Page 51 - IJB-2-2

Huijun Li, Sijun Liu and Lin Li

−∆ P The shear rate γ  in the nozzle can be calculated
2
u = (R − r 2 ) (6)
η
4 L from the following equation
Rheological study was conducted on the alginate   n
based hydrogels with different concentrations using a  2 
plate rheometer. Based on the power-law model and γ =  VR  .r (12)
22
n

2
experimental data, the constant m and n for each sam-    n  3n+  n 1 
ple can be obtained through curve fitting. These two   3n +   1  (R 2 )   
parameters will be used in further calculation to de-
duce the shear rate that the hydrogel undergoes during 2.3 Rheological Evaluations of Alginate Hydrogels
the printing process.
Assuming that in the syringe there is a uniform The rheological properties of the alginate hydrogels
flow rate (V), the volumetric flow rate (Q) of a non- were measured by using a rotational rheometer (DHR,
Newtonian fluid can be written as follows [27] TA Instruments, USA). A 40 mm parallel plate with a
1  3n+  1 measurement gap of 0.55 mm was used. First of all,
2
Q π = RV = π   n  −∆ p  n  R   n   (7) strain sweeps in the range of 0.1%−100% at the fr e-

1
 3n +  2mL  quencies of 0.1−2.0 Hz were carried out in order to
where L and Δp are length of the syringe and the determine the linear viscoelastic range of the samples.
pressure drop, respectively. The following three rheological experiments at room
Then the pressure drop can be expressed as follows temperature were adopted in order to explore the rhe-
ological properties of samples: (1) frequency sweep
  n tests over an angular frequency range of 0.01–100 rad/s
  at a constant strain of 2%; (2) steady-state flow tests
 VR 2 
p
∆= − 2mL   (8) in a range of shear rate 0.5−500 s –1 ; and (3) recovery
  n   3n+ 1   tests under a low shear rate.
   R n    
 3n +   1       2.4 Fabrication of 3D Structure
The shear rate γ  in the pipe can be described as An extrusion-based printer for scaffold fabrication
follows was employed in this study. The printing system con-
n sists of two parts: a high precision displacement pump
 
 2  (TechnoDigm, PDP 1000, Singapore) and a desktop
n
γ =   VR 3n+  1  .r (9) xyz motor (TechnoDigm, DR3331T-EX, Singapore).

   n   (R n )  The printing head was mounted on the printing system
 3n +   1    to print along the preprogrammed tracks with an ad-
Assuming that the volume of hydrogels does not justable speed (15 mm/s used in this study). The prin-
change before and after printing, ting head consists of a piston, a syringe, and a chan-

 D  2 geable nozzle. The displacement pump drives the pis-
V = V 1  1  (10) ton with a controllable speed (0.009 mm/s) to extrude
2
 D 2  hydrogels from the syringe on a glass slide. The 3D
where D 1 is the inner diameter of the syringe, V 1 is the scaffolds were fabricated at room temperature. Firstly,
speed of the piston, D 2 is the inner diameter of the a 3D structure to be printed was preprogrammed on
nozzle, and V 2 is the speed of extruded hydrogel in the the 3D printing system to define the extrusion route
nozzle, for the hydrogel. Secondly, the hydrogel was loaded
From equation (9), the shear rate γ  in the syringe into the syringe and then the syringe was installed on
can be calculated from the following equation the dispensing unit. Under the action of the piston at
the speed as mentioned previously, the hydrogel
  n loaded in the syringe was extruded through a 0.25 mm
 2  nozzle while the dispenser was moving at a defined
n
11

γ =   VR 3n+  1  .r (11) speed. Once the first layer was formed, the nozzle was
1
  n  (R n )  lift up and then continued to move along the prepro-
  3n +   1  1    grammed tracks at the same speed to form the second
International Journal of Bioprinting (2016)–Volume 2, Issue 2 57