Bioactive nano-ceramic reinforced polymer has been under study as bone substitutes since early 1980s [1–8]. When developing a bone substitute material, its stiffness is considered a crucial aspect. It is known that the bone remodeling is strongly dependent on sufficient and proper loading conditions on the bone. Thus, the load transferred from the implant or bone substitute to the surrounding bones depends on the bone substitute stiffness [9, 10]. Therefore, the replacement of hard tissue requires biocompatible as well as bioactive materials with properties similar to those of natural bone, to avoid the surrounding bone osteoporosis [11].

High Density Polyethylene (HDPE) is one of the most often used polymers in biomedical applications such as bone substitutes and orthopedic prostheses. The limitations of HDPE are its visco-elastic behavior, low modulus and poor bioactivity. Therefore, great efforts have been done to overcome these weaknesses [12–16]. To improve HDPE properties, different types of ceramic nano-particles can be added to form polymer matrix composite that can be used as alternatives to metals for bone substitutes and orthopedic implant applications [16–18]. Different types of ceramic materials such as carbon nanofibers, nano-clay and Hydroxyapatite (HA) have been used as fillers for producing HDPE composites [17–19]. For biomedical application, HA nano-particles have been widely used for improving the mechanical and bioactivity behavior of HDPE. The HA improves the composite stiffness and bioactivity, while HDPE provides toughness. An important design consideration in the long term uses of HDPE, as bone substitutes and orthopedic prostheses, is its integrity under applied load. As maintained earlier, HDPE is a viscoelastic material in nature and its deformation behavior is stress and time dependent. The HDPE viscoelastic and creep behavior easily describe its deformation under load with time and give useful data about the long term integrity of the material in such applications [17–21]. Therefore, the addition of HA nanoparticles as reinforcement may improve both bioactivity, and viscoelastic and creep behavior of HDPE.

Gamma radiation is widely used for sterilizing most of the medical products that made of plastics, because it decomposes the DNA molecules of living organisms. The material has usually been irradiated between 25 and 70 kGy [22, 23]. Nevertheless, this type of sterilization process can produce changes in the molecular structure of the polymer. It has negative effects on the entanglement density of long molecular chains as well as concentration of the tie molecules. On the other hand, the sterilization promotes crosslinking and affects the composite properties [22, 23]. Kane RJ et al. [24] studied the effects of hydroxyapatite (HA) morphology and content on the fatigue behavior of HA reinforced high density polyethylene (HDPE), which suggested that the HA whiskers directly improved the fatigue life and damage tolerance of HA reinforced polymers for synthetic bone substitutes. Soltani Z. et al. [25] studied the effects of irradiation on the mechanical and thermal properties of LDPE/HA powder which was synthesized via hydrolysis method. The samples were subjected to irradiation under 10 MeV electron beam in 75-250 kGy doses. They found that the mechanical and thermal properties more dependent on the hydroxyapatite content than on radiation. Yakov BU. et al. [26] studied the changes in the creep behavior of biaxially oriented LLDPE due to gamma irradiation. Their results showed an improvement in the creep resistance with irradiation does below 4 MR. Yeo S S. and Hsuan YG. [27] studied the tensile creep behavior of polyethylene-terephthalate (PET) and HDPE geogrids. The results showed that the creep deformation of PET geogrid is less than that of HDPE geogrid. The HDPE geogrid exhibited primary, secondary, and tertiary creep stages before rupture, whereas only primary and tertiary creep were observed in the PET geogrid.

From the literature, it can be remarked that a few studies have investigated the effects of gamma radiation dose and HA nano particles percentage on the viscoelastic, creep-recovery and relaxation behavior of HDPE/HA nano composite for biomedical applications. Therefore, the present study is a part of research project that intended to (1) fabricate homogenous HDPE/HA nano-composite, and (2) study the effects of HA nano particles percentage and gamma irradiation dose on the thermal, viscoelastic and creep-relaxation behavior of the HDPE/HA nano-composite.

The polymeric matrix used in the present work is an injection molding grade of HDPE acquired from the Saudi market. The average molecular weight ranges from 700000 to 800000 g/mol. The reported density is about 0.94 g/cm³. High purity spray dried synthetic hydroxyapatite nano particles (Fluidinova, Engenharia de Fluidos, SA, Portugal) were used as reinforcement for HDPE matrix. The typical size of HA nano particle is around 100 nm and the average aggregate size is 2.5 μm. The specific surface area of the HA nano particles is 100 m²/g.

