

Curr

ent (n

A)

Voltage (V)

-4

-3

-2

-1

0

1

Curr

ent density (

mA

/cm

2)

a b

1 10 100 1k 10k 100k1k

10k

100k

1M

10M

Impedance m

odulu

s (

Ohm

)

Frequency (Hz)

27

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