STUDY OF STRUCTURAL MODIFICATIONS IN POLY(L-LACTIDE)

INDUCED BY HIGH-ENERGY RADIATION

Dejan Milicevic and Edin Suljovrujic

Department of Radiation Chemistry and Physics, „VINČA" Institute of Nuclear Sciences -

National Institute of the Republic of Serbia, University of Belgrade, Belgrade, Serbia

30th Summer School and International Symposium on the Physics of Ionized GasesAugust 24-28, 2020

Coronary Heart Disease (CHD)

- Coronary heart disease is the leading cause of death worldwide.

- Plaque in blood accumulates against the walls of the coronary

artery, which returns oxygen rich blood from lungs to heart. If left

untreated, it could lead to blood clots/heart attacks.

CHD treatment options:

Surgery (Bypass, High Risk)

Medication treatment (Less Effective)

Angioplasty

Angioplasty:

Minimally invasive procedure; heart stents,

usually made of a metal mesh, are inserted

into the body thus enabling the blood flow

increase.

Possible problems with metalic and metal-mesh stents:

- Effective, but often delay natural healing process of the arteries.

- If metal mesh is not placed in suitably, plaque may build up on the

stent, eventually killing 1 in 200 patients in procedure.

Possible solution:

- The use of a biodegradable polymer drug

eluting stent (DES) mainly produced out of

biodegradabile polylactide. These stents

are designed to completely dissolve into the

bloodstream within 2-3 years after procedure

(no need for additional surgical procedure)

and are found to be safer than metal stents

(the risk of stent thrombosis and heart attack

are significantly reduced).

The sterility of the product - one of the main concerns with

respect to the successful application of implanted biomaterial

- Several different sterilization procedures were proposed.

- In the case of polymer implants, their high sensitivity to heat, moisture and

radiation should be taken into account when a particular method is applied.

- Amongst conventional methods, dry and moist heat sterilizations are not

suitable for thermosensitive polymers as they lead to thermal degradation and

structural changes.

- Ethylene oxide (EO) is broadly used for polymeric devices although including

drawbacks such as its toxicity, changes in morphology and the presence of gas

residues, which requires long aeration time.

- Contrary, due to the excellent penetration characteristics of high-energy

ionizing radiation (in the form of gamma radiation from a suitable radioisotopic

source such as 60Co or of electrons energized by a suitable electron

accelerator). Radiation sterilization eliminates problems which may appear

with other methods like high temperature, need for quarantine, problems

associated with residuals and EO gas penetration, uniformity of sterilization etc.

Problems with the radiation sometimes evolve as a result of its damaging effects

on materials.

Experimental: Materials

Polylactide (PLA): linear thermoplastic aliphatic polyester derived from renewable

resources such as corn starch. It is extensively employed as a biomaterial in numerous

applications including tissue engineering three-dimensional scaffolds, implantable medical

devices and carriers in controlled drug delivery systems, to the environmentally friendly

packaging ones. Degradation of this biocompatible and biodegradable polymer lead to non-

toxic products as it enters the Kreb’s cycle inside the human body and is eventually removed

as carbon dioxide and water.

(a) (b) (c)

PLA exists in two stereo-isomeric forms: (a) D-PLA (PDLA)

and (b) L-PLA (PLLA), and (c) a mixture of D- and L-lactic

acid also exists as D,L-PLA (PDLLA)

Polymer obtained from L-lactide - PLLA was used in this

study: Commercial granular poly L-lactide (Fluka, Germany;

Mw=100,000, Mn=60,000)

The polymers, which are derived from

optically active D and L monomers, are

semi-crystalline while the optically inactive

D,L-PLA is amorphous. Generally, L-PLA

is preferred in applications where high

mechanical strength and toughness are

required, e.g. sutures and orthopedic

devices. In contrast, due to the amorphous

nature of D,L-PLA, it is usually considered

for applications such as drug delivery

systems, as it is important to have a

homogeneous dispersion of the active

species within a monophasic matrix.

Experimental methods:

1. Scanning electronic microscopy (SEM)

2. Gel permeation chromatography (GPC)

3. Differential scanning calorimetry (DSC)

4. Wide-angle X-ray diffraction method (WAXD)

5. Fourier transform infrared (FTIR) spectroscopy

6. Electron spin resonance (ESR) spectroscopy

Results and discussion

Scanning electronic microscopy (SEM)

Figure 1. SEM images of (un)irradiated samples obtained by: (a-c) quenching

from the isotropic melt in the ice-water mixture and (d-f) slow cooling to the

room temperature keeping the sample between the press platens;

100m

100m 100m10m

10m 10m

(d)

(a) 0 kGy (c) 300 kGy

0 kGy

(b) 50 kGy

(e) 0 kGy (f) 300 kGy

Gel permeation chromatography (GPC)

2*104

105

Mn

[g/m

ol]

Absorbed dose [kGy]

(a)

0 100 200 300

quenchedslowly cooled

6*104

M n

,0/M

n,t

Absorbed dose [kGy]

0 100 200 3000

2

4

6

quenched

slowly cooled

(b)

Figure 2. (a) Number average molecular weight (Mn) as a function of absorbed

dose for quenched and slowly cooled samples and (b) Mn,0/Mn,t ratio as a

function of absorbed dose, where Mn,0 and Mn,t are molecular weights of the

unirradiated and irradiated samples, respectively.

Figure 3. DSC heating thermograms of (a) quenched and (b) slowly cooled

samples for various absorbed doses.