HDPE and HA nano particles were first physically mixed according to the desired weight fractions. HDPE/HA ratios of 0%, 10%, 20% and 30% wt were fed on Farrell co-rotating intermeshing twin screw extruder. The screw speed and temperature were kept constant at 12 rpm and 190°C respectively. The strings from the extruder were cooled in a water bath at about 12°C, air dried, then palletized into granules. The granules were further dried and conditioned in the lab environment for 2 days. The granules were then injection molded to get a set of standard ASTM D638 type-I specimens [28]. The nano composite specimens were exposed to gamma irradiation at doses of 0, 35 and 70 kGy at rate of 5 kGy/hr at room temperature in vacuum.

The cryo-fracture surfaces of HDPE/HA nano composites were examined using Scanning Electron Microscope (SEM Model ISM 6360A, Jeol Company, Japan) at 20 KV. The specimens were fixed with double coated carbon tape. All the specimens were coated with Platinum before scanning to dissipate the build-up of heat and electrical charges using SEM using JEOL, IFC 1600 coating machine. The melt flow index (MFI) test was performed on a Ray-Ran advanced melt flow system following ASTM D-1238 at 190°C and load of 2.16 Kg.

Calorimetric measurements for neat HDPE and its nanocomposite specimens were performed using Differential Scanning Calorimetry (DSC-7 series, Thermal Analysis System, Perkin Elmer, Covina, California, USA). Each specimen (5–7 mg) was sealed in an aluminum pan and heated from 25 to 145°C at rate of 5°C/min and then cooled down to 25°C at cooling rate of 5°C/min. The heat of melting was calculated through the integration of the area under the DSC endothermic peak of the DSC thermogram. The melting temperatures of the blends were taken at the peaks of the melting processes. The percentage crystallinity was calculated by normalizing the heat of melting to that of 100% crystalline PE (290 J/g) [21, 25–27]. Thermogravimetric analysis (TGA) of HDPE/HA was conducted using TA instrument (Q500 TGA, United States). The precisely weighed specimen was heated to 550°C at a rate of 10°C/min under nitrogen flow rate of 35 ml/min.

The tensile storage and loss modulus (viscoelastic behavior) of irradiated and non-irradiated HDPE/HA nano-composites specimens were characterized using standard Dynamic Mechanical Analysis Machine (DMA Model 2980TA Instrument, New Castle, Delaware, USA). The specimens were tested over a frequency range from 0.01 to 100 Hz at temperature 25°C. The DMA was also used to measure the creep-recovery and relaxation behavior HDPE/HA nano-composites specimens. The creep behavior of HDPE/HA specimens was obtained by applying constant stresses of 2, 5, 10 MPa for 8 hrs at room temperature. The recovery behavior was measured for 8 hrs after removing the applied stress.

The DMA machine was also used for measuring the relaxation behavior of HDPE/HA specimens. During the relaxation tests different HDPE/HA nano-composite specimens were subjected to a pre-determined strain level of 3% that was held constant while the decaying in the stress with time was recorded during test period of 3 hrs at room temperature. All the experiments were repeated at least three times, conducted in triplicate (n = 3) and error bars represent standard deviations (SD). All the values were represented as means ± SD.

Commercially available hydroxyapatite nanoparticles (~100 nm size and specific surface area of 100 m²/g.) were used as reinforcement of locally available HDPE with an average molecular weight of 700,000 to 800,000 g/mol in a different ratios of 0%, 10%, 20% and 30% wt. After twin screw extrusion and injection molding, a good distribution of HDPE and HA nanoparticles was successfully prepared and exposed to gamma irradiation at doses of 0, 35 and 70 kGy at a rate of 5 kGy/hr, at room temperature It is found that melting temperature was insignificantly affected by the addition of HA nanoparticles while the crystallinity of HDPE composites reduced due to the restriction of mobility of the molecules. The tensile storage and loss moduli, and creep resistance of the nanocomposites increased proportionally with the HA particle content. Irradiation of HDPE and its nanocomposites resulted in an increase of the tensile storage and loss moduli, creep recovery and resistance. In conclusion it was found that increasing the HA content and irradiation improved the relaxation behavior of HDPE and its nanocomposites significantly.