(a)

50 kGy

0 kGy

100 kGy

300 kGy

320 360 400 440 480

T[K]

DH

/Dt

[Jg

-1s

-1]

2 Jg-1s-1

En

do

(b)

50 kGy

0 kGy

100 kGy

300 kGy

2.5 Jg-1s-1

En

do

320 360 400 440 480

T[K]

DH

/Dt[J

g-1

s-1

]

Differential scanning calorimetry (DSC)

slowly cooledquenched

12 3

Figure 4. (a) Melting (ΔHm) and cold crystallization (ΔHcc) enthalpy and ΔHo = ΔHm

- ΔHcc of quenched samples as a function of absorbed dose and (b) ΔH0 enthalpy of

the quenched and slowly cooled samples.

Absorbed dose [kGy]

(a)

100 200 3000

En

tha

lpy [

Jg

-1]

DHm

DHO=DHm-DHCC

DHCC

10

20

30

40

50

60

Absorbed dose [kGy]

(b)

quenchedslowly cooled

En

thalp

y [

Jg

-1]

10

20

30

60

70

80

100 200 3000

quenched

Inte

nsit

y [

a.u

.]

2Ө ( o )10 20 30

(200)/(110)

(010)

(203)

(015)

5 x

20 40 60 80

(a)

50 kGy

0 kGy

100 kGy

300 kGy

(200)/(110)

(203)

10 20 30

(200)/(110)

(010)(203)

(015)

(b)

10 20 30

(c)

(d)

50 kGy

0 kGy

100 kGy

300 kGy

quenched

slowly cooled quenched

slowly cooled

Figure 5. WAXD spectra of unirradiated quenched and slowly cooled samples in the 2θ range: (a) 5°-90° and (b) 5°-35° (quenched samples

are presented magnified for clarity); the evolution of WAXD spectra with absorbed dose for (c) quenched and (d) slowly cooled samples.

Wide-angle X-ray diffraction method (WAXD)

Figure 6. Crystallinity of: (a) quenched and

slowly cooled samples obtained by DSC and

WAXD measurements, (b) quenched and

slowly cooled (un)annealed samples (in air, at

80 °C for 2 h after irradiation); (c) FWHM (B)

and crystal size as functions of absorbed dose.

DSC

quenched

slowly cooled

quenched an.

slowly cooled an.

0 50 100

20

40

60

80

Absorbed dose [kGy]

Cry

sta

llin

ity [

%]

(b)

0 50 100

20

40

60

80

DSC

WAXD

quenched

slowly cooled

Absorbed dose [kGy]

Cry

sta

llin

ity [

%]

(a)B

(°

)

0 100 200 3000,20

0,24

0,28

0,32

FWHM

Crystal size

26

30

34

38

Cry

sta

l s

ize

[n

m]

Absorbed dose [kGy]

(c)

Fourier transform infrared (FTIR) spectroscopy

Figure 7. FTIR spectra

of (a) unirradiated

quenched and slowly

cooled samples and

(b) quenched samples

for different absorbed

doses.Wavenumber [cm-1]

quenched

slowly cooled

Re

fle

cti

vit

y[a

.u.]

960 920 880 840

917868

954 860

(a)

50 kGy

0 kGy

100 kGy

300 kGy

1760 1720 1400 1300 1200 1100 1000

(b) quenched

r(CH3) + ν(C-COO)

δ(CH3); δ(CH)

ν(C-CH3); r(CH3)

ν(COC) + r(CH3)

ν(COC); ν(C-CH3)

1760 1720 1400 1300 1200 1100 1000

ν(C=O)

Electron spin resonance (ESR) spectroscopy

Figure 8. (a) ESR spectra of quenched samples (Qs); ESR

spectra of slowly cooled samples (SCs) for (b) short and (c)

prolonged period of time; (d) ESR spectra of SCs annealed

(SCA) for different periods of time

330 335 340

H [mT]

dI/

dH

(a.u

.)100 kGy(a)

T=20 oC

1 h4 h

10 h

1 h6 h

12 h

(b)

Qs

SCs

–C*(CH3)-

dI/

dH

(a.u

.)

330 335 340

0 h2 h8 h

1 h6 days

20 days

Annealing time

TO=80 oC

(d)

SCs

100 kGy

T =20 oC

100 kGy

(c)

H [mT]

SCA

Figure 9. Relative free radical concentration (FRC) as a function of (a) short and

(b) prolonged periods of time; (c) Relative FRC versus annealing time.

Rela

tive F

RC

(I/I 0

)

Time [h]

0 6 12

0.0

0.5

1.0

quenched

slowly cooled

(a)

Time [days]

0 10 20

0.0

0.5

1.0

(b)

SCs

0 5 10

0.0

0.5

1.0

Re

lati

ve

FR

C (

I/I0

)

Annealing time [h]

TO=80 oC

SCA

(c)

Re

lati

ve

FR

C (

I/I 0

)

100 kGy

CONCLUSIONS:

• Depending on the initial preparation conditions, the radiation-induced

changes in the structure and properties of PLLA, as well as the evolution of

free radicals, differ significantly.

• The low crystalline samples are found to be initially more susceptible to

the gamma radiation than the high crystalline ones.

• On the other hand, due to the presence of long-lived free radicals, the

high crystalline samples are more prone to the post-irradiation degradation.

In addition, the applied annealing treatment substantially reduces the

concentration of long-lived radicals, but can also induce additional

crystallisation.

• As the gamma radiation leads to quite foreseeable alterations of thermal

and morphological properties of PLLA, the radiation processing can be

applied with the aim of simultaneous tailoring of required functionality

(e.g. the rate of hydrolytic degradation) and achieving the sterility of final

products, especially for low sterilization doses where the observed

changes are small and tolerable (up to 25 kGy in most circumstances).

Thank you for your attention!

Questions???